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Old October 4th 10, 10:06 PM posted to uk.d-i-y
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Default Back boiler: friend or foe?

Howdy all

We have a back boiler installed behind our (60s) fireplace. However our
hot water comes from the gas CH system, which heats the water just once
a day for a couple of hours.


We hardly ever use the fire except in the depths of winter. (We _do_
enjoy it immensely, so we wouldn't consider _not_ having any fire at
all.)

I always used to think: back-boiler: bonus! Free hot water!

However recently a different thought occurred to me:
if the CH is providing a cistern full of hot water...
but the back boiler system is full of cold water ...
is it not going to slowly circulate the hot water down, by gravity,
through the back boiler, then back up to the hot water cistern ...
and slowly cool that expensive hot water?

I've a mind to take out the back boiler, in the hopes that our hot water
supply will stay hotter longer. (BTW there's only two of us in the
house now, and we don't have baths (we have showers instead!).)

I've been thinking about a wood stove for many a year, so now may be a
good time to do it. Yes, I am aware you can get stoves with back
boilers, but for the reason described above, I wouldn't fancy one.

Can anyone dispel my ignorance in this area?

Cheers
John

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Old October 4th 10, 10:12 PM posted to uk.d-i-y
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Default Back boiler: friend or foe?

Another John wrote:
Howdy all

We have a back boiler installed behind our (60s) fireplace. However
our hot water comes from the gas CH system, which heats the water
just once a day for a couple of hours.


We hardly ever use the fire except in the depths of winter. (We _do_
enjoy it immensely, so we wouldn't consider _not_ having any fire at
all.)

I always used to think: back-boiler: bonus! Free hot water!

However recently a different thought occurred to me:
if the CH is providing a cistern full of hot water...
but the back boiler system is full of cold water ...
is it not going to slowly circulate the hot water down, by gravity,
through the back boiler, then back up to the hot water cistern ...
and slowly cool that expensive hot water?

I've a mind to take out the back boiler, in the hopes that our hot
water supply will stay hotter longer. (BTW there's only two of us in
the house now, and we don't have baths (we have showers instead!).)

I've been thinking about a wood stove for many a year, so now may be a
good time to do it. Yes, I am aware you can get stoves with back
boilers, but for the reason described above, I wouldn't fancy one.

Can anyone dispel my ignorance in this area?


Heat has a tendency to rise, so it's unlikely that hot water will 'fall'
down the pipes into the cold back boiler


--
Phil L
RSRL Tipster Of The Year 2008


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Old October 4th 10, 10:32 PM posted to uk.d-i-y
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Posts: 2,735
Default Back boiler: friend or foe?

On 04/10/2010 22:06, Another John wrote:
Howdy all

We have a back boiler installed behind our (60s) fireplace. However our
hot water comes from the gas CH system, which heats the water just once
a day for a couple of hours.


We hardly ever use the fire except in the depths of winter. (We _do_
enjoy it immensely, so we wouldn't consider _not_ having any fire at
all.)

I always used to think: back-boiler: bonus! Free hot water!

However recently a different thought occurred to me:
if the CH is providing a cistern full of hot water...
but the back boiler system is full of cold water ...
is it not going to slowly circulate the hot water down, by gravity,
through the back boiler, then back up to the hot water cistern ...
and slowly cool that expensive hot water?

I've a mind to take out the back boiler, in the hopes that our hot water
supply will stay hotter longer. (BTW there's only two of us in the
house now, and we don't have baths (we have showers instead!).)

I've been thinking about a wood stove for many a year, so now may be a
good time to do it. Yes, I am aware you can get stoves with back
boilers, but for the reason described above, I wouldn't fancy one.

Can anyone dispel my ignorance in this area?


Hot water always rises. It goes against the laws of physics to fall.

IHTH

Dave

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Old October 4th 10, 10:47 PM posted to uk.d-i-y
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Default Back boiler: friend or foe?

On Mon, 4 Oct 2010 22:12:08 +0100, Phil L wrote:

Heat has a tendency to rise, so it's unlikely that hot water will 'fall'
down the pipes into the cold back boiler


Heat moves from hot places to colder places in any direction.

Hot objects expand, thus they become less dense than colder parts of
the same object. If the object can flow like in a fluid or a gas the
hotter, (less dense) will be pushed up an away by coldler (denser)
flowing in underneath.

So the hot water in the cylinder above the colder water in the back
bolier is going to stay up in the cylinder. Well the heated water in
the primary coil rather than the cylinder contents. It might be worth
checking how the gas boiler is plumbed into this system. It might be
that the flow (gravity or pumped) through the gas boiler also
introduces an unwanted and wasteful flow through the back boiler.

--
Cheers
Dave.



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Old October 5th 10, 01:26 AM posted to uk.d-i-y
cj cj is offline
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Default Back boiler: friend or foe?

Hi
You say the house is a 60's build IIR the backboilers in these houses was
(as you say) behind a coal fire with a flue diverter in the chimney to suck
heat around it when the fire was lit.
This type of system was a direct system with the same water in the hot
cylinder being routed through the back boiler .(memories of dad yelling Turn
on the Bl*&^y hot water quick before the bedroom fills with steam).
When central heating was fitted one way to heat the existing tank was to
remove the immersion heater and replace it with a heat exchanger coil which
the new gas boiler heated.
Another was to replace the existing cylinder with an indirect unit and
disconnect the back boiler totally leaving the boiler behind the fire empty.
It may be worth checking your cylinder to verify the system.

HTH
CJ.




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Old October 5th 10, 08:21 AM posted to uk.d-i-y
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Posts: 543
Default Back boiler: friend or foe?

