Finally I found the source of furnace shutdown... water in the horizontal exhaust vent. I cut the pipe open and found a lot of water... drained it and the furnace worked fine until it was filled with water again. However, for the first 4 years, this was never a problem. Everything is still well connected with no sags. Very surprising that it is happening now. There has to be something else that created this problem.

The 2.5 feet of horizontal pipe is connected to a vertical pipe going 6 feet high. So I'm assuming water was only in the horizontal part.

Our ceiling in the closet started leaking. We had a plumber come in who said the leak is related to "stack vent". Now I'm thinking, can HVAC exhaust be connected to these pipes and the problem is really originating around stack vent. I'm trying to avoid changing the slope of the exhaust as it would require taking walls down. Also, I think when I run the furnace I find more moisture/dripping water in the closet (ceiling).

Also, the exhaust pipe leaving the furnace is 2" and the exhaust coming out of the building is a 3" pipe.

My question is can HVAC exhausts be connected to other pipes? I would imagine they directly go outside the house and not mix with other vents/exhausts etc. But I'm doubting this as seems very unlikely that the furnace would run find for years, starts giving problems, and the I find water in our ceiling. Have to be really really unlucky.

Thanks for the feedback.

Your furnace and stack vent are never connected. Is it possible the plumber disturbed some insulation around the exhaust pipe, sounds like you are getting condensation.

We had a plumber come in who said the leak is related to "stack vent"
What did he do about that? Sounds like "I really don't know what is going on, pay me."

Don't know how a stack vent, which is really the vent portion above the sewer drain stack could leak.

Suggestion, until you figure this out. Cut out the ell that connects the horizontal section to the vertical. Replace it with a tee. Out of the bottom of the tee put as long a piece of pipe as you can. Glue a pipe cap on the bottom. Attach this pipe to the bottom of the tee with a couple of screws.

This would amount to a drip leg just like you have in a gas line. You can then periodically remove and empty the condensation.

I don't know much about high efficiency furnaces but I thought that they have a PVC exhaust out the side of the house and that they also have a PVC combustion air input from the outside also.

Is yours set up that way?