Wood fired stove for hot water, how are they regulated?

[usenet]

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.

[Christian McArdle]

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.

[AJH]

The ones I have seen seem to have very small heat exchange area in the back of the stove, typically just a rectangular hollow section with threaded pipes and both ends. I imagine this limits them to a few kW(t).

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.

This sounds good, with this level of heat I would suggest going for a pumped system, preferably with a loading unit. The main thing for safety would be a large bore pipe straight from hot side of boiler to F&E vent. From what I have recently learned here the cold feed should be separate and to the other side with the pump not between the two.

You no longer need them, the thermal inertia of the store should be sufficient to absorb the heat. The buffers I see, one 1000ltr the other 3000ltr in conjunction with solar panels, signal the boiler at a lower switchover temperature, typically DHW set point at 85C and buffer switchover at 55C.

AJH

[usenet]

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.

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?

[mcbrien410]

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.

[Andy Dingley]

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.

[Holly, in France]

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

[Christian McArdle]

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

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.

[usenet]

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

[robkgraham]

< 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

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

[Mr X]

In article , Christian McArdle writes

Why? Why is pumping not satisfactory?

[Aidan]

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).

[Christian McArdle]

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.

[Dave Liquorice]

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...

[Mr X]

In article , Dave Liquorice writes

Ok, thanks for that. See my other post.

[Mr X]

In article , Aidan writes

Thank you. See my reply to Christian

[Mr X]

In article , Christian McArdle writes

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

Ok

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 here:

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.