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Confused HVAC system


JesseWBryant
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I didn't either. Thats why I had to post it to see if anyone else has. The funny thing is, my client originally did not want to get an inspection as it is a new home. Her mother-in-law talked her into it by paying the inspection fee for her. As I was describing the HVAC issue to her, she said "I'm really glad I got that inspection!" Converting clients one day at a time.

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Originally posted by Bain

Are you certain the system isn't a heat pump, with gas auxiliary heat?

I had one like that about a month or so ago. The only difference was that the heat only cycled up to the point of ignition and then started over. The vent fan would start, then the starter would glow, then it would stop, and then start the cylce over again. Weird to see.

The technician that came while I was there said it was a board at the furnace as Jim indicated it might be.

The unit I saw was in the attic and who knows how long it had been doing that. I knew something was up as soon as I pulled the stairs down because the vent fan started just as I opened the attic.

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But Jim, what if the furnace was supplementing the heat produced by the condenser? The condenser would remain energized even though the burners were ignited. If someone dialed the thermostat to 90 degrees to check the system, the auxiliary heat would likely kick in while the compressor remained on.

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Originally posted by Bain

But Jim, what if the furnace was supplementing the heat produced by the condenser? The condenser would remain energized even though the burners were ignited. If someone dialed the thermostat to 90 degrees to check the system, the auxiliary heat would likely kick in while the compressor remained on.

No. Heat pumps with gas back up don't ever run both at the same time. There really isn't an "auxilliary" function with these systems. When it gets too cold out, the heat pump stops and the gas furnace takes over.

They're designed like this because the gas heat exchanger is always upstream of the indoor heat pump coil. If you were to run both at the same time, the heat pump's coil would be awash in hot air fresh from the gas heat exchanger. It then wouldn't be able to dump any of its heat and it would develop high head pressures.

- Jim Katen, Oregon

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  • 2 weeks later...

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