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???"
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?