There seems to be floating about among some people I know online the
idea that keeping the central heating on all the time is cheaper than
turning it off at night. Not sure where it comes from, but seeing as
they all frequent the same/ related e-mail lists, maybe from there.

The basic idea seems to be that it costs more to heat the house back up
in the morning, than to keep it at the same temp.

now I know it depends on things like internal and external temp (we have
a programmable stat set at 12-13 C, it hardly seems to turn on except
when down to about freezing outside from what I can tell),

But in principle it seems rubbish, it must use more energy to keep
something hot, than to let it cool and then reheat., but a couple of
people had been hard to convince, esp. as a 'plumber told me it)

Or is my brain addled?? :-)
--
Chris French

On 19 Dec, 16:12, chris French
wrote:
There seems to be floating about among some people I know online the
idea that keeping the central heating on all the time is cheaper than
turning it off at night. Not sure where it comes from, but seeing as
they all frequent the same/ related e-mail lists, maybe from there.

The basic idea seems to be that it costs more *to heat the house back up
in the morning, than to keep it at the same temp.

now I know it depends on things like internal and external temp (we have
a programmable stat set at 12-13 C, it hardly seems to turn on except
when down to about freezing outside from what I can tell),

But in principle it seems rubbish, it must use more energy to keep
something hot, than to let it cool and then reheat., but a couple of
people had been hard to convince, esp. as a 'plumber told me it)

Or is my brain addled?? :-)
--
Chris French

ISTR hearing that the story began with experiments on immersion
heaters and was erroneously extrapolated to central heating which is,
of course, completely different.

Some time ago I carried out experiments both in my home and church
heating systems. I concluded that there was very little in it in
either case, but that it was slightly cheaper to switch off at night.
It also depends, of course, whether you're in all day or just the
evenings. You'll just have to experiment yourself. The hard bit is
finding two periods with identical outside temperatures.

Chris

chris French wrote:
There seems to be floating about among some people I know online the
idea that keeping the central heating on all the time is cheaper than
turning it off at night. Not sure where it comes from, but seeing as
they all frequent the same/ related e-mail lists, maybe from there.

The basic idea seems to be that it costs more to heat the house back
up in the morning, than to keep it at the same temp.

now I know it depends on things like internal and external temp (we
have a programmable stat set at 12-13 C, it hardly seems to turn on
except when down to about freezing outside from what I can tell),

But in principle it seems rubbish, it must use more energy to keep
something hot, than to let it cool and then reheat., but a couple of
people had been hard to convince, esp. as a 'plumber told me it)

Or is my brain addled?? :-)

The rate of heat loss from a system always increases as the temperature
difference between inside and outside increases. In fact it's pretty much
proportional to that difference. You therefore MUST always use more energy
keeping that temperature difference constantly high than letting it cool for
a while and reheating it again as needed.

However with a reasonably well insulated house the net extra cost is very
low. If we assume in cold weather an 18 degree C difference between inside
and outside when the heat is on, 16 hours of heat on during the day and 8
with it off overnight and a 4 degree total drop in temp of the house
overnight (i.e. an average of 2 degrees) the net extra cost of keeping the
heating on all the time is only about 4%. That's less than you'd save just
turning the thermostat temperature down by 1 degree.

For a poorly insulated house the savings would be greater but still not very
much. For the saving to be as much as 10% the house temperature would have
to drop by 10 degrees C overnight for which you'd probably have to leave all
the windows open.

As with most things where some people think one thing and some another the
reason is simple. There's not much difference in the two approaches.

The two biggest influences on heating costs are the temperature you set and
the insulation of the house. Turning the heat off at night is negligible in
comparison to either of those.
--
Dave Baker

"chris French" wrote in message
...
There seems to be floating about among some people I know online the
idea that keeping the central heating on all the time is cheaper
than
turning it off at night. Not sure where it comes from, but seeing as
they all frequent the same/ related e-mail lists, maybe from there.

The basic idea seems to be that it costs more to heat the house
back up
in the morning, than to keep it at the same temp.

now I know it depends on things like internal and external temp (we
have
a programmable stat set at 12-13 C, it hardly seems to turn on
except
when down to about freezing outside from what I can tell),

But in principle it seems rubbish, it must use more energy to keep
something hot, than to let it cool and then reheat., but a couple of
people had been hard to convince, esp. as a 'plumber told me it)

Or is my brain addled?? :-)
--
Chris French


Yes - it's a load of tommyrot - obviously more energy used keeping it
hot all the time.

Now I did work in an oil refinery where if they let the pipes cool it
got very expensive, as the tarry gunge set solid, never to move again
G

AWEM

On Fri, 19 Dec 2008 15:41:51 UTC, wrote:

ISTR hearing that the story began with experiments on immersion
heaters and was erroneously extrapolated to central heating which is,
of course, completely different.

It's not valid for them anyway.

You have something which, left to its own devices, loses heat over time.
The rate of heat loss will reduce as its temperature gets nearer to that
of its surroundings (heat transfer rate depends on difference in
temperature).

If left to cool, there is reducing heat loss over whatever time elapses.
If kept at same temperature, there is heat loss over the same time, but
it's constant (-ish) because the temperature difference does not reduce
- i.e. more heat is lost over the same time.

In both cases, that heat loss has to be made up. That's how much
fuel/cost will be involved. More heat lost - more cost.
--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
http://www.diybanter.com

On 19 Dec, 16:21, "Bob Eager" wrote:
On Fri, 19 Dec 2008 15:41:51 UTC, wrote:
ISTR hearing that the story began with experiments on immersion
heaters and was erroneously extrapolated to central heating which is,
of course, completely different.

It's not valid for them anyway.

You have something which, left to its own devices, loses heat over time.
The rate of heat loss will reduce as its temperature gets nearer to that
of its surroundings (heat transfer rate depends on difference in
temperature).

If left to cool, there is reducing heat loss over whatever time elapses.
If kept at same temperature, there is heat loss over the same time, but
it's constant (-ish) because the temperature difference does not reduce
- i.e. more heat is lost over the same time.

