What’s a Passive House designer to do when their client specifies a woodburner in their new home? Our first response is to point out that this is not a good match. Woodburners put out a lot of heat, which a Passive House doesn’t need. Overheating is a virtual certainty. We can back up the technical data with stories about New Zealand Passive House owners who have lit their high-spec woodburners once since they moved in.
There’s the obvious fact that burning wood emits CO2, methane and other greenhouse gases, so it’s contributing to global warming. Wood smoke is full of particulates, which create air pollution that is seriously bad for people’s health. A reverse-cycle air conditioner is a far superior, cleaner and more useful heat source compared to a woodburner. But some clients still insist.
Scope and terms
The scope of the following discussion doesn’t include open fireplaces. These are hopelessly inefficient and polluting and have no place being used indoors ever. Power-vented wood pellet stoves, which use an electric fan to power the draft, are out of scope too. Note these are easier to deal with on many levels. This discussion focuses on naturally-vented woodburners where the draft of the exhaust pulls in fresh air. Secondly, I’ll outline how to safely specify a woodburner if you must. Note woodburner, wood fire and wood stove seem to be used interchangeably to refer to the same instrument of heating.
There are four technical issues to consider.
- Overheating risk
- Need for room sealed combustion
- Safety due to CO introduced into the home by depressurization
- Thermal losses when the wood stove is not operating
It is very, very easy to overheat any type of high-performance home with a wood stove. A well-designed Passive House has a heating load of 10W/m² of home; even high-performance homes that don’t reach Passive House certification targets might only require 25W/m² of heating load. A 100m² common area needs only 1.0-2.5kW to maintain a comfortable temperature even on a very cold day. Even a small wood stove typically puts out 18kW or more. That’s five to ten times more heat than is needed and temperatures will get extremely uncomfortable very quickly.
Address this risk, at least partially, by specifying one of the imported wood stoves specifically designed for buildings with a very low heating load requirement. For example the Bionic Wood Fire from Denmark has a nominal heat output of only 8.5kW.
Woodburners cause the opposite problem too, of creating thermal loss when not in use. That flue is a big metal pipe that can bring cold air in through the meticulously crafted thermal envelope. Without a damper, the heat losses would be severe. Dampers usually are fitted, but this doesn’t prevent all heat loss. If closed when the wood stove is not burning, the flue is warming the air it contains. What does warm air do? It rises. It bubbles out through the flue, passes through the thermal envelope and is replaced by falling cold air. The flow rate is limited but still this warm air rises out and cold air falls in to replace it.
The solution is to account for this in your design from the very outset. PHI has modelled it and advises that when targeting Passive House certification, thermal loss should be conservatively modelled by a conductance of 50W/(m²K), with reference to the opening area. For example, a 100mm diameter pipe means a loss of 0.4W/K. This is small enough that it can be managed if taken into account at the initial design stage. If it’s a late add-on that wasn’t allowed for, it will almost certainly require redesign, especially in colder New Zealand climates.
The flue diameter is very important. Some common flues used in New Zealand are closer to 300mm. Using PHI’s number, the losses would be 3.5W/K, nine times more than a flue with a 100mm diameter. Additionally these sorts of flues could, depending on the location of the damper, allow unimpeded airflow even when the damper is closed. In such a case, the heat loss when the wood fire is not operating could be higher than the 50W/(m²K) with reference to the flue diameter.
Safety due to CO potentially introduced into the home by depressurisation
Woodburners in high-performance buildings create a risk of carbon monoxide poisoning. PHI considers it is not safe to simultaneously operate either a woodburner and the MVHR system in a well-sealed building without taking additional precautions. Note this also applies to imported room-sealed woodburners and wood pellet stoves.
If the MVHR system supply air flow drops below the exhaust air flow rate (due to supply fan failure, an air filter blockage or even the defrost cycle), the building interior can depressurise. Kitchen extract hoods, toilet extract fans and clothes driers can also cause depressurisation
It only needs to be 4Pa lower than outside for the CO produced inside the woodburner to leak into the building interior.
Building depressurisation via the MVHR is believed by PHI to be a very small risk—but it is a real risk. I personally consider that the overall risk of CO poisoning is lower in a high-performance building compared to Code-minimum buildings that include unvented appliances, woodburners or pellet stoves that aren’t room sealed and (worst) attached garages with unsealed access doors. But more important is PHI’s determination that a naturally vented flue is required in all certified buildings, including LEBs.
Room sealed combustion
Using a room sealed woodburner heads off a whole raft of problems. Room sealed means supply air is ducted from outside directly into the firebox; it’s not drawn from the room like we’re used to in old, leaky houses. It’s also much more efficient, as cold air isn’t pulled into the home when the woodburner is on. Third, even if wind causes backdrafting, woodsmoke won’t be pulled into the home.
I’ve noticed some modern homes are accidentally airtight enough to need a window cracked into order to get the woodburner to catch. If you duct the supply air into the stove directly, you’re not trying to pull supply air through a relatively tight building enclosure. Room sealed type stoves deserve to be far more widely specified, even in Code-minimum homes.
The above discussion covers general additional technical considerations beyond that required by fire safety codes and manufacturers’ recommendations. If you’re stuck with specifying a wood burner in a project targeting Passive House certification, Sustainable Engineering’s team recommends involving a fire safety professional. The odds might be low, but the impact of getting this wrong are severe.The easiest, safest, and cheapest way to remove the risk of CO poisoning is specify an all-electric home: no gas, and no woodburning either.
If you are considering using a wood/pellet stove in a building that is to be certified as a Passive House or PHI Low Energy Building, I highly recommend two articles on Passipedia: “Combustion Heat in the Passive House”, and “Biomass Heating in Passive Houses”. These are only available to iPHA members. These walk through the technical quantification of the risk and discuss several solutions. The solutions available for a certified building include monitoring of the pressure difference or CO levels and switching off the ventilation system if the allowable levels are exceeded. With these safeguards in place, there is also potential to use a woodburner or pellet stove that is not room sealed.
As for higher-performing buildings that fall short of the Passive House performance levels? The risk of CO poisoning is difficult to determine but below an air leakage rate of 2 ACHn50 there is increased risk of depressurisation in the case of faulty MVHR operation and the above precautions are recommended. This does not mean that above 2 ACHn50 you are safe! Fatalities in Code-minimum construction do occur, due to many complicated situations ranging from running the car in the attached garage, to depressurisation of the home via exhaust fans, to gas oven malfunctions. These can certainly occur in high performance buildings too unless the risk is designed out.