I recently switched to natural gas heat, and in doing so found out the hard way (via CO alarms) that my chimney isn't drafting properly. After numerous inspections and experiments, it seems the culprit is the power vent in the attic. When it turns on, it essentially sucks air through the house and down the chimney, causing the boiler to spill exhaust right into the basement.

I have already installed a powered fresh air supply (Fan in a Can, it's called) next to the boiler, so my next step is to address the attic ventilation situation. I don't want to simply turn off the fan and the leave the attic as is... it surely needs some ventilation. So the question is, what do I do?

For reference, the roof in that area is square with a peak in the center, so there is no place for a ridge vent. The power vent is about halfway up the side of the roof. My initial thought was to install soffit vents around the perimeter (3 of the 4 sides) and see if that provides enough air flow for the power vent, but that would be a lot of money down the toilet if it didn't work (I wouldn't be able to do that myself).

My next thought was to replace the power vent with a standard square vent, then add the soffits and maybe another square vent on another side of the roof.

I am thoroughly confused, as you can tell. Any suggestions?

 

It sounds like you do not have ANY intake venting, usually located low in the attic. A powered exhaust fan is typically powerful enough to exceed even normal attic vent area to the extent that it depressurizes the attic and draws air from the house.

The problem (besides using a powered fan) is the failure to air seal between the house and the attic. Regardless of what you provide for intake vent area, the attic will go negative and even if it doesn't backdraft the chimney, it will be stealing conditioned air, expensive. Your solution is a band aide in the wrong place and potentially a dangerous one as a new fan failure could result in a house full of CO.

Let's back up to the original problem, a hot attic and concerns that the excessive heat is affecting the house below. The correct solution for that is air sealing and lots of insulation. Then, the concern about ventilation turns to moisture passing through the ceiling plane and needing to be vented out before it condenses in the winter time. Your location would help us identify related conditions.

What have you done for air sealing?
What is in place for a vapor barrier in the ceiling?
Is there currently any intake venting?
Where are you located (climate wise)?

Bud

 

The house is circa 1958, Long Island, NY.

There looks to be one soffit vent (there are two mesh screens on one overhang, one for the bathroom fan, the other presumably going to the attic.) The other, non-accessible half of the attic has a ridge vent and a single gable vent. There is a hole connecting the two attic areas where A/C ducting has been run.

No sealing has been done, and there is no vapor barrier, unless you count paper faced 50-year-old batts. A cursory check shows that, at least in that half of the attic, there are a few gaps over fixtures that could be sealed up with foam, but to seal up the tops of the walls and the area leading into the other part of the attic would be a job for a professional.

I know sealing the attic from the living space is the best solution. I just wanted to hear some ideas.

 

From your post I can tell you have been reading. Air sealing the tops of the walls is part of the process, but secondary to finding the major leaks. Often a plumbing passage or space around a chimney can be accounting for 80% of the total ceiling leakage. If you have a basement attack what you can from down there, not the best but helps and is easier. You can plug a straw from either end.

Your venting sounds like circa 58. Since I know you are not going to take out that powered fan then what you need to do is provide lots of intake area for that attic. In time you can address the issues we have discussed, but the make-up air you installed is not fail safe and you need to rethink that.

Another annoying note is that CO detectors don't work very well. They have tried to make low level units but expensive and not popular. But current run of the mill CO detectors are very slow to respond and only at higher levels.

Below is a link to more reading:
http://www.buildingscience.com/docum...-sealing-guide

Bud

 

I have a '59 split level that had a similar problem. After better sealing and installing more soffit vents.... it's perfect now. Air needs to be let in at the bottom of the roof and soffit vents are the answer.