In both cases, that heat loss has to be made up. That's how much
fuel/cost will be involved. More heat lost - more cost.
--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
http://www.diybanter.com

ISTR there was some point about older (solid wall) houses that fair
worse if unheated for periods as the walls get damp from rain etc and
so become even worse at insulating until they are dried out again -
which takes more heat than if they were kept up to temp (and losing
heat) throughout same period - even so still seems a bit
"mathematical" to me to be relied on.....

Jim

In article , chris French
writes
There seems to be floating about among some people I know online the
idea that keeping the central heating on all the time is cheaper than
turning it off at night.

But in principle it seems rubbish, it must use more energy to keep
something hot, than to let it cool and then reheat.,
Of course you are right.

but a couple of
people had been hard to convince, esp. as a 'plumber told me it)
Arguing with idiots is seldom satisfying.
--
fred
BBC3, ITV2/3/4, channels going to the DOGs

"Bob Eager" wrote in message
...
On Fri, 19 Dec 2008 15:41:51 UTC, wrote:

ISTR hearing that the story began with experiments on immersion
heaters and was erroneously extrapolated to central heating which is,
of course, completely different.

It's not valid for them anyway.

You have something which, left to its own devices, loses heat over time.
The rate of heat loss will reduce as its temperature gets nearer to that
of its surroundings (heat transfer rate depends on difference in
temperature).

If left to cool, there is reducing heat loss over whatever time elapses.
If kept at same temperature, there is heat loss over the same time, but
it's constant (-ish) because the temperature difference does not reduce
- i.e. more heat is lost over the same time.

In both cases, that heat loss has to be made up. That's how much
fuel/cost will be involved. More heat lost - more cost.

I'm confused. Is that a yes or a no to the original question?

tim

"chris French" wrote in message
...
There seems to be floating about among some people I know online the idea
that keeping the central heating on all the time is cheaper than turning
it off at night. Not sure where it comes from, but seeing as they all
frequent the same/ related e-mail lists, maybe from there.

The basic idea seems to be that it costs more to heat the house back up
in the morning, than to keep it at the same temp.

now I know it depends on things like internal and external temp (we have a
programmable stat set at 12-13 C, it hardly seems to turn on except when
down to about freezing outside from what I can tell),

But in principle it seems rubbish, it must use more energy to keep
something hot, than to let it cool and then reheat., but a couple of
people had been hard to convince, esp. as a 'plumber told me it)

I discovered that in my, very leaky, house if I had the heating off whilst I
was out all day (or asleep), when I wanted it back on in order to make the
room warm enough, quickly enough, I had to turn the boiler temperature up
and that this did use more gas.

So, in order to make the room warm enough when I got home (awake), using the
normal boiler temp, I set the heating to come hours before and it still
wasn't warm enough. I progressively made the "on" time earlier and earlier
until it ended up only being off for about 2 hours in the day and 4 at
night.

I gave up at that point and it's now on all day, costing me about half a
meter unit a day.

tim

wrote in message
...
On 19 Dec, 16:12, chris French
wrote:
There seems to be floating about among some people I know online the
idea that keeping the central heating on all the time is cheaper than
turning it off at night. Not sure where it comes from, but seeing as
they all frequent the same/ related e-mail lists, maybe from there.

The basic idea seems to be that it costs more to heat the house back up
in the morning, than to keep it at the same temp.

now I know it depends on things like internal and external temp (we have
a programmable stat set at 12-13 C, it hardly seems to turn on except
when down to about freezing outside from what I can tell),

But in principle it seems rubbish, it must use more energy to keep
something hot, than to let it cool and then reheat., but a couple of
people had been hard to convince, esp. as a 'plumber told me it)

Or is my brain addled?? :-)
--
Chris French

ISTR hearing that the story began with experiments on immersion
heaters and was erroneously extrapolated to central heating which is,
of course, completely different.

--------------------------------------------------------------------

so what was the result of the immersion heater and in what way was it
completely different (I agree that it is, I interested in what the tester
thought was the difference)

tim

jim wrote:
On 19 Dec, 16:21, "Bob Eager" wrote:
On Fri, 19 Dec 2008 15:41:51 UTC, wrote:

ISTR hearing that the story began with experiments on immersion
heaters and was erroneously extrapolated to central heating which is,
of course, completely different.

It's not valid for them anyway.

You have something which, left to its own devices, loses heat over time.
The rate of heat loss will reduce as its temperature gets nearer to that
of its surroundings (heat transfer rate depends on difference in
temperature).

If left to cool, there is reducing heat loss over whatever time elapses.
If kept at same temperature, there is heat loss over the same time, but
it's constant (-ish) because the temperature difference does not reduce
- i.e. more heat is lost over the same time.

In both cases, that heat loss has to be made up. That's how much
fuel/cost will be involved. More heat lost - more cost.

ISTR there was some point about older (solid wall) houses that fair
worse if unheated for periods as the walls get damp from rain etc and
so become even worse at insulating until they are dried out again -
which takes more heat than if they were kept up to temp (and losing
heat) throughout same period - even so still seems a bit
"mathematical" to me to be relied on.....

Jim

But switching the heating off overnight doesnt see a house lose much
temperature. To get the effect you mention it would need to be off a
month or more.


NT

chris French wrote:
There seems to be floating about among some people I know online the
idea that keeping the central heating on all the time is cheaper than
turning it off at night. Not sure where it comes from, but seeing as
they all frequent the same/ related e-mail lists, maybe from there.

The basic idea seems to be that it costs more to heat the house back up
in the morning, than to keep it at the same temp.

now I know it depends on things like internal and external temp (we have
a programmable stat set at 12-13 C, it hardly seems to turn on except
when down to about freezing outside from what I can tell),

But in principle it seems rubbish, it must use more energy to keep
something hot, than to let it cool and then reheat., but a couple of
people had been hard to convince, esp. as a 'plumber told me it)

Or is my brain addled?? :-)

I've heard one reasonable idea for when this might be true - in obscure
circumstances.

If when cool the walls drop below the dewpoint they will start soaking
up water. Wet walls are not as good an insulator as dry ones.

So if when the heating is off your walls start getting damp, then *and
only then* it might be worth leaving the heating on. Or more likely,
buying a dehumidifier to keep the mould under control!

