Wood stove (15kw) with water boiler (12kw) - how to calculate volume it can heat?

I'm lost - I have found a neat little wood stove - a Bruno Techna - which might just be a good solution to my heating problems. But for the life of me, I can't work out the room volume which can be heated by hot air.

I assumed subtract the energy needed to heat water (12kw) from the total (15kw), but that leaves me with 3kw - sure, enough to heat a 30m2 room, but I have a nagging feeling that a 15kw woodstove would have to be super-insulated to direct 80% of the energy exclusively into the hot water system.

So there is obviously a lot I am not understanding.

Thanks for your comment Michael - interesting, and mine is more like RT1850 ;-)

- I was really trying to understand output of a wood burning stove with boiler, and Nigel put me on to a site which gave me the answer I needed.

to work out the size of a heating solution you need to calculate the heat loss of the house.

In a house that is well insulated the 100w m2 doesn't apply.

In new rt2012 houses here in france you can get away with maybe 40w/m².

What is your house insulation like?

Brilliant! Yes thanks - this shows me that it is indeed 'normal' that only 3kw goes to heating the room even if the boiler is at 12kw.

Thanks again.

Not sure if this will help you.

Yes exactly - I am well aware of the 100w /M2 with Wall height of 2.5m.

I didn't calculate - 12kw is what the manufacturer claims.

Hi William, I am looking at the Techna model - which does not exectly follow the 'bullerjan' design.

Thanks for your lead I didn't know that a specialist Bruno shop existed - I expect they can answer my questions.

Thanks again. BTW did you find the 30% reduction in consumption which Bruno advertises?

Perhaps I'm not looking in the right place, but that calculator does not divide between water heating and space heating.


How did you calculate that you need 12 kw to heat your water - how big is the tank?. Or are we talking about a central heating system?

Richard try this site contact there is Eric he speaks some English. We got our bruno from them and am very pleased with it.

On the site it gives the cubic metres that each stove will heat.


The good thing with the one we bought is that the pipes around the outside act as convectors as they draw the cold air from the floor and direct it to the ceiling in the same way convector radiators do.

Hope this helps


I haven't heated water from a wood stove, but as a room general heating calculator my open plan kitchen/living room is 6mx6mx2.5m high, same for the upstairs (stairs open from living room) and I have upgraded from a 6kw to 8 kw stove and have found the 8 to be more than enough. The 6 required background heating on very cold/windy days, the 8 doesn't. I also had an air vent installed behind the stove to draw fresh air from behind the heater and this has cut out any thermal draught from the stairs.

My gite is 10mx6mx2.5 and has a 12kw wood stove, which again is perfectly fine (also open plan/stairs leading from the main room).

Good luck, it may ultimately be cheaper and easier to keep water/heating separate? Or have something like an Aga in the kitchen that will heat water?