Hi,

Our DHW can be heated by the boiler (oil fired) or by an immersion
heater on economy seven. I'm never sure which is the cheapest way. So
that I don't have to post every time the price of fuel/electricity
goes up, can anyone explain how I can work it out?

The system was already installed when we moved in, so I don't know
what make the cylinder/immersion/etc. is, nor do I know whether it has
been there 3 years or 33 years!

Is it safe to assume that all 3kW immersion heaters are the same? Do
you assume 100% efficiency?

If so, is it a simple calculation of working out how many much water
the cylinder holds and then multiplying that by the temperature
difference of the water and 4.2?

The water is heated to 60C but I don't know what the temperature of
the incoming water is until I get a thermometer out.

So energy (J) = volume of cylinder in ml x (60 - incoming temp) x 4.2

If the immersion is 3kW, work out how long the immersion heater needs
to be on to give that number of Joules heat and that way work out how
many kWh of electric I use?

That's more straightforward than the boiler calculation. I don't
really know where to begin with that. It's oil fired (kerosene if that
makes any difference) and that is only a couple of years old, so it's
a condensing model.

If the boiler had a fixed output of so many kW, I suppose I could
follow the calculation I did for electricity but don't all boilers
modulate now? So I wouldn't know what the output was.

The other issue is heat loss. The boiler is on the far side of the
kitchen, so the pipes have to go up the wall and across the ceiling,
under the landing, to the airing cupboard. So I guess there's anything
between 6m and 8m of pipe (all 22mm; some copper, some polybutylene).
I have tried to lag this as much as possible.

I used the "economy" pipe wrap. This claims to reduce heat loss by 70%
but that still means I could be losing a third. I didn't use the
thicker "bylaw" insulation; that claims to reduce loses by over 75%
but doesn't say how much over 75%. If it is a case of 70% vs. 75%,
does that 5% make much difference? I think I went with the economy
version not just because it was cheaper but because I don't think
there was room to use that thicker stuff.

Once the hot water gets to the cylinder, I don't know anything about
what is inside. Would I be right to think that some cylinders have
more efficient heat exchangers than others?

I guess people with gas boilers have an easy way to do this: read the
meter before and after heating the water! If only I had an oil meter,
I could find out how many litres were used at x pence per litre. I
have seen some oil meters in BES but they are over a hundred pounds,
which is an expensive way of finding out!

One reason for asking is I'm wondering whether to abandon economy
seven. We try to run the dishwasher and washing machine overnight to
take advantage of it, but I imagine it is the immersion heater that is
the biggest load. If that was gone, I could switch to a traditional
tariff and enjoy a lower rate during the day.

Economy seven was ok before we had children but now it's harder to
schedule washing to be on overnight as the washing machine and
dishwasher need to be on much more frequently now!

TIA

Fred wrote:
Hi,

Our DHW can be heated by the boiler (oil fired) or by an immersion
heater on economy seven. I'm never sure which is the cheapest way. So
that I don't have to post every time the price of fuel/electricity
goes up, can anyone explain how I can work it out?

Very roughly 1/10th of a liter of oil is one unit of electricity.

Which puts the break even price about 4.7p per unit electricity.

On Sun, 22 Aug 2010 15:43:19 +0100, The Natural Philosopher
wrote:

Very roughly 1/10th of a liter of oil is one unit of electricity.

Thanks everyone for your replies. A few of them have mentioned litres
and that's the problem I have. I don't have an oil meter. If only I
did, it would be so simple!

Thanks again.

On 26 Aug, 13:58, Fred wrote:
On Sun, 22 Aug 2010 15:43:19 +0100, The Natural Philosopher

wrote:
Very roughly 1/10th of a liter of oil is one unit of electricity.

Thanks everyone for your replies. A few of them have mentioned litres
and that's the problem I have. I don't have an oil meter. If only I
did, it would be so simple!

Thanks again.

Someone on this group previously suggested that if you recorded how
long the boiler was firing for, then some mathematics on the jet size
would reveal how much oil you were using. I quite liked this idea, but
I couldn't easily find an hour meter that I could recalibrate (using
the aforesaid maths) to read directly in litres.