Andy

On Fri, 19 Dec 2008 19:18:04 UTC, "tim....."
wrote:


"Bob Eager" wrote in message
...
On Fri, 19 Dec 2008 15:41:51 UTC, wrote:

ISTR hearing that the story began with experiments on immersion
heaters and was erroneously extrapolated to central heating which is,
of course, completely different.

It's not valid for them anyway.

You have something which, left to its own devices, loses heat over time.
The rate of heat loss will reduce as its temperature gets nearer to that
of its surroundings (heat transfer rate depends on difference in
temperature).

If left to cool, there is reducing heat loss over whatever time elapses.
If kept at same temperature, there is heat loss over the same time, but
it's constant (-ish) because the temperature difference does not reduce
- i.e. more heat is lost over the same time.

In both cases, that heat loss has to be made up. That's how much
fuel/cost will be involved. More heat lost - more cost.

I'm confused. Is that a yes or a no to the original question?

In summary - anyone who says that it's cheaper to leave it on, rather
than turn it off and then heat it up again later, is talking nonsense.
No matter whether it's an immersion heater, central heating or whatever.

--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
http://www.diybanter.com

"Bob Eager" wrote in message
...
On Fri, 19 Dec 2008 19:18:04 UTC, "tim....."
wrote:


"Bob Eager" wrote in message
...
On Fri, 19 Dec 2008 15:41:51 UTC, wrote:

ISTR hearing that the story began with experiments on immersion
heaters and was erroneously extrapolated to central heating which is,
of course, completely different.

It's not valid for them anyway.

You have something which, left to its own devices, loses heat over
time.
The rate of heat loss will reduce as its temperature gets nearer to
that
of its surroundings (heat transfer rate depends on difference in
temperature).

If left to cool, there is reducing heat loss over whatever time
elapses.
If kept at same temperature, there is heat loss over the same time, but
it's constant (-ish) because the temperature difference does not reduce
- i.e. more heat is lost over the same time.

In both cases, that heat loss has to be made up. That's how much
fuel/cost will be involved. More heat lost - more cost.

I'm confused. Is that a yes or a no to the original question?

In summary - anyone who says that it's cheaper to leave it on, rather
than turn it off and then heat it up again later, is talking nonsense.
No matter whether it's an immersion heater, central heating or whatever.

That's what I thought, but it wasn't clear

tim

On Fri, 19 Dec 2008 12:26:00 -0800 (PST), wrote:

But switching the heating off overnight doesnt see a house lose much
temperature. To get the effect you mention it would need to be off a
month or more.

Depends a bit on the house and its construction. Our heating is off over
night from 2300 to about 0600. The start time varies as we have a
programmable stat with start optimsation. Temp might get down to 15C on a
windy night with temps outside down to -5C.

What we don't do is turn the heating off if we go away for a few days,
there is this uninsulated 18" thick solid stone wall right through the
middle of the building, this holds a lot of heat and if we let it get cool
it takes days, literally, before the place is comfortable again. The air
will be warm but 32 tonnes (guess 8 x 8 x .5m) of stone has one helluva
thermal mass. Advantge is that it keeps the place cool in summer and warm
in winter.

--
Cheers
Dave.

"Bob Eager" wrote in message
...
On Fri, 19 Dec 2008 15:41:51 UTC, wrote:

ISTR hearing that the story began with experiments on immersion
heaters and was erroneously extrapolated to central heating which is,
of course, completely different.

It's not valid for them anyway.

You have something which, left to its own devices, loses heat over time.
The rate of heat loss will reduce as its temperature gets nearer to that
of its surroundings (heat transfer rate depends on difference in
temperature).

If left to cool, there is reducing heat loss over whatever time elapses.
If kept at same temperature, there is heat loss over the same time, but
it's constant (-ish) because the temperature difference does not reduce
- i.e. more heat is lost over the same time.

In both cases, that heat loss has to be made up. That's how much
fuel/cost will be involved. More heat lost - more cost.

If you can do the maths you will find its always cheaper to turn it off and
reheat unless you have a very odd tariff.
As it cools it loses less heat so it will always lose less heat over the
same period when off than when on.
This is true for CH and immersion heaters.

chris French wrote:
There seems to be floating about among some people I know online the
idea that keeping the central heating on all the time is cheaper than
turning it off at night. Not sure where it comes from, but seeing as
they all frequent the same/ related e-mail lists, maybe from there.

The basic idea seems to be that it costs more to heat the house back up
in the morning, than to keep it at the same temp.

now I know it depends on things like internal and external temp (we have
a programmable stat set at 12-13 C, it hardly seems to turn on except
when down to about freezing outside from what I can tell),

But in principle it seems rubbish, it must use more energy to keep
something hot, than to let it cool and then reheat., but a couple of
people had been hard to convince, esp. as a 'plumber told me it)

Or is my brain addled?? :-)

The devil is in the detail, and whether your boiler is markedly more
efficient at a nice slow trickle of heat than a full bore blast to get a
cold house warm

I did a few calcs on this and by and large if your house has a fairly
large thermal mass, there isn't a lot on it either way, if its cardboard
and rockwool, then the balance tends to tip in favour of only heating it
when you need it, as the warm up time is pretty fast. BUT if yor biler
is markedly less efficient at flat out heating the balance even there
tips the other way,.

i finally gave up worrying about it.

Bob Eager wrote:
On Fri, 19 Dec 2008 19:18:04 UTC, "tim....."
wrote:

"Bob Eager" wrote in message
...
On Fri, 19 Dec 2008 15:41:51 UTC, wrote:

ISTR hearing that the story began with experiments on immersion
heaters and was erroneously extrapolated to central heating which is,
of course, completely different.
It's not valid for them anyway.

You have something which, left to its own devices, loses heat over time.
The rate of heat loss will reduce as its temperature gets nearer to that
of its surroundings (heat transfer rate depends on difference in
temperature).

If left to cool, there is reducing heat loss over whatever time elapses.
If kept at same temperature, there is heat loss over the same time, but
it's constant (-ish) because the temperature difference does not reduce
- i.e. more heat is lost over the same time.

In both cases, that heat loss has to be made up. That's how much
fuel/cost will be involved. More heat lost - more cost.
I'm confused. Is that a yes or a no to the original question?

