Combustion air, negative pressure, and power vent

Old 02-16-10, 10:42 AM
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Question Combustion air, negative pressure, and power vent

First off, this is a cross post. I posted this question in the boiler forum but think that it is on point here as well. I hope this is OK.


Tjernlund SS1 power venter is hooked to Lennox boiler firing a Beckett AFG. When the boiler is fired frequently, as it has been during this cold, northeastern winter, exhaust fumes enter the house from the barametric damper and the intake on the burner after the burner powers down and the SS1 finishes its post-purge.

The house is only 5 years old and sealed tight. When the burner fires, the barametric damper swings completely open and draws mass amounts of air up the flu. It seems like the stack pressure can only be maintained properly by drawing lots of air through the damper. This obviously creates a pressure problem when the boiler kicks off.

After the SS1 finishes its post-purge, cold air is sucked back into the stack, picks up ugly fumes, and exits the barametric damper, drifting into the living area of my house.

First, would an outside combustion air kit help out with this problem? Field makes the CAS-2B to fit the Beckett AFG and it seems logical that this would at least mitigate the pressure problem. Tjernlund makes a forced air intake that seems a bit like overkill but would no doubt provide even more combustion air. Does anyone have advice as to whether an outside air kit would help and which one of these (or another) would be best?

Second, if an outside combustion air kit is used, won't air still rush in and out of the barametric damper? If so, is there a way I can also run this outside? I have not seen a 3-way flu system that I am describing (intake, damper, and exhaust) -- although Field's "sealed combustion system" comes close -- so I assume that the damper must stay exposed to the basement. Any thoughts on this?

Third, I am considering putting an oil flu vent between the power vent and the damper but do not know how to wire it to operate the OFV with proper timing and to accomodate the post purge cycle of the power vent. Does anyone have thoughts on this addition to the system?
Old 02-16-10, 05:32 PM
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Future Reference

This problem was covered quite well in johngalt's thread in the boilers forum.

For anyone's future reference here is a link to that thread:
Old 02-25-10, 04:29 PM
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The fumes were so bad, I redesigned the post-burn turn-off delay and ran it out to 20 minutes. I sealed off the whole burner intake area and put a blower in which vented to the outside and this removed most of the blowback but not enough to get rid of everything. It was tolerable - that's all. Finally, when the Tjernlund stopped working and I went to look at it. everything was so corroded, trying to rebuild wouldn't make any sense, unless I re-engineered it. 2nd rate company. I finally gave up on my Tjernlund. Poor engineeering, bad metal choices.

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