If one has a wood fired stove (as opposed to a dedicated boiler) how
does the hot water/CH system work? What happens to the 'spare' heat
when there's too much wood on the fire and no one is having a bath?

We're thinking of having a second wood fired stove in our house as we
have almost unlimited supplies of wood and it would be good to use it
to heat water as well. However I'm not sure how practical this is in
the real world as regards controllability etc.

We were initally looking at stoves with a much larger hot water
capacity than heat output to the room but we may modify this on
hearing answers to the above questions.

--
Chris Green

If one has a wood fired stove (as opposed to a dedicated boiler) how
does the hot water/CH system work? What happens to the 'spare' heat
when there's too much wood on the fire and no one is having a bath?

The hot water cylinder gets hotter and hotter until it boils, unless you
have an additional radiator for heat dumping. Having a full radiator circuit
is a good idea. It is very likely that if you want heat on the fire, then
heat in the radiators would also be beneficial.

It is best done using a heat bank system, with primary gravity ciculation
for the solid fuel back boiler. You can then add an additional gas/oil
boiler for convenience and the radiators will work off either.

Christian.

Christian McArdle wrote:
If one has a wood fired stove (as opposed to a dedicated boiler) how
does the hot water/CH system work? What happens to the 'spare' heat
when there's too much wood on the fire and no one is having a bath?

The hot water cylinder gets hotter and hotter until it boils, unless you
have an additional radiator for heat dumping. Having a full radiator circuit
is a good idea. It is very likely that if you want heat on the fire, then
heat in the radiators would also be beneficial.

Would one radiator be enough to dump all the excess heat? Presumably
a big boiler (we're thinking of something capable of maybe 15kW water
heating) might need more than one radiator to be sure of dumping
enough heat. We have a big house - six bedrooms, so that's perfectly
feasible.


It is best done using a heat bank system, with primary gravity ciculation
for the solid fuel back boiler. You can then add an additional gas/oil
boiler for convenience and the radiators will work off either.

So you have a heat store of some sort which can be heated by either
the gas/oil or the wood boiler? Presumably at it's simplest this is
just a big cylinder with two indirect circuits? Where do the heat
dump radiators figure in this scenario?

--
Chris Green

Christian McArdle wrote:
If one has a wood fired stove (as opposed to a dedicated boiler) how
does the hot water/CH system work? What happens to the 'spare' heat
when there's too much wood on the fire and no one is having a bath?

The hot water cylinder gets hotter and hotter until it boils, unless you
have an additional radiator for heat dumping. Having a full radiator circuit
is a good idea. It is very likely that if you want heat on the fire, then
heat in the radiators would also be beneficial.

I installed a Bont ESSE Mk 4 solid fuel stove back in 1983 and
it was still running this lunchtime. The rating was 12kWatts to water.
At the rear of the water jacket there are four outlets. I used the
left hand pair for the indirect heating loop in a HW cylinder plus the
leak or dump radiator. The right hand pair of outlets feed the CH
circuit with the pump in the top outlet pushing the hot water around
the radiators.

When the CH pump is switched off, all of the heat goes to the
HW cylinder. This has a thermostat switch fitted to turn the pump on
when the cylinder gets too hot. I use Anthracite smokeless which comes
all the way from China (Hey, we used to have coal mines didn't we) and
is called Chinacite. I also burn fallen Beech boughs and any other
wood that Nature throws my way. I find that solid fuel gives a more
even heat compared with the full ON or full OFF state of gas/oil
boilers. Our radiators are never very hot but are on almost 24/7.

Chris.

On 15 Nov 2005 11:44:16 GMT, wrote:

If one has a wood fired stove (as opposed to a dedicated boiler) how
does the hot water/CH system work?

Much the same as for a coke solid fuel stove and back boiler. There's
gravity circulation to a heat store and a permanent radiator. In
practice you need to turn the circulation pump on (even in Summer) if
you do "over-do it" on occasion and get boiling and kettling.

With wood you also have some other options to rapidly throttle output. A
quick shovelful of sawdust will cut heat output for a while (it drops
the temperature, cutting wood gasification and the secondary burn
output). The damper is also more rapid-acting than on a coke stove.

