woodstove in a passive house ?

[Ted Nickell]

Jesper, I have installed four masonry heaters in four superinsulated houses. They work very well. These are Tulikivi soapstone stoves from Finland. They burn kindling once or twice a day, each fire lasting 30-40 minutes. They are closed up the rest of the time (flue and combustion air from outdoors) so the airtightness of the house isn't compromised much. Each stove is sized for the house it goes into. These masonry heaters can be combined with other heat sources, such as a coil at the HRV. One thing I like about them is that when the fire is going, it's going all out. It's never damped down for a slow burn, so it's exempt from EPA regulations because it doesn't pollute. One last thing: After your morning fire you close everything up and this pile of hot rocks just sits there radiating heat all day long while you're off at work with no worries about a fire burning in your absence.

Ted Nickell

[Mark Siddall]

Ted,
Thanks for the Tulikivi link. Are you aware of any other manufacturers? Also do you have a systems strategy diagram for the masonry heater HRV combo? Sound like a very interesting way of avoiding any negative impact upon IAQ.

Mark

[Mark Siddall]

Ted,
Having had a look at the Finish soapstone fires, as impressive as they are, I have now developed some concerns about the possibility of back drafts into the living space. Theory is once the fire begins to bed down, just before in burns out, there is the risk that the HRV system will draw polluted air into the house thus having a detrimental impact upon IAQ. (A similar risk may existing at the beginning of the combustion cycle also.)
Does anyone have any thoughts on this?
Are there any studies that assess the risk of fires in houses with HRV that could support or refute my concerns?

Cheers,
Mark

[eric wilson]

What about an outdoor wood-fired boiler? This would minimize infiltration and IAQ concerns, although there appears to be some controversy around them: http://www.woodheat.org/technology/outboiler.htm

[Mark Siddall]

Eric,
Due to concerns about air bourne particulates, if choosing bio-mass, I would tend to favour the woodchip (a farourite) or wood pellet option (slightly less eco.) They burn cleaner and with a balance flue can be with in the house (thus avoiding unnecissary efficiency losses.)

Mark

[Ted Nickell]

Hi, Mark

Sorry to take so long to reply to your two questions.
First, in the absence of a diagram, here is what I do. I first design a house with an open floorplan. I put the masonry heater as near the center of the house as I can. It radiates heat, but heat is also being distributed throughout the house by the air movement caused by the HRV supplying fresh air to some rooms and exhausting stale air from other rooms.
Second, I haven't experienced any backdrafting of the Tulikivi caused by the HRV. I assume this is because the HRV has balanced airflow (intake and exhaust). Also, the Tulikivi draws like no other fireplace. Start the fire. Wait three seconds. Close the glass door. Whump--the roar is immediate. I don't have any experience with any other masonry heaters, but I'm guessing they perform similarly. A backdraft can be caused by a range hood exhaust blower, so you have to be sure not to use it while a fire is going in the firebox.

Ted Nickell

[jesper]

Hi Ted,

It seems that one way to make a passive house affordable is by saving big on the heating system, but the Tulikivi heaters seems pretty expensive (according to my marginal research on line). Do you have an idea what a stove might cost for a 1500 sq. ft. house ? Do you have someone build the stove or do you do it yourself (thinking this might be a way to shave a bit of the price) ? Mark has mentioned a balanced flue( which I assume has the fresh air supply as well as the smoke stack); what do you prefer (the balanced flue, or a separate air supply) ?
Thanks,
Jesper

[Mark Siddall]

It struck me that with an airtightness of 0.6ach @50pa the permited leakage area less than the size of the palm of a hand. How airtight is the masonry heater when in use or not in use? If your not careful in one go you could use all the airtightness "allowance" if the masonry heater can vent directly between inside and out.

Another thing, if there is no backdraft i.e. the HRV is suitably balanced, and the masonry heater is drawing combustion air from inside the house then it seems to me that one of two things has to happen i) the space undergoes negative pressure, thus draws combustion air, via infiltration, into the house, or ii) the HRV has to be able to provide the additional combustion air (some sort of intelegent variable flow control linked to the masonry heater). I am not aware of HRV systems that can cope with the item ii concept, thus it seems to me that infiltration is the likely means by which combustion air will be provided. As this cold air has not passed via the heat exchanger I would have concerns about i) cold drafts ii) energy consumption above the PH standard. A balanced flue i.e. intake and exhaust linked direct to external environment would address this by avoiding the cause of infiltration and work out cheaper than developing sophisticated (unreliable?) control systems.

Does anyone have any thoughts on this? (Is there something I'm not thinking of?)

Mark
P.S. I would echo Jesper's concerns about cost. I understood that superinsulated houses, and PH project, become "cost effective" when you designed out the chimney and hydronic heating system.

[ubus]

To operate a fireplace safely in a Passive House, 3 conditions must be met:
1. The fireplace must be ventilated independently (external air supply)
2. The whole system (fireplace/flues) must be airtight
3. A flap valve in the flue must prevent back drafts


Greetings from New Zealand.
Kerstin Rosemeier

[ubus]

Almost forgot: the Passivhaus Institut also requires a differential pressure switch for ventilation systems that aren't certified for the use with fireplaces. Certification is fothcoming in Europe in the near future. For uncertified systems (i.e. systems without inbuilt checks) it is essential to exclude large pressure differentials in case of a malfunction of the ventilation system (even airtight flues and stoves are only airtight up to a certain pressure differential). Since there is the very real threat of suffocation, this seems a reasonable precaution.