Greg Taylor, my contact at Stovax, has highlighted a problem for those wanting to heat near-airtight houses with wood burning stoves. As things stand, they are not compatible.
How come? It's to do with air supply to the wood burners. In common with many people, I had been assuming that this was a problem easily addressed by having a room-sealed stove, just like a gas fire or boiler with a balanced flue. But this is not how a wood burner works because it needs re-fueling and, everytime you open the door to re-fuel, you upset the airflow balance and the smoke will tend to come out into the room, rather than go up the flue. It's referred to as spillage. So whilst you can have an external air supply ducted towards the wood burner, there are as yet no off-the-shelf wood burners which can operate in a way that is genuinely room-sealed.
Now our building regs (Part J) recognise this problem and in their next iteration (due to come into effect in October 2010) the problem of reconciling airtightness and flue efficiency is covered. In the previous version (2002), Part J only called for open vents on appliances rated at 5kW or more. But the new regs now take all sizes of wood burner into account and, for the first time, relate it all to the overall airtightness score of the house.
Look at row 4 of Table 1 in p29. It covers the minimum air supply required to keep stoves burning safely and efficiently and it states that for:
Appliances, such as a stoves, cookers or boilers, with no flue draught stabiliser, you must have permanently open vents:
• Option 1 (the leaky house) - If design air permeability >5.0m3/(h.m2) then 550mm2/kW of appliance rated output above 5kW
• Option 2 (the airtight house) - If design air permeability <5.0m3/(h.m2) then 550mm2 per kW of appliance rated output.
So let's take the second option, the house with a good airtightness score, and a wood stove with a 4kW output, typical of what might be required for a low energy house. Part J is intimating that you would need a 4 x 550mm2 hole in order to satisfy the air requirements for the stove. That's 2200mm2, which is 47x47mm, 2 inches by 2 inches. You'd end up having to fit a product like this in your wall. A hole, permanently open, in your elaborately constructed airtight structure. Doesn't that upset you just a little? And won't it make the room cold when the stove isn't in use. ANSWER: yes.
At the moment, there appears to be no solution. A mechanical ventilation system isn't acceptable as an air supply (it might breakdown); even a hit or miss vent isn't OK (it might be set to miss). It has to be a permanently open vent. Not that this is really safe in operation because there is a very high chance that (cold) people will take matters into their own hands and simply block off the vent.
There may well be a techno-fix for this problem; for instance a valve that closes off when the stove door is opened, or something of that ilk. It's being worked on across Europe, and when it becomes clear that the issue can be resolved, then Part J and its attendant BS 8303 may/will be re-written. In the meantime, it's a conundrum that I don't think people have fully got their heads around.