If you have a big two-box stove with wood-gas burning in the second
chamber rather than an internal baffle, then they usually have a bypass
valve for easy lighting. Opening this removes about 1/3rd of the heat
output. Although it also reduces efficiency and encourages tar buildup
in the flue, so don't make a habit of it.


--
Cats have nine lives, which is why they rarely post to Usenet.

AJH wrote:
On 15 Nov 2005 13:16:09 GMT, wrote:

Would one radiator be enough to dump all the excess heat?

Our old rayburn had just one radiator plumbed in to the gravity fed
system, I guess after that you rely on any boiling water venting to
header, though I don't recall this ever happening.

I do! In two different houses!! Water through the ceiling jobs. I'll
leave the rest of you to discuss options/solutions, but we managed to
sort out both of these systems without too much trouble - although the
current one still has a back up system of turning on a pump manually
quick when the stove begins to bang. It's on the 'to do' list :-)

--
Holly, in France
Holiday Home in Dordogne
http://la-plaine.chez.tiscali.fr/

Would one radiator be enough to dump all the excess heat?

Not if it needs to dump 15kW. That would be one almighty radiator. Check to
see what the minimum dump radiator size is.

So you have a heat store of some sort which can be heated by either
the gas/oil or the wood boiler? Presumably at it's simplest this is
just a big cylinder with two indirect circuits? Where do the heat
dump radiators figure in this scenario?

No, for a solid fuel appliance you will need reliable direct gravity flow.
The heat bank will need to be above the appliance. You should also have
enough radiators above the heat bank to gravity flow to that too. Sticking
the heat bank mounted high in an adjacent cupboard is good, and run the
upstairs rads on gravity. The downstairs are then pumped. Due to the thermal
mass of the heat bank and its open vented nature, this should be good enough
even if the upstairs rads don't have quite enough output.

Christian.

Andy Dingley wrote:
On 15 Nov 2005 11:44:16 GMT, wrote:

If one has a wood fired stove (as opposed to a dedicated boiler) how
does the hot water/CH system work?

Much the same as for a coke solid fuel stove and back boiler. There's
gravity circulation to a heat store and a permanent radiator. In
practice you need to turn the circulation pump on (even in Summer) if
you do "over-do it" on occasion and get boiling and kettling.

With wood you also have some other options to rapidly throttle output. A
quick shovelful of sawdust will cut heat output for a while (it drops
the temperature, cutting wood gasification and the secondary burn
output). The damper is also more rapid-acting than on a coke stove.

If you have a big two-box stove with wood-gas burning in the second
chamber rather than an internal baffle, then they usually have a bypass
valve for easy lighting. Opening this removes about 1/3rd of the heat
output. Although it also reduces efficiency and encourages tar buildup
in the flue, so don't make a habit of it.

Thanks for all the feedback and ideas everybody, we'll now go away and
ruminate some more! :-)

--
Chris Green

big snip

Two caveats to bear in mind. One is that to be sure that the water
heat output is what it says it is - ie you are talking about 12 kW; is
that total in which case a significant proportion goes to the room. Or
is this the water heat in which case just be aware of manufacturer's
specmanship as again if 12kW is going to water then there's an enormous
amount going to the room. Consider the surface area of the stove and
the surface area of the heating tank in the stove.

Secondly jusr remember that electricity isn't permanant and you may
loose your CH pump. Possibly worth while considering a back up petrol
gene. of a kW or so, so that you can have the house warm and some basic
lighting, and not have to worry about the system overheating.

Another option by the way is to use a Dunsley Neutraliser

http://www.dunsleyheat.co.uk/linkupsys.htm

which allows mixed heating systems without going to the heatstore
solution - I have one for a log stove / oil burner set up and it works
well with suitable thermostats to control the system.

Rob

In article , Christian
McArdle writes

Would one radiator be enough to dump all the excess heat?

Not if it needs to dump 15kW. That would be one almighty radiator. Check to
see what the minimum dump radiator size is.

So you have a heat store of some sort which can be heated by either
the gas/oil or the wood boiler? Presumably at it's simplest this is
just a big cylinder with two indirect circuits? Where do the heat
dump radiators figure in this scenario?

No, for a solid fuel appliance you will need reliable direct gravity flow.

