Need to buy a clue--condensate drain @ LPG furnace

[David Meiland]

I'm used to seeing air handlers with electric heat strips and heat pumps, and there's always a condensate drain. Rarely see gas furnaces and yesterday saw an American Standard 90 w/o heat pump, with a 1/2" CPVC drain line from the cabinet to daylight. There was a drip coming from it. What is the source of *that* condensate? Is it just what drips back out of the vent pipe?

[AHI in AR]

The high efficiency furnaces gain their efficiency because they recover waste heat which normally goes up and out the exhaust vent. In doing so, the moisture generated during burning of natural gas is too cold to stay in a vapor state so it condenses back out. That explains the drain. It also explains the use of a plastic exhaust vent.

[inspector57]

I'm used to seeing air handlers with electric heat strips and heat pumps, and there's always a condensate drain. Rarely see gas furnaces and yesterday saw an American Standard 90 w/o heat pump, with a 1/2" CPVC drain line from the cabinet to daylight. There was a drip coming from it. What is the source of *that* condensate? Is it just what drips back out of the vent pipe?

Water is a by product of combustion of LP and natural gas. Low efficiency units send the moisture out the vent stack (thus the vapor cloud during cold weather) while high efficiency (Condensing Furnaces) cool the discharge gas to the point that moisture condenses out and is drained away instead of going out the vent pipe.

[asihi]

Here's a good article about condensing furnaces.

http://www.ashireporter.org/HomeInspect ... stems/1871