Hi Everyone
Wife & I are in the process of building a passive home in North Bay, Ontario. We’ve built the garage and plan on building the house next summer. We’re looking for a masonry design with a cooktop & oven such as the Cabin Stove. Since the home is very tight I can’t have the stove drawing air from inside the home, and need advise on how to incorporate using exterior air in the stove. Air can either be drawn down along side the chimney or through an exterior wall. The stove will be located in the middle of the house backing onto a utility room. Air intake would need to be at the back since the front and sides are open to the room.
Hi Doug,
Real passive houses sports what we call here (Netherlands) a balanced ventilation. Which means air is extracted but also blown in, by two separate low-running blowers and duct systems. Between those two streams there’s a heat exchanger so the incoming air is warmed up by the extracted air.
I live in a passive house and we have a means of make-up air from outside. Half of the time it worked, the other half it exhausted air instead of supplying, depending on wind circumstances. We had to fight the heater for half a season.
What we finally did is this: the ventilation and heat exchanger unit has the possibility of three programmable presets. We used the third one to tune the incoming blower to run 7% faster than the extracting one. In our case this was just enough to compensate for the chimney draw. Do yourself a favor and find out whether this is about the same for your situation. In our case it works very reliable, 100 times out of 100. As long as we don’t forget to switch it on, of course.
Thanks for your response.
Most passive homes use a central ventilation heat exchange like you describe. We’re using a decentralized system. Each room has its own ventilation heat exchange unit that’s controlled from a main control centre. I have a total of 6 units (not counting the bath units). While some units are bring air in the other units are blowing air out and every 90 seconds the units switch directions. It’s fully programmable so I could have 4 units bring air in and 2 out, however they will switch to 4 out & 2 in and then back again every 90 sec.
I know I could install a make-up air vent, but there would by no heat capture and I’d just be sucking -20 C air directly into the home.
I could also just install an air-tight wood stove that uses outside air.
If doable, our preference is to have a cook stove with an outside air source.
@Doug, great topic you’ve started! And @peterberg10, fantastic to see you in the forum. For those that don’t know, @peterberg10 has had a tremendous impact on modern custom stove building.
Although my general approach would be to shy away from needing an air exchanger unit in the first place, I understand that they are an important part of modern passive house design and therefore it is good to develop a knowledge base around it.
Do I assume correctly that the systems being discussed (either central or room-specific) are connected to exhaust fans such as kitchen, bathroom and dryer vents so that when these appliances are switched on they bump up the supply of air (above the minimum for healthy air exchange) to accommodate for the increased exhaust? And do they do this by supplying ambient room air rather than an appliance specific supply? If so, the challenge of a solid fuel combustion unit like a masonry heater or cookstove that naturally draws the air it needs is that it doesn’t have a switch to communicate with the air exchanger. @peterberg10’s solution seems like a very feasible one where you manually trigger the increased air supply and either theoretically or empirically tune the amount. @Doug, does the air exchange control system you are looking at provide for a manual adjustment for the zone the stove is in like the one @peterberg10 is talking about?
In the event of a power outage, you would have the age-old fallback of cracking a window to supply air to the fireplace. 
In any case, the nice thing about a masonry heater is that it continues to supply radiant heat long after the need for any combustion air flow thanks to its thermal mass battery.
I can imagine that there are sophisticated wood combustion units in Europe that electronically communicate with an air exchanger (using flow sensors in the chimney?) although, again, that is generally outside of my personal design intent or philosophy.
This discussion is an interesting new variant of the age old discussion on whether or not to pipe outside air to a unit. And there is also the specific application for the cookstove with some thermal mass @Doug is hoping to include in his home. I hope to further comment on these.
This sounds like a really good idea because you could wash the cold incoming air with otherwise wasted heat going up the chimney. One issue is that this approach doesn’t meet the current North American code. In the ASTM1602, the outside air intake is perscribed to come from below the level of the firebox. Presumably, this is to prevent situations where the outside air intake acts as a chimney itself.
Again, I am curious how outside air is perscribed in Europe. @Theo.Dubois, do you have thoughts on the subject?
I can not find anything (yet) oubout getting outside air from avove. But I would reckon the heater could easily mistake in and out this way and turrn around the draft.
Can’t you install the air supply in the floor?
Otherwise there’s, besides cracking a window for a short time, a possibility to get the air in through a narrow slot just below the ceiling. The cold air that enters the house then “sticks” to the ceiling and gets heated there without causing unpleasant drafts. Search for “coanda effect” for more info.
I know a Dutch architect (Rens Pijnenborgh) who uses this successfully in his designs for residential buildings.
There are different brands in Europe who produce electronic devices which manager air intake, smoke exhaust fan and central ventilation. (Ortner ABS, Hoxter HOS AU, Leda LEDATRONIC and LUC )
Hi Max,
You wrote:
Do I assume correctly that the systems being discussed (either central or room-specific) are connected to exhaust fans such as kitchen, bathroom and dryer vents so that when these appliances are switched on they bump up the supply of air (above the minimum for healthy air exchange) to accommodate for the increased exhaust?
No, not correctly, you are overthinking it. All fed-in air is blown into the living quarters and extracted in the wet spaces like kitchen, bathroom and toilet. Ideally, both ventilators are running at the same speed and in and out vents are the same number.
The ventilation unit is running at 25 or 30% of capacity continuously. It sports a Boost preset for a programmable period of time (say 20 minutes) that runs 100% of capacity. By functioning like this, there’s no kitchen fan, bathroom or dryer vents, the ventilation unit is all that and more.
In order to save most of the heat (92%) that is produced in-house, all air exhausting is done through the heat exchanger. There are large filters in the unit itself and in the vent grids that extract air. All those need to be replaced regularly. The unit itself is signalling also when it’s time to change the filters out.
The house is ventilated continuously without opening even a window. Now you might think: and in summer? There’s a summer bypass, which opens automatically in case it’s outside warmer than inside. In part of our summer that won’t be enough, so we also employ a way of night cooling. With a window in the bedroom and another one higher in the house, opened in such a way the bedroom window is facing the wind direction. Warm air is exhausted through the upper window in this way.
Our mass heater is incorporated as I mentioned in such a way that there’s no cold draft at all.
Of course we need to flick the switch of the unit ourselves and switch it off when the heater’s air inlet is closed. A couple of times per winter we tend to forget to do so. The heater is getting up to speed in about 5 to 10 minutes, unless the switch hasn’t been flicked. The fire will come up very, very slow in that case, or eventually smoke enters the room.
regards, Peter
Thanks for all the responses. In response to bath and dyer venting; our bathroom vents are self-contained units with a heat exchange and pulls in outside air while venting inside air, and we have a heat-pump dyer that doesn’t need venting.
My preference is to find a masonry stove design that will incorporate using outside air. One suggestion I got was to just have the outside air installed close to the air input for the stove and not attached. I’m just not crazy about having -20 C air coming directly into the house. Although feeding -20 C air directly into the fire box may not be a good idea either.
I’ve also been looking at the new breed of air-tight cook stoves that draw outside air. If I don’t find a masonry design that works for me I may go that route.