This intermittent problem could be caused by one of several factors, GBA senior editor Martin Holladay points out, including a chimney that does not extend above the ridge of the roof or a restricted or undersized outdoor air duct. Another possibility is that a range hood fan, a bathroom exhaust fan, or a clothes dryer is putting the house under negative pressure, Holladay writes.
Wood Fire Direct Melbourne Dorsett adds another wrinkle in the form of a code provision that would seem to prohibit exactly the kind of stove installation that Agnew has: that is, an air intake at an elevation higher than the fire box.
But Agnew can put some of these theories to rest from the start. The chimney is, in fact, high enough; it extends 2 feet above the ridge. And the Vermont Energy Code Handbook, Agnew adds, specifically allows his below-grade stove installation providing certain conditions are met.
But the comments have him wondering whether the stove is getting enough combustion air. “I can imagine the stove using slightly more air than is being allowed in, hence it works great for about 15 to 30 minutes,” Agnew says, “but then perhaps it reaches a point where the negative pressure in the house becomes greater than the draft pressure and the draft reverses directions therefore bringing the smoke in the firebox and cold air from outside down the chimney, through the stove and into the house.”
This might help explain the condensation collecting on the floor near the air intake. As to a fix, he wonders whether his only option is to open a window to allow more air into the house. “I would be a little disappointed if it was,” he says. “Maybe there is a better stove out there for a tight house?”
Heating Consultant Melbourne suggests the installation may be hampered by the stack effect, in which air leaks in at the bottom of the house and exits near the top of the house.
The outside air duct is probably allowing air inside even when the stove is not running, and should Agnew open the door of the stove when it’s not burning any wood he’d be likely to feel cold air coming down the chimney.
“That’s the way ‘stack effect’ pressures work may be functioning as an air inlet whenever it can (i.e. unless you overcome the pressure with a roaring fire). Whenever the range hood, dryer, bath fans, built-in vacuum, etc., are in use, the problem is worse. In addition, your HRV may be unbalanced and causing a negative pressure with Wood heaters Melbourne“