A related question. I am looking at a 12Kw stove with 5kw to room, and the balance of 7kw to the boiler. I have a 300litre thermal store. Do you think I would need a relief radiator in the system as well or would the thermal store be big enough?
Mike
I do not believe that you will be able to boil that amount of water with a 5kw unless you try very very hard.
You should find this thread very interesting
//www.diynot.com/forums/viewtopic.php?t=196170
Below is the interesting bit from cbell: I saved it to file some time ago after thinking it a very useful post:
I'll get shouted at for saying this, but unless your wood-burner is absolutely massive and your house microscopic you really, honestly don't need to worry about boiling the water in the heat store unless you are catastrophically stupid.
This is because boiling 210 litres of water requires a *lot* of heat. The figures work out like this:
The specific heat capacity of water is 4200 Joules / kg / deg C, or in plain english it takes 4200 Joules to raise the temperature of 1 kg (= 1 litre) of water by 1 deg C.
Lets say that your store is already at 50 deg C, then to raise 210 litres by another 50 deg C to boiling point requires
210 x 4200 x 50 = 44.1e6 Joules, = 44.1e3 kiloJoules (kJ)
Stove outputs will be rated in kiloWatts (kW). 1 watt = 1 Joule for 1 second, so 1 kiloWatt = 1 kiloJoule for 1 second.
So let's say that your stove generates heat in the water jacket at the rate of 10kW, then it will take 44.1e3 kJ divided by 10 kW seconds to deliver all that heat, which is 4410 seconds, or 73.5 minutes, or about 1 and a quarter hours.
So you would have to blaze your stove at its rated full capacity for an least an hour and a quarter to boil that tank if it was already at 50 deg C, and for at least 2 hours if the water in the tank was at room temperature.
In practice the system will lose heat in pipework and elsewhere, meaning that it will take longer to heat the water, and it is pretty much impossible to run a wood-burner at full capacity for that length of time anyway since it consumes the wood too fast. So you would probably have to run the thing flat out for between 2 and 3 hours, refuelling it at least two times, while simultaneously not drawing any heat at all out into the CH system, for the tank to get anywhere near boiling.
If you *did* chose to do this you would get lots of audible warning from the stove itself as the water in its boilers started to hiss and bubble.
You would have to be completely nuts (and deaf) to get into this situation, and since a wood-burner has to be manually refuelled it simply is not a problem in practice.
As I said at the top of this post all of the above is based on actual experience: we have a wood-burner rated at 20kW, of which (nominally) 3kW goes to the room and 17kW to the water jacket, and an oil-fired boiler rated at 15kW. Last winter the wood-burner heated the whole house so long as I fed it enough wood.
That last point is worth considering: we burned about 1.5 tons of wood last January when it was really cold, and averaged about 1 ton/month through the winter as a whole. That's a lot of wood, and if you haven't got anywhere to store it you need to think long and hard about this whole exercise.
