Q&A / 

Furnace Pulls Cold Air from Bath Exhaust Fan

Barbara rents a house or apartment in Guilford, CT. But she's got a problem with her new furnace.

"My landlord just installed a gas furnace. When it is running, cold air is leaking into the bathroom through the ceiling exhaust. When the furnace is at rest, no cold air is coming in. Why? and am I safe???"

Here's Barbara's new furnace. You can't see the cold air. Photo credit: Barbara

Here's Barbara's new furnace. You can't see the cold air. Photo credit: Barbara

Barbara, the most important thing is yes, you're probably safe, but it's possible you do have a life-safety issue.

But something is most certainly wrong with the installation if the furnace is getting its combustion air from the bathroom exhaust fan.

Most modern high-efficiency furnaces have the PVC intake and exhaust pipes as you show in the photo you sent.

I can only assume that somehow the combustion air pipe is clogged or partially restricted so that it's starved for air.

There are other reasons why the furnace is sucking air from your exhaust fan and the issue with that is it could suck other exhaust fumes containing carbon monoxide into your home.

You need to have the contractor come back and verify the furnace is installed properly.

Was it inspected? What does the building inspector have to say about this?


4 Responses to Furnace Pulls Cold Air from Bath Exhaust Fan

  1. Tim,

    I'll keep this short, with an offer to explain at length if you're interested....

    I doubt very much whether combustion air is the issue. It appears to be a two-pipe condensing furnace and unlikely that's it's the problem. Even if the furnace was not getting combustion air from outside, the amount of air flow would not be sufficient to cause a noticeable amount of cold air coming through the bath exhaust fan.

    The problem is most likely an unbalanced forced air system. The air handler (furnace fan) is the driving force causing air to be pulled in through the bath fan. Why is the system unbalanced? There are a number of reasons, all easily checked by someone who knows how. Here are two likely possibilities:

    One of the most common causes is a single return system (i.e., a distribution system with supply registers but no returns in the bedrooms.) When the bedroom doors are closed, the bedrooms are pressurized and the rest of the house is depressurized – sucking hard to find the air it needs. The furnace fan looks for air from any place it can find it, and the bath fan provides an easy route for supplying this air.

    The second very likely reason is a disconnected supply duct outside the conditioned space, usually in the crawl space or attic. The mnemonic we building scientists use to remember what happens with a supply leak (more correctly, supply leaks greater than return leaks) is “Supply leaks suck!”

    There are some potentially very serious health and safety issues with both of these scenarios which I will be glad to discuss if anyone is interested. The homeowner definitely should be interested, especially if she has a conventional (non sealed combustion) gas water heater. This can be very dangerous!

    Hey, that was my short answer.... 🙂

  2. If the combustion air pipe was starved or restricted, there are usually pressure switches that would disable the furnace from working. Doesn't sound like a combustion air issue, unless there has been some serious monkey business with the furnace board.

    Even then, that small of a furnace doesn't pull many cfm from the inducer motor for combustion. It is more likely that something associated with the ducts & air handler would be capable of depressurizing the bathroom enough to pull enough air that could be felt.

  3. Excellent answer Bill (long and short)!

    Please review my site for additional information/understanding on how the home works.

    I also agree that the two major HVAC problems in today’s homes are leaking duct work and improper balancing of the system. As you know in my home all duct work is sealed and I have high and low louvered “return registers” in all living spaces to allow balancing the systems on the return side for summer and winter for nearly 40 years.

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