15 Jun 2010

Wood burning issues

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.


  1. Sounds like a lot of reinventing of the wheel. Over here in Canada, where airtightness specifications greatly exceed those in the UK (for example, R2000 specifies no more than 1.5ACH@50Pa) there are plenty of sealed wood stoves that meet R2000 requirements. Indeed, I have one and can report that there is minimal spillage on refueling. The specs for the fireplace: http://www.icc-rsf.com/en/rsf/technical-specifications-of-the-rsf-opel2-fireplace state that it is suitable for R2000 applications - and a note in the user manual says that if spillage does occur (most usual time for this is on lighting) to just open a window until the pressure is equalized.

    The only time I ever had a problem was once when trying to light the fire with the cooker hood on high speed. Once this was turned off and the fire was burning properly, then there were no more problems, even with the cooker hood running. And I do have an airtight house.

  2. @PaulM 1.5 ach @ 50Pa is not very airtight at all - Passivhaus equates roughly to 0.6 ach, almost a third of what you are working with.

    I once heard of a brief which asked for Passivhaus with OPEN fireplaces. In the end I think the fireplace won :o/

  3. 1.5ACH@50 is the maximum leakage for R2000 houses - granted it's not passivhaus but it's still approximately 4x better than the "Option 2 (the airtight house) - If design air permeability <5.0m3/(h.m2) then 550mm2 per kW of appliance rated output." in Mark's article. Heck, if they're talking about open air bricks for that level of leakage then it's hopeless for anything better.

    Trouble with ACH figures is that they're volume dependent. A large house with 1.5ACH@50 or even 0.6 could still have enough leakage volume to supply an open fireplace - but in the usually tiny houses in the UK, even 5m^3/m2/h may be too small (though I somehow doubt that).

    I would think that any dwelling with a fireplace should have some sort of CO sensor just to be safe - though the pragmatic advice in the manual of my fireplace to open the window temporarily is good I think though it surprises me that it needs to be stated.

  4. Paul,

    What happens in Canada is of great interest here. It may well be that Part J is insisting on a crude solution to a problem which barely exists. But as it stands, you won't be able to fit a wood burner into a near-airtight house without punching a hole in the wall.

  5. I understand the desirablility of an airtight house to reduce heat loss when there is very little input, but surely when you are using an woodstove, the game changes a bit. Certainly stoves are controlable to an extent, though in my limited experience the new clean burning stoves less so that the older ones. However when they are alight, stoves just produce loads of heat, to the extent that a slightly open window might be very welcome! In any case, controlling air flow and ventilation is a big part of operating a woodstove.

    The 'ventilation problem' seems to me to be an entirely artificial one and a close parallel to the nonsense about 'low energy only lightfittings'. It is simply not realistic to attempt such a high degree of control over how people use their houses even if it were desirable.

    Woodstoves are not automatic heating devices that can be started and then left alone. Anyone installing and using one is already taking responsibility for not setting light to the place, surely looking after the ventilation is not too much to ask.

    However, covering arses is usually higher up the to do list than making sense, so if the regulations do insist on fixed ventilation, it will just mean another can of expanding foam in the vent after the BS has gone. What a waste. Sigh.

  6. Mark BennettJune 22, 2010

    It is clear from reading the approved document that a room sealed wood burning stove doesn't require any permanently open air vents. (See diagram 8 on page 18.) The table referred to in the article is clearly assuming an open flued appliance and I suspect that this is really just an oversight in the drafting of the document.

    The Stovax argument is that at some point you have to open the stove to fuel it and this means it cannot be effectively room sealed. This is clearly not the intent of the regulation and I can't imagine any reasonable BCO enforcing it in this way.

    I do have to wonder what your and Stovax's motivation is in trying to "scaremonger" in this way. If you read it strictly as you and they are indicating then it basically means the end of log burning stoves in low energy houses. The only option would be room sealed pellet burners. This would seriously affect Stovax's business.

    The alternative is to abandon the strive to airtight buildings, which will have prevent us achieving PassivHaus levels of performance and mean that the current dream of "zero carbon" housing must be firmly declared as rabid fantasy.

    I can't see what's in it for Stovax in this scenario either.

    Do Stovax offer a room sealed wood burning appliance? Perhaps the aim is to spread FUD and persuade people that room sealed appliances won't comply in any case so they might as well have an open-flued appliance instead?

  7. Mark Bennett

    Stovax's point is that there are currently no truly room sealed wood burners on the market in Europe. I think this is because there has been no demand, because airtight housing is a new idea. I don't think this amounts to scaremongering, especially as there is some confidence that a room-sealed wood burner isn't impossible to build. Indeed, the Canadians already seem to have them.

    I just wish to point out that as things stand there seems to be an issue for here for people commissioning airtight homes and yet wanting a wood burner. I get asked this question a lot and to my mind it remains unresolved.

  8. Mark BennettJune 23, 2010

    There are plenty of manufacturers in Europe (and UK) claiming that their stoves are room sealed, why are Stovax saying that this is not the case?