In summary - anyone who says that it's cheaper to leave it on, rather
than turn it off and then heat it up again later, is talking nonsense.
No matter whether it's an immersion heater, central heating or whatever.



Well if the boiler was 100% efficient, and constant efficiency, yes.


However that's not the case in the real world.

The other issue is diifferential heating: houses coming up to
temperature may have some parts hotter than need be, that will lose more
heat, depending where the thermostats are.

Quite apart from any considerations of cost etc., IMO it is bad for
the health to be subjected to constant temperature. The human organism
needs change in all things...


J^n

On Sat, 20 Dec 2008 08:03:43 UTC, "dennis@home"
wrote:

"Bob Eager" wrote in message
...
On Fri, 19 Dec 2008 15:41:51 UTC, wrote:

ISTR hearing that the story began with experiments on immersion
heaters and was erroneously extrapolated to central heating which is,
of course, completely different.

It's not valid for them anyway.

You have something which, left to its own devices, loses heat over time.
The rate of heat loss will reduce as its temperature gets nearer to that
of its surroundings (heat transfer rate depends on difference in
temperature).

If left to cool, there is reducing heat loss over whatever time elapses.
If kept at same temperature, there is heat loss over the same time, but
it's constant (-ish) because the temperature difference does not reduce
- i.e. more heat is lost over the same time.

In both cases, that heat loss has to be made up. That's how much
fuel/cost will be involved. More heat lost - more cost.

If you can do the maths you will find its always cheaper to turn it off and
reheat unless you have a very odd tariff.
As it cools it loses less heat so it will always lose less heat over the
same period when off than when on.
This is true for CH and immersion heaters.

Which is what I said.

--
The information contained in this post is copyright the
poster, and specifically may not be published in, or used by
http://www.diybanter.com

If you can do the maths you will find its always cheaper to turn it off
and
reheat unless you have a very odd tariff.

I think that is an interesting point, and may be behind the "immersion
heater" story. With a well insulated tank, and a policy of taking a large
bath every evening, and on "Economy 7", you might a disbenefit in switching
off for part of the "cheap energy" window.

"Dave Baker" wrote in message
...

8

However with a reasonably well insulated house the net extra cost is very
low. If we assume in cold weather an 18 degree C difference between inside
and outside when the heat is on, 16 hours of heat on during the day and 8
with it off overnight and a 4 degree total drop in temp of the house
overnight (i.e. an average of 2 degrees) the net extra cost of keeping the
heating on all the time is only about 4%. That's less than you'd save just
turning the thermostat temperature down by 1 degree.


Sorry but that does not make sense.
If its off for 8 hours out of 24 you will save nearly one third.
You may save more as it usually colder at night and will use more energy
than in the day.
It will take slightly more energy to raise the temp back to the norm in the
morning but it shouldn't take much.

On 19 Dec, 16:16, "Dave Baker" wrote:

For a poorly insulated house the savings would be greater but still not very
much. For the saving to be as much as 10% the house temperature would have
to drop by 10 degrees C overnight for which you'd probably have to leave all
the windows open.

Oh dear, ours can easily drop from 22C down to 12C overnight with no
heating (this is with outside temps above freezing, we're in
centralish London, with rarely a frost). Single-glazed sash windows
are probably to blame, along with less-than-optimal loft insulation.

jkn wrote:
Quite apart from any considerations of cost etc., IMO it is bad for
the health to be subjected to constant temperature. The human organism
needs change in all things...


J^n

Well the healthiests time of my life was at college with a wonderful
free and totally overspecced central heatings system.

On Dec 19, 1:16*pm, "Andrew Mawson"
wrote:
"chris French" wrote in message

...





There seems to be floating about among some people I know online the
idea that keeping the central heating on all the time is cheaper
than
turning it off at night. Not sure where it comes from, but seeing as
they all frequent the same/ related e-mail lists, maybe from there.

The basic idea seems to be that it costs more *to heat the house
back up
in the morning, than to keep it at the same temp.

now I know it depends on things like internal and external temp (we
have
a programmable stat set at 12-13 C, it hardly seems to turn on
except
when down to about freezing outside from what I can tell),

But in principle it seems rubbish, it must use more energy to keep
something hot, than to let it cool and then reheat., but a couple of
people had been hard to convince, esp. as a 'plumber told me it)

Or is my brain addled?? :-)
--
Chris French

Yes - it's a load of tommyrot - obviously more energy used keeping it
hot all the time.

Now I did work in an oil refinery where if they let the pipes cool it
got very expensive, as the tarry gunge set solid, never to move again
G

AWEM- Hide quoted text -

- Show quoted text -

Well right now it's minus 8 C outside (it was minus 12 earlier today).
Inside it's somewhat less than plus 20 degrees. Probably around 67 F.

When retiring in a couple of hours time will definitely turn down the
thermostats in the two rooms we use most. And the TV will be off so
it's wasted heat will not be present either. The temp. since it is not
all windy tonight will drop slowly and by morning the house will be
cooler by quite a few degrees and thus losing less heat per unit of
time than right now while I am still up and about.

It won't take long, maybe half an hour with the thermostats turned
back up, in the morning for the two or three most used living areas to
warm back up. And if I was leaving for the day I might even leave them
turned down until returning tomorrow evening! As a single retiree home
most of the time it is possible to keep thermostats in the less used
rooms turned lower.

A basement workshop, and an attached storeroom garage are only heated
occasionally when in use. The bathroom has a row of six inefficient 40
watt bulbs; much of the time since they are on when bathroom is in use
they are sufficient to heat the room so the 500 watt electric
baseboard heater rarely cuts in.

It's quite obvious that one heats the areas necessary to the degree
needed and anything more costs more to heat. At any temperature the
heat loss from wind IS a significant factor; not so much because wood
frame houses here are leaky, they are not. If anything are often too
well sealed and must use air/heat exchangers; but because wind carries
away heat from the structure.

On Fri, 19 Dec 2008 16:16:44 -0000, "Andrew Mawson"
wrote:


Now I did work in an oil refinery where if they let the pipes cool it
got very expensive, as the tarry gunge set solid, never to move again
G

So would they actually have to cut away the pipes and replace or could
they get them clear somehow?

Dad mentioned something similar the other day. Apparently it's just as
inconvenient when a ship hold (Shell tankers in his case) full of
something that should be liquid (like pitch or tar I think he said)
goes solid.