On Thu, 26 Aug 2010 13:58:52 +0100, Fred wrote:

Very roughly 1/10th of a liter of oil is one unit of electricity.

Thanks everyone for your replies. A few of them have mentioned litres
and that's the problem I have. I don't have an oil meter.

You don't need one. Read your electric meter or do the maths on the
heaters consumption against time and divide the number of electric
units by ten to get the number of litres of oil equivilant. Then
correct that number by the boilers effciency to get the number of
litres oil input. It'll be an approximation but close enough.

--
Cheers
Dave.

On Fri, 27 Aug 2010 11:15:09 +0100 (BST), "Dave Liquorice"
wrote:

You don't need one. Read your electric meter or do the maths on the
heaters consumption against time and divide the number of electric
units by ten to get the number of litres of oil equivilant. Then
correct that number by the boilers effciency to get the number of
litres oil input. It'll be an approximation but close enough.

Thank you very much. Of course you are right, I don't need to know how
many kW of oil the boiler is actually using, if I know how many kW of
electric are used, I only need to know how much the same number of kW
of oil would cost (after efficiency is taken into consideration).
Thanks again.

On 22 Aug, 11:03, Fred wrote:
Hi,

Our DHW can be heated by the boiler (oil fired) or by an immersion
heater on economy seven. I'm never sure which is the cheapest way. So
that I don't have to post every time the price of fuel/electricity
goes up, can anyone explain how I can work it out?

The system was already installed when we moved in, so I don't know
what make the cylinder/immersion/etc. is, nor do I know whether it has
been there 3 years or 33 years!

Is it safe to assume that all 3kW immersion heaters are the same? Do
you assume 100% efficiency?

If so, is it a simple calculation of working out how many much water
the cylinder holds and then multiplying that by the temperature
difference of the water and 4.2?

The water is heated to 60C but I don't know what the temperature of
the incoming water is until I get a thermometer out.

So energy (J) = volume of cylinder in ml x (60 - incoming temp) x 4.2

If the immersion is 3kW, work out how long the immersion heater needs
to be on to give that number of Joules heat and that way work out how
many kWh of electric I use?

That's more straightforward than the boiler calculation. I don't
really know where to begin with that. It's oil fired (kerosene if that
makes any difference) and that is only a couple of years old, so it's
a condensing model.

If the boiler had a fixed output of so many kW, I suppose I could
follow the calculation I did for electricity but don't all boilers
modulate now? So I wouldn't know what the output was.

The other issue is heat loss. The boiler is on the far side of the
kitchen, so the pipes have to go up the wall and across the ceiling,
under the landing, to the airing cupboard. So I guess there's anything
between 6m and 8m of pipe (all 22mm; some copper, some polybutylene).
I have tried to lag this as much as possible.

I used the "economy" pipe wrap. This claims to reduce heat loss by 70%
but that still means I could be losing a third. I didn't use the
thicker "bylaw" insulation; that claims to reduce loses by over 75%
but doesn't say how much over 75%. If it is a case of 70% vs. 75%,
does that 5% make much difference? I think I went with the economy
version not just because it was cheaper but because I don't think
there was room to use that thicker stuff.

Once the hot water gets to the cylinder, I don't know anything about
what is inside. Would I be right to think that some cylinders have
more efficient heat exchangers than others?

I guess people with gas boilers have an easy way to do this: read the
meter before and after heating the water! If only I had an oil meter,
I could find out how many litres were used at x pence per litre. I
have seen some oil meters in BES but they are over a hundred pounds,
which is an expensive way of finding out!

One reason for asking is I'm wondering whether to abandon economy
seven. We try to run the dishwasher and washing machine overnight to
take advantage of it, but I imagine it is the immersion heater that is
the biggest load. If that was gone, I could switch to a traditional
tariff and enjoy a lower rate during the day.

Economy seven was ok before we had children but now it's harder to
schedule washing to be on overnight as the washing machine and
dishwasher need to be on much more frequently now!

