Failing Exhaust Duct work

By January 27, 2019 Fixing things, HVAC, Indoor Air Quality

Estimated reading time: 2 minutes, 16 seconds

A client called to report a hot exhaust wall in their bathroom. Naturally fire and Carbon monoxide poison come to mind. The gas water heater exhaust and the 80% furnace exhausts commingle and go up and out of the home in a double walled oval exhaust flu. The home was built in the ’70s and has already had the furnace replaced once. Was it a bad install or just warn out?

The thin 2 x 4 wall was hot in one spot. A thermal image was showing over 100 degrees in one spot. How were we going to see what was going on, we did not want to tear out the wall but we could have. The chase was lined in metal so we were not too worried about fire but the hot spot was concerning for sure.

Differential diagnosis

  1. Is there is an obstruction? We took the cap off at the roof and look for any kind of obstruction.  Theirs was clean as a whistle.
  2. Is there flu rot or disconnection? Look down and see. But how?

The Incredible Journey

How could we inspect the flu without tearing it out? How could we get that small? I figured we would need to get into my private stash of magic dust and get really small ala Cheech and Chong.  Just then it dawned on Brant to send his cell phone for a ride. He taped a smart phone to a rope using the video feature started the trip. Much like the incredible journey, he found out what what was going on.

There it was a serious crack and a break in the flu connection. There was also a substantial amount of sand looking material that I believe was product of combustion deterioration and minerals that settled in the base blocking the majority of the water heater exhaust.

The fix

We felt better after vacuuming the (sand like material) out and putting things back together while we researched a more permanent fix. The oval double wall is still available in 5′ lengths that can be connected as they get set down from the attic. We ended up replacing all of the pipes and the transitions from round to oval and back again.


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