They normally pumped steam through pipes in the tanks when the sea /
outside temp dropped below a certain point and this would keep the
cargo liquid. However if the heating system failed (or they forgot to
turn it on) and the cargo did go solid, the steam heating system
wasn't able to take it back to liquid again (poor convection) and it
would all have to be cut out by hand. :-(

T i m

terry wrote:

It's quite obvious that one heats the areas necessary to the degree
needed and anything more costs more to heat. At any temperature the
heat loss from wind IS a significant factor; not so much because wood
frame houses here are leaky, they are not. If anything are often too
well sealed and must use air/heat exchangers; but because wind carries
away heat from the structure.



It is NOT obvious that 'heating fast' is as boiler efficient as 'keeping
warm'.
It is NOT obvious that 'heating fast' will not in many cases overshoot
the 'needed temperature' by quite a margin. If parts of the structure
take longer to warm up.

This is a case where simplistic analysis fails, because the devils are
in the details.

The other case we discussed here recently and was very hard to pin down,
was 'what is the most fuel efficient way to accelerate a car to its
cruising speed'.

Popular myth has it 'as gently as possible'
simplistic analysis says that since the energy gained is constant, it
doesn't matter whether its a short sharp burst or a prolonged gentle shove.
Detailed analysis implies its about operating the car engine in its most
efficient part of the power band.

Which in the case of a petrol engine is assuredly NOT low throttle
setting at low RPM. Although possibly it is for a Diesel.


Nor yet full revs and wide open ;-)

dennis@home wrote:



"Dave Baker" wrote in message
...

8

However with a reasonably well insulated house the net extra cost is very
low. If we assume in cold weather an 18 degree C difference between
inside and outside when the heat is on, 16 hours of heat on during the
day and 8 with it off overnight and a 4 degree total drop in temp of the
house overnight (i.e. an average of 2 degrees) the net extra cost of
keeping the heating on all the time is only about 4%. That's less than
you'd save just turning the thermostat temperature down by 1 degree.


Sorry but that does not make sense.
If its off for 8 hours out of 24 you will save nearly one third.
You may save more as it usually colder at night and will use more energy
than in the day.
It will take slightly more energy to raise the temp back to the norm in
the morning but it shouldn't take much.

I agree with your sound logic, in theory the heat losses (and hence energy
costs) through the night will be higher if the house is heated but a recent
check on meter readings suggest that the case might not be quite so
straightforward.

We have the heating set to run from 08:00 to 23:30. The overnight set point
is 13C so it's effectively turned off overnight unless the weather gets
very cold. The programmer uses optimum start so I don't know exactly what
time it normally starts up but the maximum advance is 2 hours so I'll
assume it starts up on average a bit less than this at 06:30 (I certainly
don't intend waking up at 06:00 to find out). Average gas consumption for
the last week has been about 300 ft^3 per day but extra meter readings last
thing at night and at about 08:00 for a couple of nights show that about
100 ft^3 are consumed each morning in bringing the house back up to
temperature leaving 200ft^3 for the remaining 15.5 hours, i.e. 200/15.5 =
12.9 ft^3 per hour to maintain a steady temperature. So if we left the
heating on all the time the daily consumption would be 12.9x24 = 309.6
ft^3 - remarkably close to what we're using already.

O.K. the losses through the night would be a bit higher due to the outside
temperature being lower but I don't expect it would make a huge difference.
I don't intend to put it to the test though because we don't like sleeping
in a hot bedroom but don't want to turn the bedroom TRV down because we
want the room to have warmed up by the time we have to get up in the
morning.

--
Mike Clarke

The Natural Philosopher wrote:
terry wrote:

It's quite obvious that one heats the areas necessary to the degree
needed and anything more costs more to heat. At any temperature the
heat loss from wind IS a significant factor; not so much because wood
frame houses here are leaky, they are not. If anything are often too
well sealed and must use air/heat exchangers; but because wind carries
away heat from the structure.



It is NOT obvious that 'heating fast' is as boiler efficient as 'keeping
warm'.
It is NOT obvious that 'heating fast' will not in many cases overshoot
the 'needed temperature' by quite a margin. If parts of the structure
take longer to warm up.

This is a case where simplistic analysis fails, because the devils are
in the details.

The other case we discussed here recently and was very hard to pin down,
was 'what is the most fuel efficient way to accelerate a car to its
cruising speed'.

Popular myth has it 'as gently as possible'
simplistic analysis says that since the energy gained is constant, it
doesn't matter whether its a short sharp burst or a prolonged gentle shove.
Detailed analysis implies its about operating the car engine in its most
efficient part of the power band.

Which in the case of a petrol engine is assuredly NOT low throttle
setting at low RPM. Although possibly it is for a Diesel.


Nor yet full revs and wide open ;-)


It also appears to me that the room (well, our lounge) feels warmer when
the radiator is hot (rather than warm). Letting the room cool down, then
heating up relatively fiercely can *feel* warmer than keeping it at a
steady state.

Very difficult to factor in things like that with any degree of accuracy.

Many years ago, Porsche were reported as having done fuel economy tests
in town. Result: Best economy achieved by accelerating fairly briskly to
around 30, then keeping steady speed.

As TNP wrote, nothing is obvious in the real world.

--
Rod

Hypothyroidism is a seriously debilitating condition with an insidious
onset.
Although common it frequently goes undiagnosed.
www.thyromind.info www.thyroiduk.org www.altsupportthyroid.org

The Natural Philosopher wrote:
Well the healthiests time of my life was at college with a wonderful
free and totally overspecced central heatings system.


That's not the heating, that's because ethanol is an antiseptic.

Andy

Andy Champ wrote:
The Natural Philosopher wrote:
Well the healthiests time of my life was at college with a wonderful
free and totally overspecced central heatings system.


That's not the heating, that's because ethanol is an antiseptic.

Andy
I suspect it was the antibiotic value of cannabis actually.

The message
from Mike Clarke contains these words:

Sorry but that does not make sense.
If its off for 8 hours out of 24 you will save nearly one third.
You may save more as it usually colder at night and will use more energy
than in the day.
It will take slightly more energy to raise the temp back to the norm in
the morning but it shouldn't take much.