TIA

An immersion heater is 100% efficient as all all electric heating
devices.
There will bw losses from the tank but they will be the same losses
for oil and electricity.
The answer to your question is simple. If you have the oil heating on
for the house, use it to heat your water.
If (in Summer for example) the house heating is off, then use your
immersion heater. It's definitely not economical to fire up the
boiler just to get a tank of hor water.

Check out your lagging on the hot water tank. Add more if you can.
If you want to save, get an instantaneous electric shower and use that
in preference to the IH. Get an electric undersink heater(s) for
Summer use, again to avoid having to use the IH. Consider getting a
solar panel for hot water.
Doing the above, you will only need the IH for a bath in Winter.

On the money side you need to do some very careful calculations,
especially after doing the above.
Quite often the *******s give you cheap eco7 but charge more for
daytime electricity than you would otherwise pay. You need to burn
lots of night electricity to save anything. I suspect you are already
in this catagory.

This means you could actually save by ditching the economy 7.

harry wrote:
On 22 Aug, 11:03, Fred wrote:
Hi,

Our DHW can be heated by the boiler (oil fired) or by an immersion
heater on economy seven. I'm never sure which is the cheapest way. So
that I don't have to post every time the price of fuel/electricity
goes up, can anyone explain how I can work it out?

The system was already installed when we moved in, so I don't know
what make the cylinder/immersion/etc. is, nor do I know whether it has
been there 3 years or 33 years!

Is it safe to assume that all 3kW immersion heaters are the same? Do
you assume 100% efficiency?

If so, is it a simple calculation of working out how many much water
the cylinder holds and then multiplying that by the temperature
difference of the water and 4.2?

The water is heated to 60C but I don't know what the temperature of
the incoming water is until I get a thermometer out.

So energy (J) = volume of cylinder in ml x (60 - incoming temp) x 4.2

If the immersion is 3kW, work out how long the immersion heater needs
to be on to give that number of Joules heat and that way work out how
many kWh of electric I use?

That's more straightforward than the boiler calculation. I don't
really know where to begin with that. It's oil fired (kerosene if that
makes any difference) and that is only a couple of years old, so it's
a condensing model.

If the boiler had a fixed output of so many kW, I suppose I could
follow the calculation I did for electricity but don't all boilers
modulate now? So I wouldn't know what the output was.

The other issue is heat loss. The boiler is on the far side of the
kitchen, so the pipes have to go up the wall and across the ceiling,
under the landing, to the airing cupboard. So I guess there's anything
between 6m and 8m of pipe (all 22mm; some copper, some polybutylene).
I have tried to lag this as much as possible.

I used the "economy" pipe wrap. This claims to reduce heat loss by 70%
but that still means I could be losing a third. I didn't use the
thicker "bylaw" insulation; that claims to reduce loses by over 75%
but doesn't say how much over 75%. If it is a case of 70% vs. 75%,
does that 5% make much difference? I think I went with the economy
version not just because it was cheaper but because I don't think
there was room to use that thicker stuff.

Once the hot water gets to the cylinder, I don't know anything about
what is inside. Would I be right to think that some cylinders have
more efficient heat exchangers than others?

I guess people with gas boilers have an easy way to do this: read the
meter before and after heating the water! If only I had an oil meter,
I could find out how many litres were used at x pence per litre. I
have seen some oil meters in BES but they are over a hundred pounds,
which is an expensive way of finding out!

One reason for asking is I'm wondering whether to abandon economy
seven. We try to run the dishwasher and washing machine overnight to
take advantage of it, but I imagine it is the immersion heater that is
the biggest load. If that was gone, I could switch to a traditional
tariff and enjoy a lower rate during the day.

Economy seven was ok before we had children but now it's harder to
schedule washing to be on overnight as the washing machine and
dishwasher need to be on much more frequently now!

TIA

An immersion heater is 100% efficient as all all electric heating
devices.
There will bw losses from the tank but they will be the same losses
for oil and electricity.
The answer to your question is simple. If you have the oil heating on
for the house, use it to heat your water.
If (in Summer for example) the house heating is off, then use your
immersion heater. It's definitely not economical to fire up the
boiler just to get a tank of hor water.