    Is it because they temporarily become unsealed when loading fuel? Again, I doubt that a BCO will enforce this.

    Or are Stovax claiming that the other manufacturers are misleading their customers?

    The other point that comes up from reading the approved document is that in a room sealed appliance the supply and exhaust vents need to be reasonably balanced to ensure that you don't get backdrafts. I have not yet found an off the shelf solution for this that will cope well with large diameter flues without having excessively long supply pipes.

  9. what about using a wood-pellet stove which doesn't require the door to be opened for every fuel re-charge ?

  10. Surely there can be nothing legislatively done when the door is opened and the seal is broken when you need to reload the fuel and add more wood to the fire?

  11. John FidlerMarch 02, 2011

    Mark. Read your comments in June 2010 "Wood Burning Issues. We are building a SIPs house in Leics. at present with Mech. Venting& were going to put in a little Charnwood Country 4. Now read J doc. Seems we need to get a model with direct air intake - crazy situation! Has the thinking evolved on this question. Thanks!

  12. John,

    Part J still seems to present a problem but I met David Wright from Schiedel at Eco Build today who says there is already a technical fix for this. Scheidel make a clay balanced flue for woodburners called Swiftair, and there are German or Austrian stoves that are designed to work with it, all designed specifically to work in an airtight house.



  13. Calvin BarrowsJune 19, 2011


    The Swiftair would not suit many because it cannot be retro-fitted easily. My discussions indicate that most agree that the regs requires some ventilation from outside. One sourse has identified a "Black-Hole" ventilator which fits in the wall and is draft free all the time without any hit & miss facility. Baffles stop pressured air from outside causing drafts but suction from inside stimulates air flow. They are made but at the moment I am unable to sourse the smaller ones needed for a wood burning stove - namely about 50 mm x 50 mm.

  14. I have a German built timberframe house in Ireland I want to install a balanced flue wood burning stove but I cant find one. The chimney supplied with the house has an inner flue for smoke and an outer flue for fresh air intake and terminates on the wall in a250mm hole

    any ideas were I can get such a stove


  15. I am currently facing a similar dilema.Interestingly my BCO has said it is ok for my sealed MVHR refurb/extension of old farmhouse to install a wood burning stove/boiler as long as it has its own ducted air supply. He is ok with the point of having to open the door to refuel as long as the MVHR system is set to give a positive plenum inside. My problem though is sourcing such a boiler that does not rely on gravity feed as I have issues with running the pipework so it rises.There are boilers that will allow non gravity flow with inbuilt safety controls but not any with their own ducted air supply. Anyone who knows different will can help solve my problem.

    1. We are proposing to use a Woodfire f12 on our own build. This has a ducted air supply and quench coil with thermal safety valve to allow a pumped circuit on the boiler. In fact there are quite a lot of wood stoves with these features now available in the UK, usually of continental origin (La Nordica).

  16. @ Mark Bennett - see section 2.2 on page 29 - doesn't this mean that all solid fuel appliances are required to have permanent openings in the enclosure?

    We struggle with this in the US as well. My understanding is that all stoves take at least some air from indoors because it raises burn efficiency and lowers emissions if the final stage of combustio nis done with indoor (warm) air. This makes EPA certification easier. I've heard Germany has passed regulation that wood stoves in air tight homes must be closed combustion, and a rep from Jydepejsen told me that they have three air tight models (we can't get them in the US - EPA cert too pricey).

  17. Perhaps Stovax could make thus clear in their marketing material? As someone who recently purchased one of their insets with an external air kit I was told (by their authorised dealer) would stop the drafts we experienced with the previous old inset, I am pretty annoyed.

  18. @Anonymous could you clarify your comments… are you still experiencing drafts with your new Stovax, and if so which model do you have?
    We’re in the process of finalizing on a wood burning stove for a reasonably airtight house and have been advised by one of the major suppliers that the Stovax Riva Studio 2 is not airtight and could lead to drafts (even with an external air supply). They have instead recommended a significantly more expensive Barbas Cuatro-3 90. Other suppliers are saying that the Stovax is suitable...
    Any comments or views on this from anyone here would be much appreciated.

  19. Hi There, I see this thread has been mostly dead but I'll add my comments anyway. I've design a super insulated an air tight house and lots of planning has gone into it. I got my eyes and fingers tired from all the googling I did trying to find a wood burner with a sealed combustion system with low emissions and wetback. I've found only one in NZ www.ethos.co.nz. I have found out that the direct vent type of wood burner are usually not sealed but do your own research.
    Even tho I've just applied for my building consent and used ethis wood burner technical details I highly dislike what it looks like. I feel on the crossroads of what I need vs what I want.

  20. I have just returned from The sella ronda disrict in the Dolomites, Northern Italy. In one of the mountain restaurants i noticed what appeared to be a balanced flue wood burning stove. On questioning the owner he was very surprised we in the UK could not have such a stove. obviosly it could not be used where smoke or fumes could cause problems with neighbours or nearby windows or doors, but would suit me.
    Does anyone know how to get hold of such a stove?