Why? Why is pumping not satisfactory?
--
Mr X

Mr X wrote:

Why? Why is pumping not satisfactory?

I'd guess the pump/power can fail leaving the fuel burning but the
system unable to dissipate the heat. Temperature then increases, water
boils, pressure then increases if steam cannot be dissipated quickly
enough.

However, I'm not certain that a pumped system is prohibited.

Gravity circulation has fewer/no moving parts to fail. Circulation rate
increases as the temperature rises (assuming the heat dump can
dissipate the heat).

Why? Why is pumping not satisfactory?

Because if the pump fails and there is no reliable convection flow, the
water will boil and might explode.

You only need reliable gravity circulation for enough radiators for dumping
heat. In some cases this might be only a single upstairs rad, with the
others all fully pumped. The bathroom radiator is traditional for this.

You might also be allowed to have no heat dump radiators at all if the heat
bank is large and is connected to cold water with a float valve (to replace
losses to steam as it boils).

Our building regulations are very strict on things blowing up. For example,
when we have pressured hot water cylinders, these require a whole plethora
of safety systems that other jurisdictions don't bother with. There was
probably a few high profile explosions back in the depths of time which led
to the rules becoming very strict or something.

Christian.

On 5 Dec 2005 08:12:14 -0600, Mr X wrote:

No, for a solid fuel appliance you will need reliable direct
gravity flow.

Why? Why is pumping not satisfactory?

Because if the power or pump fails and the fire burning on you stand
very good chance of it boiling madly or even exploding. Gravity just
works.

IF you happen to be in AND you notice the failure you MIGHT be able to
damp the fire down quick enough. That there are rather too many
variables in there...

--
Cheers
Dave. pam is missing e-mail

In article om, Dave
Liquorice writes

On 5 Dec 2005 08:12:14 -0600, Mr X wrote:

No, for a solid fuel appliance you will need reliable direct
gravity flow.

Why? Why is pumping not satisfactory?

Because if the power or pump fails and the fire burning on you stand
very good chance of it boiling madly or even exploding. Gravity just
works.

IF you happen to be in AND you notice the failure you MIGHT be able to
damp the fire down quick enough. That there are rather too many
variables in there...

Ok, thanks for that. See my other post.
--
Mr X

In article om, Aidan
writes


Mr X wrote:

Why? Why is pumping not satisfactory?

I'd guess the pump/power can fail leaving the fuel burning but the
system unable to dissipate the heat. Temperature then increases, water
boils, pressure then increases if steam cannot be dissipated quickly
enough.

However, I'm not certain that a pumped system is prohibited.

Gravity circulation has fewer/no moving parts to fail. Circulation rate
increases as the temperature rises (assuming the heat dump can
dissipate the heat).

Thank you. See my reply to Christian
--
Mr X

In article , Christian
McArdle writes

Why? Why is pumping not satisfactory?

Because if the pump fails and there is no reliable convection flow, the
water will boil and might explode.

Ok. That's the conclusion I came to after thinking about it for a while.

You only need reliable gravity circulation for enough radiators for dumping
heat. In some cases this might be only a single upstairs rad, with the
others all fully pumped. The bathroom radiator is traditional for this.

Ok

You might also be allowed to have no heat dump radiators at all if the heat
bank is large and is connected to cold water with a float valve (to replace
losses to steam as it boils).

Ok

Our building regulations are very strict on things blowing up. For example,
when we have pressured hot water cylinders, these require a whole plethora
of safety systems that other jurisdictions don't bother with. There was
probably a few high profile explosions back in the depths of time which led
to the rules becoming very strict or something.

Victorian times - in the age of steam and boilers.
The regulations still apply to any pressure vessels and for good reason.

Thank you to all who replied and so quickly as well.

I am considering the purchase of a Raeburn solid fuel range for a future
project and was looking at the only two Raeburns for coal and wood he

http://www.aga-rayburn.co.uk/37_193.htm
http://www.aga-rayburn.co.uk/37_196.htm

.... and was puzzling about the specification calling for "Gravity" on
the DHW side, saw this thread and realised the reason, with you gents
who replied confirming it for me.

Thank you again.
--
Mr X