Venting high efficiency furnace

Old 02-19-08, 06:17 AM
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Venting high efficiency furnace

Alright today's question, although I believe I'm pretty sure on the response...
Do high efficiency furnaces have to be vented directly outside or can they still be vented through a chimney?

My furnace is centrally located(next to my chimney) and the nearest exterior wall is about 10 feet away(through a bathroom - nonetheless)...

Two contractors came out, one laughed, said stick w/ an 80, other said he's seen it done(seen being the operative word)...

According to things I've read here it's a must to go directly outside, but just want to be sure.

Old 02-19-08, 06:26 AM
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Most 90% furnaces can be vented vertically, and through a chimney if desired. But the chimney must be decommissioned and not used to vent other appliances.

Read the installation manual for the furnace you are interested in to check on venting that particular furnace vertically.

I had an interesting repair job on a furnace venting certically. In very cold weather, the furnace didn't operate reliably, and would shut off before heating up the home very much.

After some careful study, I found the pressure switch was opening when moisture condensed on the inside of the plastic PVC pipe in very cold weather. The inducer motor forced air to flow vertically out the vent pipe, which tended to support the water droplets adhearing to the inside of the pipe. The water droplets thus created an obstruction to the airflow through the vent pipe, which caused the pressure switch to open and shut off the burner.

When you turned off power to the furnace, shutting off the inducer motor, you could hear a rush of water coming down the pipe as those suspended droplets drained out.

The problem was undersized vent pipe which permitted that problem to occur.

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