Poisiones gas escaping

Any idea’s as to why the gas furnace is partially exhausting out the water heater exhaust?


Blockage in the flue above, I had a birds nest in a flue just like that once. I’ve also had people turn on the whole house fan while I was testing the furnace because they were hot. :). That drew in the combustion air real quick.

Why do you say partially? Does it resolve itself when things warm up? Cold flues sometimes take a few seconds to get flowing properly.

Sounds like something simple we can look into, a possible partial blockage in the exhaust pipes.

I can relate to the whole house fan…I was in an attic and the client decided to find out what that switch was…about peed my pants…

If it is a tight home then the furnace is lowering the pressure in the home so you could be getting a back feed as it is pushing more air out of the house then what is leaking in .
An open supply pipe from the outside to near the furnace could be the simple fix .
I would recommend they get a Furnace man in to fix it.

Hi Gerald,
After 1 minute and 32 seconds my Tiff meter is no longer indicating an alert.
That would be quite a force of hot air/gas to get to the Y junction then travel (at the same time) upwards out the exhaust and downwards out the hot water exhaust.

I checked it three times, and each time was the same results. The exhaust pipes were already warmed up.

Always during the initial startup?

I thought about this before I did my third test, so I opened the door to the garage/outside. Not a fresh air intake issue.


On the second test I shut down the furnace and turned up the hot water heater, then kicked on the furnace, and all was as it should be.

The exhaust going through the attic.