Check out your lagging on the hot water tank. Add more if you can.
If you want to save, get an instantaneous electric shower and use that
in preference to the IH. Get an electric undersink heater(s) for
Summer use, again to avoid having to use the IH. Consider getting a
solar panel for hot water.
Doing the above, you will only need the IH for a bath in Winter.

On the money side you need to do some very careful calculations,
especially after doing the above.
Quite often the *******s give you cheap eco7 but charge more for
daytime electricity than you would otherwise pay. You need to burn
lots of night electricity to save anything. I suspect you are already
in this catagory.

This means you could actually save by ditching the economy 7.

In any case water heating is typically a trivial expenditure.

I think it cost about 50p to heat a steaming hot really full bath of 150
liters.

A shower if done carefully is a few pence.

Electricity night rate and oil are very close. I use oil simply because
its no more expensive and is cheaper than day rate.

And I can run it 24x7 at no cost penalty, and always have hot water.

On Mon, 23 Aug 2010 11:48:40 +0100, The Natural Philosopher
wrote:

In any case water heating is typically a trivial expenditure.

That's what I am beginning to think. After all, it is only 3kW. I
think eco7 is probably better suited to people with storage heaters in
every room, rather than one heater in the airing cupboard.

Thanks.

harry wrote:

Check out your lagging on the hot water tank. Add more if you can.

With a tank which came with reasonably thick foam insulation, is
it practical or worthwhile to add more?

Chris
--
Chris J Dixon Nottingham UK


Have dancing shoes, will ceilidh.

Chris J Dixon wrote:
harry wrote:

Check out your lagging on the hot water tank. Add more if you can.

With a tank which came with reasonably thick foam insulation, is
it practical or worthwhile to add more?

Chris
not a lot, no.

On 23 Aug, 13:23, Chris J Dixon wrote:
harry wrote:
Check out your lagging on the hot water tank. *Add more if you can.

With a tank which came with reasonably thick foam insulation, is
it practical or worthwhile to add more?

Chris
--
Chris J Dixon *Nottingham UK


Have dancing shoes, will ceilidh.

I have put boards down the front of my IH cupboard and filled the
cupboard with roof lagging.

On 23 Aug, 13:51, harry wrote:
On 23 Aug, 13:23, Chris J Dixon wrote:

harry wrote:
Check out your lagging on the hot water tank. *Add more if you can.

With a tank which came with reasonably thick foam insulation, is
it practical or worthwhile to add more?

Chris
--
Chris J Dixon *Nottingham UK


Have dancing shoes, will ceilidh.

I have put boards down the front of my IH cupboard and filled the
cupboard with roof lagging.


HARRY ENFIELD: You don't wanna do that...

....that cupboard is going to be a fire risk, the lagging will insulate
the electric cables too, and may attack the sheath chemically,
especially if the cables get hot, which the ones supplying the IH will
do, as they are carrying a fairly heavy current for long periods.

On Mon, 23 Aug 2010 01:59:45 -0700 (PDT), harry
wrote:

If you have the oil heating on for the house, use it to heat your water.
If (in Summer for example) the house heating is off, then use your
immersion heater. It's definitely not economical to fire up the
boiler just to get a tank of hor water.

Thanks. Is the same true of a gas boiler? Only in my limited
experience gas users don't seem to have/use immersion heaters, they
use the boiler. the only people I know with economy 7 are people
without gas.

If you want to save, get an instantaneous electric shower and use that
in preference to the IH. Get an electric undersink heater(s) for
Summer use, again to avoid having to use the IH. Consider getting a
solar panel for hot water.
Doing the above, you will only need the IH for a bath in Winter.

Lots of interesting ideas, thanks. I'm not sure about the electric
shower: sometime quality of shower is more important than quantity of
fuel used to heat water, or perhaps I have just been unlucky with the
electric showers I have had (poor flow)?

Under sink heaters would be a good idea regardless, because I think
most people have a few seconds when the water flows cold before they
get any hot out and under sink heaters would remove this. However if
the object of this exercise is to save money, I don't think the fuel
savings would outweigh the cost of several under sink heaters. Aren't
they about £100 each?

I'm in a hard water area. I thought under sink heaters did not like
hard water and the elements needed changing annually?

Thanks again.