I agree with your sound logic, in theory the heat losses (and hence energy
costs) through the night will be higher if the house is heated but a recent
check on meter readings suggest that the case might not be quite so
straightforward.

Hardly sound logic to suggest that the heat needed to rewarm the house
is minimal.

We have the heating set to run from 08:00 to 23:30. The overnight set point
is 13C so it's effectively turned off overnight unless the weather gets
very cold. The programmer uses optimum start so I don't know exactly what
time it normally starts up but the maximum advance is 2 hours so I'll
assume it starts up on average a bit less than this at 06:30 (I certainly
don't intend waking up at 06:00 to find out). Average gas consumption for
the last week has been about 300 ft^3 per day but extra meter readings last
thing at night and at about 08:00 for a couple of nights show that about
100 ft^3 are consumed each morning in bringing the house back up to
temperature leaving 200ft^3 for the remaining 15.5 hours, i.e. 200/15.5 =
12.9 ft^3 per hour to maintain a steady temperature. So if we left the
heating on all the time the daily consumption would be 12.9x24 = 309.6
ft^3 - remarkably close to what we're using already.

O.K. the losses through the night would be a bit higher due to the outside
temperature being lower but I don't expect it would make a huge difference.
I don't intend to put it to the test though because we don't like sleeping
in a hot bedroom but don't want to turn the bedroom TRV down because we
want the room to have warmed up by the time we have to get up in the
morning.

Consider a really simplified model in which the outside temperature is
constant and the house requires a single unit to keep it up to
temperature and loses one degree for every hour without heat. Heating on
for 24 hours would require 24 units. Heating on for 16 hours would
require 16 units plus whatever it takes to get it back up to
temperature. In our simplistic model the temperature loss is 8 degrees
which requires 8 units to reverse. 16 + 8 = 24. So no saving.

In the real world there is a small saving because the heat loss is a
function of the temperature difference so during the time the heating is
off the rate of heat loss will decay.

--
Roger Chapman

"roger" wrote in message
...
The message
from Mike Clarke contains these words:

Sorry but that does not make sense.
If its off for 8 hours out of 24 you will save nearly one third.
You may save more as it usually colder at night and will use more
energy
than in the day.
It will take slightly more energy to raise the temp back to the norm in
the morning but it shouldn't take much.

I agree with your sound logic, in theory the heat losses (and hence
energy
costs) through the night will be higher if the house is heated but a
recent
check on meter readings suggest that the case might not be quite so
straightforward.

Hardly sound logic to suggest that the heat needed to rewarm the house
is minimal.

We have the heating set to run from 08:00 to 23:30. The overnight set
point
is 13C so it's effectively turned off overnight unless the weather gets
very cold. The programmer uses optimum start so I don't know exactly what
time it normally starts up but the maximum advance is 2 hours so I'll
assume it starts up on average a bit less than this at 06:30 (I certainly
don't intend waking up at 06:00 to find out). Average gas consumption for
the last week has been about 300 ft^3 per day but extra meter readings
last
thing at night and at about 08:00 for a couple of nights show that about
100 ft^3 are consumed each morning in bringing the house back up to
temperature leaving 200ft^3 for the remaining 15.5 hours, i.e. 200/15.5 =
12.9 ft^3 per hour to maintain a steady temperature. So if we left the
heating on all the time the daily consumption would be 12.9x24 = 309.6
ft^3 - remarkably close to what we're using already.

O.K. the losses through the night would be a bit higher due to the
outside
temperature being lower but I don't expect it would make a huge
difference.
I don't intend to put it to the test though because we don't like
sleeping
in a hot bedroom but don't want to turn the bedroom TRV down because we
want the room to have warmed up by the time we have to get up in the
morning.

Consider a really simplified model in which the outside temperature is
constant and the house requires a single unit to keep it up to
temperature and loses one degree for every hour without heat. Heating on
for 24 hours would require 24 units. Heating on for 16 hours would
require 16 units plus whatever it takes to get it back up to
temperature. In our simplistic model the temperature loss is 8 degrees
which requires 8 units to reverse. 16 + 8 = 24. So no saving.

In the real world there is a small saving because the heat loss is a
function of the temperature difference so during the time the heating is
off the rate of heat loss will decay.

In the real world you have to take into account the heat capacity to work
out how long it takes to reheat the house, not the heat loss. I.e. your
simple model doesn't work.

roger wrote:

Consider a really simplified model in which the outside temperature is
constant and the house requires a single unit to keep it up to
temperature and loses one degree for every hour without heat. Heating on
for 24 hours would require 24 units. Heating on for 16 hours would
require 16 units plus whatever it takes to get it back up to
temperature. In our simplistic model the temperature loss is 8 degrees
which requires 8 units to reverse. 16 + 8 = 24. So no saving.


Now consider the actual boiler output. The plus whatever it takes to get
it back to temperature may push teh boiler into continuous flat out
mode. How much less efficient is it then?

Or you may have to raise the outlet temperature to get it to warm up
faster. More loss of effiiency.

Analsysing transient conditions using steady state analysis is a very
precarious exercise.


In the real world there is a small saving because the heat loss is a
function of the temperature difference so during the time the heating is
off the rate of heat loss will decay.


Don't be too sure. Red hot rads heating up room against a cold wall,
lose a lot of heat..then the wall itself gets locally very hot leading
to more heat losses through that part of it. Until the room stabilises.


The more concentrated the heat and the higher temp the heat the more
there is a chance of excess loss over simple steady state analysis. what
about ultra hot feed pipes in the loft? sure they are insulated, but the
hotter they are, the more they lose..

The better your insulation the more likely you are to get little benefit
from time switching and actually stray into excess transient losses.

The message
from "dennis@home" contains these words:

In the real world you have to take into account the heat capacity to work
out how long it takes to reheat the house, not the heat loss. I.e. your
simple model doesn't work.

Time is not really an issue unless the heating system is inadequate. It
is the quantity of energy required to reheat the house that matters and
that depends on how much has been lost.