On 4 Oct, 22:06, Another John wrote:
Howdy all

We have a back boiler installed behind our (60s) fireplace. *However our
hot water comes from the gas CH system, which heats the water just once
a day for a couple of hours.

We hardly ever use the fire except in the depths of winter. (We _do_
enjoy it immensely, so we wouldn't consider _not_ having any fire at
all.)

I always used to think: back-boiler: bonus! * Free hot water!

However recently a different thought occurred to me: *
if the CH is providing a cistern full of hot water...
but the back boiler system is full of cold water ...
is it not going to slowly circulate the hot water down, by gravity,
through the back boiler, then back up to the hot water cistern ...
and slowly cool that expensive hot water?

I've a mind to take out the back boiler, in the hopes that our hot water
supply will stay hotter longer. *(BTW there's only two of us in the
house now, and we don't have baths (we have showers instead!).)

I've been thinking about a wood stove for many a year, so now may be a
good time to do it. *Yes, I am aware you can get stoves with back
boilers, but for the reason described above, I wouldn't fancy one.

Can anyone dispel my ignorance in this area?

Cheers
John


Our wood burning stove with a back boiler has its own valves and pump
and only pumps water when the thermostat attached to its pipes
indicates it's hot enough. I have the thermostat set at 45C and it
certainly warms the radiators.

Jonathan
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Old October 5th 10, 09:33 AM posted to uk.d-i-y
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Default Back boiler: friend or foe?

On Tue, 5 Oct 2010 00:21:19 -0700 (PDT), Jonathan wrote:

Our wood burning stove with a back boiler has its own valves and pump
and only pumps water when the thermostat attached to its pipes
indicates it's hot enough. I have the thermostat set at 45C and it
certainly warms the radiators.


And what happens when you have a good fire burning and you get a
power cut? Just when you need the heat from the wood burner you have
to put it out to stop the boiler boiling or possibly exploding.

The circuit from a wood burner or other stove should be gravity and
have a passive heat dump for when the cylinder has reached 90C...

--
Cheers
Dave.



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Old October 5th 10, 09:35 AM posted to uk.d-i-y
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Posts: 9,210
Default Back boiler: friend or foe?

On 5 Oct, 01:26, "cj" wrote:
Hi
You say the house is a 60's build IIR the backboilers in these houses was
(as you say) behind a coal fire with a flue diverter in the chimney to suck
heat around it when the fire was lit.
This type of system was a direct system with the same water in the hot
cylinder being routed through the back boiler .(memories of dad yelling Turn
on the Bl*&^y hot water quick before the bedroom fills with steam).
When central heating was fitted one way to heat the existing tank was to
remove the immersion heater and replace it with a heat exchanger coil which
the new gas boiler heated.
Another was to replace the existing cylinder with an indirect unit and
disconnect the back boiler totally leaving the boiler behind the fire empty.
It may be worth checking your cylinder to verify the system.

HTH
CJ.


The above has a point. However I think if it had been disconnected
you would have become aware of it.
(a) Your water in the cylinder wouldn't get hot when you lit a fire.
(b) The firebacke boilers had no drain point you would have heard the
rumble of boilng water in the boiler after a while when you lit a
fire if it had been disconnected.
After a few hours the boiler would have melted /burned through/
sagged./ when it boiled dry.

There used to be an alternative way of dealing with the cyinder's lack
of heat exchanger. That was to install a " sidewinder" heat exchanger,
a dodgy device with mechanical joints.
Bloody hell, they still make them!
http://www.trademate.co.uk/ProductEx.../yorkshire.pdf
page nine.
These were very popular back in the sixties/seventies. They worked
quite well but sometimes leaked.
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Old October 5th 10, 09:58 AM posted to uk.d-i-y
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Default Back boiler: friend or foe?

On 5 Oct, 09:33, "Dave Liquorice"
wrote:
On Tue, 5 Oct 2010 00:21:19 -0700 (PDT), Jonathan wrote:
Our wood burning stove with a back boiler has its own valves and pump
and only pumps water when the thermostat attached to its pipes
indicates it's hot enough. I have the thermostat set at 45C and it
certainly warms the radiators.


And what happens when you have a good fire burning and you get a
power cut? Just when you need the heat from the wood burner you have
to put it out to stop the boiler boiling or possibly exploding.

The circuit from a wood burner or other stove should be gravity and
have a passive heat dump for when the cylinder has reached 90C...

--
Cheers
Dave.


No, two radiators are permanently attached on a separate loop, gravity
fed to dissipate any excess heat and are sized for that purpose.

Jonathan
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Old October 5th 10, 10:07 AM posted to uk.d-i-y
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Posts: 9,210
Default Back boiler: friend or foe?

On 5 Oct, 09:33, "Dave Liquorice"
wrote:
On Tue, 5 Oct 2010 00:21:19 -0700 (PDT), Jonathan wrote:
Our wood burning stove with a back boiler has its own valves and pump
and only pumps water when the thermostat attached to its pipes
indicates it's hot enough. I have the thermostat set at 45C and it
certainly warms the radiators.


And what happens when you have a good fire burning and you get a
power cut? Just when you need the heat from the wood burner you have
to put it out to stop the boiler boiling or possibly exploding.

The circuit from a wood burner or other stove should be gravity and
have a passive heat dump for when the cylinder has reached 90C...

--
Cheers
Dave.

It is in fact illegit to have it any other way. All solid fuel fired
heating systems have to ave a convection heat sink. Usually the DHW
cylinder but sometimes a radiator.


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