Image Credit: Clark Agnew
More Q&A Spotlight
Clark Agnew should be the envy of his neighborhood. He has a tight house, a high-efficiency wood stove with its own fresh-air intake, and access to free firewood. A heat recovery ventilator (HRV) keeps indoor air healthy. What’s not to like?
But, as he writes in a Q&A post at Green Building Advisor, the situation is far from ideal.
“I have run the stove about 6 or 7 times since we moved in,” Agnew writes. “Three of those times it has backdrafted.”
The stove has been installed in Agnew’s basement. It gets combustion air from a 4-inch-diameter vent to the outside, although the line is not connected directly to the stove. Agnew hasn’t had any problems getting a fire started, and the stove “seems to draft wonderfully.”
But periodically, the stove backdrafts.
“The first time was after about 10 to 20 minutes. I was just sitting there enjoying the heat and flames with my 2-year-old, and all of a sudden the fire goes out and smoke starts billowing out from all corners of the stove,” he writes. Plus, there seemed to be some kind of condensation coming out of the air intake.
On another occasion, the stove ran fine until Agnew’s wife turned on a bathroom fan. Once again, smoke billowed from the stove — until the fan was turned off.
“Third time, the stove had been running for about 20 to 30 minutes,” Agnew continues. “We were up reading books with my 2-year-old and the smoke alarm started going off. I run downstairs and once again, the fire is out and smoke is billowing out, mostly from the top, next to the air intake lever. I can feel air coming in the fresh air vent 1 foot away. After 30 to 60 seconds of this it just…
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11 Comments
Since the wind doesn't always blow in the same direction...
... it's a bit difficult to install the air intake on the windward side of the house as prescribed, unless the site-specific prevailing wind is very consistent. (It can be very consistent in some areas, but it's really all over the place in most.)
A venturi-type stack cap draws harder whenever the wind blows, and isn't much affected by wind direction, as long as it's at least a couple feet above the ridge of the roof.
Pacific Energy Vista works for us with outside air intake
Hi - we have a pacific energy vista wood stove with a 4" outside air duct that feeds fairly directly to stove. The stove pedestal sits over the 4" hole so there is a bit of leakage but not huge. The air duct goes under slab and pops up in wall and comes out about 7 feet up the outside wall. We used the double wall pipe up 2 storeys then the insulated chimney through attic space (another storey). The stove has great draft and once its going and door is shut I can turn on the 200cfm kitchen fan and its OK. If I even have a 110 cfm bath fan on when trying to light fire, the air coming down the chimney will blow out the lighter. We have very dry wood. My inlaw suite has the same stove but on one storey. That is fussier, but still works as long as there are no exhaust fans on when lighting. Sometimes it can help to warm up the chimney with a couple of lit candles inside the stove prior to actually lighting the fire to get the air moving up the chimney instead of down. Our Venmar EKO HRV is balanced.
what HRV?
HRVs aren't *supposed* to negatively pressure the house on defrost
cycles, they should shut off the exhaust-side fan and simply
recirculate the interior air for a while to warm up the core.
What kind of HRV is it? Has the manufacturer been contacted?
_H*
Yes even my cheapo builder
Yes even my cheapo builder model's defrost does a recirc mode...
But i suppose maybe the Canadians got Defrost modes right sooner.... (venmar)
Could have a failing or leaky damper in the HRV?
Response to post #3
Hobbit, I suspect you meant that in defrost mode the HRV should shut off the intake duct, not the exhaust fan. Otherwise the unit would continue to suck cold outside air, the cause of the need for defrosting. In my HRV, a Lifebreath 195ECM, there is a damper-closed port on the bottom of the cabinet, right under the intake air port, that opens to the inside air during defrost. The motor-driven damper swings 90 degrees, up from the inside air port and closes off the outside air port. Thus already warmed inside air flows through the HRV and back out to the house, while "stale" air from the house continues to flow through the HRV in the opposite direction and to the outside, as usual.
defrost
Well, that's the problem then: the exhaust side has to SHUT
OFF when that recirc-only scenario is in progress, or the
interior air gets pulled out. My Fantech does the right thing,
moving a damper that closes off the exterior intake and swaps
to a vent right on the side of the HRV that pulls from wherever
the unit is located.
I'd contact Lifebreath and call them on this. If it was a
misguided design, maybe they have a retrofit fix that'll
kill the exhaust-fan relay during defrost...
_H*
Might want to think about your chimney
I'd bet that a smaller diameter insulated stainless chimney liner will take care of this. Not too difficult to install--you can get drop preinsulated stainless down, or use uninsulated and pour castable vermiculite insulation mix around it. Although the fact that this is an interior chimney is certainly helpful with draft, you do have three strikes against: the cross sectional area of an 8X8 combined with the thermal mass of the masonry added to the squeeze a tight house puts on air supply. We've seen this phenomenon even with a woodboiler that had an induced draft fan and was operating in a leaky house. If you dump into a big masonry chimney, the smoke cools down and wants to sit around and hang out instead of going up. Keep it warm (insulated) and moving (appropriate diameter) and it'll probably do what it's supposed to.
It's an 8" round not an 8x8... (response to Bennett)
... but yes, still oversized for this stove, which is in-turn oversized for the house.
A right-sized stove with a ducted intake air and a correctly sized flue liner for the new stove, filling the space between the stainless liner and the 8" round with rock wool really will take care of the problem, and work better in every possible way. The behemoth stove probably turns the place into a sauna if you run it a few consecutive hours at the mid to high-fire where it's particulate pollution is lowest.
Response to post #6
You wrote both: "the exhaust side has to SHUT OFF when that recirc-only scenario is in progress" and "My Fantech does the right thing, moving a damper that closes off the exterior intake and swaps
to a vent right on the side of the HRV that pulls from wherever the unit is located." The latter sounds exactly like what my Lifebreath is doing, and also sounds like your exhaust side is not shut off at all.
I see no reason why interior air can't be exhausted from the house during the defrost period of an HRV, other than the small short-term loss of heat recovery from that outgoing air. True, without inflow of exterior air, the unit goes out of balance, depressurizing the house slightly and resulting in the sort of air infiltration that a blower door test induces, although at a lower rate, and the exhaust rate may not be as high. But for sure there shouldn't be continued flow of cold outside air through the unit while it attempting to defrost itself. The slight depressurization of the house during defrost could conceivably contribute to backdrafting of the woodstove without a directly connected outside air duct to seal off the woodstove.
Draft inducer
I've been looking at this for a while, but still haven't tried it. It sounds like just what you need.
http://heat-booster.com/
depresurization
What readings did you get with the manometer? I have a stove that I used in a leaky house and never had a problem with it. Then installed it in my new tight house with its own fresh air intake and if I do not open a window when I turn a fan on it will suck smoke into the house. I finally got my hands on a blower door and I checked for depressurization with the fans on. Just the bath fan -5.3 pa, dryer and bath fan -16 pa. Bath fan, dryer, and range hood on low -32 pa. and with the hood on high it was at -76.1 pa. I think the stove is the path of least resistance and smoke gets sucked in. The blower door reading was 320 cfm50 at 50pa. I am ok with opening a window but I would like to make it more idiot proof without making things complicated or expensive.
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