--
Roger Chapman

The message
from The Natural Philosopher contains these words:

roger wrote:

Consider a really simplified model in which the outside temperature is
constant and the house requires a single unit to keep it up to
temperature and loses one degree for every hour without heat. Heating on
for 24 hours would require 24 units. Heating on for 16 hours would
require 16 units plus whatever it takes to get it back up to
temperature. In our simplistic model the temperature loss is 8 degrees
which requires 8 units to reverse. 16 + 8 = 24. So no saving.

Now consider the actual boiler output. The plus whatever it takes to get
it back to temperature may push teh boiler into continuous flat out
mode. How much less efficient is it then?

You have your condensing boiler set so it always condenses. If it is a
non condensing boiler then the difference would not be significant.

Or you may have to raise the outlet temperature to get it to warm up
faster. More loss of efficiency.

Marginal.

Analsysing transient conditions using steady state analysis is a very
precarious exercise.

But unless you have the heating firing for 100% of the time you do not
have anything approaching a steady state in the first place. Having the
heating off for 8 hours at a time is closer to normal operation than
continuous firing, it is just that the hysterisis is larger.


In the real world there is a small saving because the heat loss is a
function of the temperature difference so during the time the heating is
off the rate of heat loss will decay.


Don't be too sure. Red hot rads heating up room against a cold wall,
lose a lot of heat..then the wall itself gets locally very hot leading
to more heat losses through that part of it. Until the room stabilises.


The more concentrated the heat and the higher temp the heat the more
there is a chance of excess loss over simple steady state analysis. what
about ultra hot feed pipes in the loft? sure they are insulated, but the
hotter they are, the more they lose..

The better your insulation the more likely you are to get little benefit
from time switching and actually stray into excess transient losses.

But the better the insulation the less the heating system has to do to
restore the status quo. Scalding radiators are not needed. It may not be
entirely impossible to engineer a heating system in an extremely well
insulated house to use more energy if it is switched off for a lengthy
period but, other things being equal, the only way that is going to
happen is for a single firing of the boiler to produce a larger
temperature overshoot on the heating phase than the temperature drop on
the non heating phase. ISTM that that is much more likely in normal
operation than it is when the temperature drop is significant.

Of course deliberately buggering up the operation of a condensing boiler
would make the task easier but even the 10% loss of efficiency may not
be enough even if the set-up could be designed to give maximum
efficiency in normal operation and maximum inefficiency in sustained
firing.

--
Roger Chapman

roger wrote:
The message
from The Natural Philosopher contains these words:

roger wrote:
Consider a really simplified model in which the outside temperature is
constant and the house requires a single unit to keep it up to
temperature and loses one degree for every hour without heat. Heating on
for 24 hours would require 24 units. Heating on for 16 hours would
require 16 units plus whatever it takes to get it back up to
temperature. In our simplistic model the temperature loss is 8 degrees
which requires 8 units to reverse. 16 + 8 = 24. So no saving.

Now consider the actual boiler output. The plus whatever it takes to get
it back to temperature may push teh boiler into continuous flat out
mode. How much less efficient is it then?

You have your condensing boiler set so it always condenses. If it is a
non condensing boiler then the difference would not be significant.

Or you may have to raise the outlet temperature to get it to warm up
faster. More loss of efficiency.

Marginal.

Analsysing transient conditions using steady state analysis is a very
precarious exercise.

But unless you have the heating firing for 100% of the time you do not
have anything approaching a steady state in the first place. Having the
heating off for 8 hours at a time is closer to normal operation than
continuous firing, it is just that the hysterisis is larger.


In the real world there is a small saving because the heat loss is a
function of the temperature difference so during the time the heating is
off the rate of heat loss will decay.


Don't be too sure. Red hot rads heating up room against a cold wall,
lose a lot of heat..then the wall itself gets locally very hot leading
to more heat losses through that part of it. Until the room stabilises.


The more concentrated the heat and the higher temp the heat the more
there is a chance of excess loss over simple steady state analysis. what
about ultra hot feed pipes in the loft? sure they are insulated, but the
hotter they are, the more they lose..

The better your insulation the more likely you are to get little benefit
from time switching and actually stray into excess transient losses.

But the better the insulation the less the heating system has to do to
restore the status quo. Scalding radiators are not needed.

Scalding radiatators happen when any system that is not modulated, goes
into heat mode.

It doesn;t matter whethert tu re one degree or 100 degrees below TRV
temp, thermostat temp. The rads and the boiler will be working full on.

What modulates the radiator surface temperature is in fact the time
they are on 'heat' because they have thermal mass.

Thats teh whole point.

If you had e.g. a house made of polystrene with hot air heating,
ultimately probably te lowest theral mas possible, then I would almost
agree with your perspective.

But the more mass there is in the building, or the heating system, the
more a high heat regime will tend to overheat parts of it with respect
to other parts. And leave the possibility of higher heat loss from those
parts during the transient phase.


It may not be
entirely impossible to engineer a heating system in an extremely well
insulated house to use more energy if it is switched off for a lengthy
period but, other things being equal, the only way that is going to
happen is for a single firing of the boiler to produce a larger
temperature overshoot on the heating phase than the temperature drop on
the non heating phase. ISTM that that is much more likely in normal
operation than it is when the temperature drop is significant.

Of course deliberately buggering up the operation of a condensing boiler
would make the task easier but even the 10% loss of efficiency may not
be enough even if the set-up could be designed to give maximum
efficiency in normal operation and maximum inefficiency in sustained
firing.


Well the figures and experiences of people in high insulated houses tend
to bear out the proposition that it makes very little difference on a
modern house whether its run 24x7 or timed.

The worse the insulation and the lower the thermal mass, the more the
timed solution saves you. Simply because it both loses far more heat
(and gets icy cold when the heating is off) and because its not got much
to heat up mass wise, so it comes up to temp quickly.

Houses that keep heat in them well, don't benefit much, and as I have
been trying to say, the heatloss from parts of the system that dont heat
the house as such at all, can be a lot higher when their average
temperature is now at 65C-70C for prolonged periods, I.e heating pipes
in the loft or walls. If you have to use high boiler temps to get the
house to heat up, you WILL be losing more heat during that phase.

A pipe, even in a couple of inches of foam, is nowhere near as well
insulated as the rest of the house should be.

This is not a simple analysis.

