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What do I do about my backdrafting oil-furnace?

Patrick McCombe | Posted in Mechanicals on

Last weekend after noticing my bath fan wasn’t working. I went into my attic to see what was going on. I found the vinyl flex duct that was snaking across the attic floor full of water, so I swapped the fan for a high-quality model and did a nice job running 4-in. aluminum pipe to the outside. The fan is rated at 70 cfm and now my bath doesn’t fill with steam and my windows are no longer fogged up in the morning.

While I was connecting the duct and wiring to my new fan, I had the R38 fiberglass batts pulled away from the drywall ceiling and found a 3” x 10” thermal bypass. I could see all the way to the basement! Feeling smug, I got a piece of foam and my foam gun and plugged the hole. I even filled some electrical holes while I was at it. I replaced the batts carefully and went to get a shower. The fan worked great.

While I was showering with my new fan, I was running the clothes dryer and we had the range hood running full blast (You can see where this is going?) I come out of the shower and I smell that distinctive smell of an unvented kerosene space heater. I say to myself “XXXX!”. I immediately know exactly what I did. I turn off all the fans and the smell dissipates and the oil forced-air furnace starts drafting again.

I’ve been experimenting and now I’m confident it’s only when I’ve got the bath fan, dryer, and range hood running that there’s a problem. Even when two of the three are running, the furnace seems to draft properly, although I don’t have real way to measure the amount of draft. My house is tiny (700 sf ) and on one level, although I wouldn’t describe it as especially air tight. I haven’t done a blower door test, but I will this weekend. My guess is that the leaks will amount to a few hundred CFM.

The question is, what do I do? I thought I’d install a makeup air vent to the outside in the basement with an electric damper that’s controlled by the thermostat. When the thermostat calls for heat it will open the damper and close when the thermostat is satisfied. I explained the situation to a smart coworker and he suggested I could skip the electric damper and install a cape damper instead.

I know I should also air seal the basement ceiling to keep my fans from pulling air down the chimney. Am I on the right track? I’d really appreciate your thoughts on my situation.

PS: I do have two good hardwired CO detectors with battery backup. Thanks for the help guys.

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  1. Aj Builder, Upstate NY Zone 6a | | #1

    Most oil burner guns now have an outside air kit you can purchase along with directions to get install all.

    The easy answer though is to not run all the fans at once, but you know that.


  2. GBA Editor
    Martin Holladay | | #2

    Almost undoubtedly, the fan that represents the straw that breaks the camel's back is your range hood fan. That fan is likely to have a higher cfm rating than your bathroom exhaust fan or your clothes dryer.

    The problem you describe is well known, and has been addressed by newer versions of the building codes. Large range hood fans require powered makeup air units. To read more on this issue, see Makeup Air for Range Hoods.

  3. GBA Editor
    Patrick McCombe | | #3

    You're right Martin. The range hood is rated at 300 CFM on high, while the bath fan is 70 CFM. A quick web search reveals a typical dryer is 150 to 200 CFM.

  4. Aj Builder, Upstate NY Zone 6a | | #4

    Step one, crack window, step two turn range fan on high. Reverse procedure to shut down. Added benefit, lower risk of diabetes from added exercuse?

    Added benefit to oil burner kit is reduced burner noise.

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