I went to a no-heat call last week, a single story back-split with a walk-
out basement, around 1500 sq.ft.
As I got close to the furnace room, I noticed the high humidity and the
walk-out glass patio doors covered in moisture.
Sure enough, the B vent was blocked with a birds nest just above the "Y"
fitting for the water heater and the furnace.
The furnace was operating intermittently (Smart Valve issue), and the water
heater was pumping out the flue gases, both of which were spilling at the
My CO detector showed zero.
I asked them how long had they had the humidity problem in the finished
basement and they said for several MONTHS!. They had a de-humidifier
No headaches, no nausea, no burning eyes from alkides.
I took apart the B vent, removed the nest, and fixed the furnace.
I'm amazed that these people didn't wake up dead from CO poisoning!
The house is about 20 years old, so it's not as air tight as some newer
ones, but it's not exactly made with barn boards either.