Boilers modulate as well, because it is both better to run for longer
periods than to run at lower power levels. The reasons are simple: high
power means less efficiency as the condensers - if fitted - work less
well, and also the alternative to modulation - cycling - leads to start
up losses when the boiler fires.

And if you use the most efficient heater there is - a heat pump - the
efficiency severely degrades when called for high output temperatures.

I m not saying that its always better to run a system this way or that
way, just trying to point out that the simple analysis runs into trouble
the more modern the house is.

There is a law of diminishing returns. And sometimes the transient
behaviour of the system makes it a net loss.

roger wrote:

Now consider the actual boiler output. The plus whatever it takes to get
it back to temperature may push teh boiler into continuous flat out
mode. How much less efficient is it then?

You have your condensing boiler set so it always condenses. If it is a
non condensing boiler then the difference would not be significant.

For our non-condensing Worcester Bosch 350 the difference is the other way
round:

71% efficient at 9 Kw
79% efficient at 25 Kw
80% efficient at 35 Kw

--
Mike Clarke

The Natural Philosopher wrote:

It doesn;t matter whethert tu re one degree or 100 degrees below TRV
temp, *thermostat temp. The rads and the boiler will be working full on.

That would only be true if the TRVs snapped shut in a binary fashion. In
practice these are analogue devices and close progressively as the set
temperature is approached, gradually throttling the water flow. If they are
all approaching the target temperature the boiler will modulate down due to
the reduced flow.

--
Mike Clarke

The message
from The Natural Philosopher contains these words:

snip

The more concentrated the heat and the higher temp the heat the more
there is a chance of excess loss over simple steady state analysis. what
about ultra hot feed pipes in the loft? sure they are insulated, but the
hotter they are, the more they lose..

The better your insulation the more likely you are to get little benefit
from time switching and actually stray into excess transient losses.

But the better the insulation the less the heating system has to do to
restore the status quo. Scalding radiators are not needed.

Scalding radiators happen when any system that is not modulated, goes
into heat mode.

It doesn;t matter whethert tu re one degree or 100 degrees below TRV
temp, thermostat temp. The rads and the boiler will be working full on.

What modulates the radiator surface temperature is in fact the time
they are on 'heat' because they have thermal mass.

Thats teh whole point.

I don't have any figures to hand but I would have thought that the
thermal mass of the radiator (as opposed to the water within it) would
be negligible. The temperature of the radiators does take time to get up
to full working temperature but not so long that the set temperature
isn't reached on a normal firing cycle. Given a 10 degree drop across
the boiler it is going to take several circuits of the heating loop to
get the boiler output up to the boiler set temperature unless the set
temperature is very low.

If you had e.g. a house made of polystrene with hot air heating,
ultimately probably te lowest theral mas possible, then I would almost
agree with your perspective.

But the more mass there is in the building, or the heating system, the
more a high heat regime will tend to overheat parts of it with respect
to other parts. And leave the possibility of higher heat loss from those
parts during the transient phase.

Surely a passive high mass acts as a damper smoothing out the overshoot.
I am not sure passive is the right word but I am trying to distinguish
between a house with plenty of solid masonry in its internal walls and a
concrete slab with underfloor heating below which is really a radiator
needing serious control gear to prevent a significant overshoot, but
even there you have a large time constant in the equation.


It may not be
entirely impossible to engineer a heating system in an extremely well
insulated house to use more energy if it is switched off for a lengthy
period but, other things being equal, the only way that is going to
happen is for a single firing of the boiler to produce a larger
temperature overshoot on the heating phase than the temperature drop on
the non heating phase. ISTM that that is much more likely in normal
operation than it is when the temperature drop is significant.

Of course deliberately buggering up the operation of a condensing boiler
would make the task easier but even the 10% loss of efficiency may not
be enough even if the set-up could be designed to give maximum
efficiency in normal operation and maximum inefficiency in sustained
firing.


Well the figures and experiences of people in high insulated houses tend
to bear out the proposition that it makes very little difference on a
modern house whether its run 24x7 or timed.

If you go back to my very simplified model there is no difference. Don't
forget that this exchange grew from Dennis' claim that turning the
heating off for 8 hours would save very nearly one third of daily fuel
use.

The worse the insulation and the lower the thermal mass, the more the
timed solution saves you. Simply because it both loses far more heat
(and gets icy cold when the heating is off) and because its not got much
to heat up mass wise, so it comes up to temp quickly.

As I see it the thermal mass isn't much of an issue being a function
both of the heat required to keep the house up to temperature when the
heating is on constantly and of the heat required to bring the house
back up to the set temperature when the heating is off for an extended
period. What is true is that a poorly insulated house will leak heat
much more quickly than a well insulated one and thus save more of the
heat that would have leaked out had the heating remained on (IYSWIM).

Houses that keep heat in them well, don't benefit much, and as I have
been trying to say, the heatloss from parts of the system that dont heat
the house as such at all, can be a lot higher when their average
temperature is now at 65C-70C for prolonged periods, I.e heating pipes
in the loft or walls. If you have to use high boiler temps to get the
house to heat up, you WILL be losing more heat during that phase.

A pipe, even in a couple of inches of foam, is nowhere near as well
insulated as the rest of the house should be.

Pipes per se give out very little heat compared with radiators even when
they are not lagged.

This is not a simple analysis.

Boilers modulate as well, because it is both better to run for longer
periods than to run at lower power levels. The reasons are simple: high
power means less efficiency as the condensers - if fitted - work less
well, and also the alternative to modulation - cycling - leads to start
up losses when the boiler fires.

I would expect only minor differences in boiler efficiency if the same
system was in use in both timed and constant mode which would be dwarfed
by the difference in heat losses.

And if you use the most efficient heater there is - a heat pump - the
efficiency severely degrades when called for high output temperatures.

I m not saying that its always better to run a system this way or that
way, just trying to point out that the simple analysis runs into trouble
the more modern the house is.

There is a law of diminishing returns. And sometimes the transient
behaviour of the system makes it a net loss.

But the more modern the house the more sophisticated the control system
has to be. I do not believe for one moment there is a single centrally
heated house in the country where running the heating 24 hours a day
results in less fuel usage than having the heating off for a solid 8
hours in 24 and very few where it would be possible to even engineer
such an outcome.

--
Roger Chapman