Replies

1. GBA Editor
   Martin Holladay|Jan 28, 2009 09:49am|#1

   Sean,
   You don't provide enough information to understand the problem. What is freezing: the condensate drain or the flue? If a condensate drain runs through an unconditioned space, it can freeze. If the problem is in the furnace flue, three possibilities come to mind:
   1. The flue termination might be blocked by snow.
   2. If a screen was installed on the flue termination, ice could have accumulated on the screen and blocked the flue. No screen should be installed at the flue termination.
   3. If the flue isn't sloped correctly — it should slope back toward the furnace — condensate could accumulate in the flue and eventually freeze.
   Martin Holladay

2. 
   Michael Chandler|Jan 28, 2009 11:00pm|#2

   Sean, I'm going to guess that you have terminated your PVC flue with a straight pipe with a bird screen in it which is accumulating frost and choking off the ability of the blower to expel combustion gases from the heat exchanger leading to an air fault on the burner.

   The solution should be to terminate in a sanitary tee installed upright with two bird screens which should double your screen area. The intake pipe can still terminate with a single bird screen but you may want to do this with an elbow facing down to keep freezing rain and snow off of that screen. I know it looks ugly but I think this is what it's going to take to frost proof you're terminations.

   Martin's point about checking the pitch of the flue to confirm that it is pitched back to the burner and it's condensate line is well taken. I'd check the installation manual about eliminating the bird screen though, a horizontal run of PVC pipe seems like a nice summer accommodation for a house wren to me.