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I have a 13 SEER heat pump about one year old. Everything's been working fine, but the AC suddenly started heating my house spontaneously (no changes to thermostat, etc).

The outside ambient temperature is 80°F but the incoming air measures 116° at the vent. The odd part is that the outside condensing unit is blowing 68°F (cold?).

What might be causing this?

Coincidentally, I've been meaning to install a Nest thermostat I purchased awhile back. If this is a problem with the thermostat, I would just go ahead and do the installation now. But if this is more likely a problem with the system, I'd just as soon have it repaired before changing anything.

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2 Answers 2

Your heat pump switches between heating and cooling mode through a reversing valve. It sounds like that is where your problem is; it could be that your thermostat has gone bad, or it could be something else.

The outside condensing unit is blowing 68 degrees because the system is pulling heat out of the outside air, and pumping it into the house.

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We've had the shuttle valve fail both ways. Trying to create thicker blocks of ice in the winter and tremendous heating in the summer. Time for your friendly local HVAC tech to show up and fix it. The thermostat just tells it that it isn't finished and since its doing the exact opposite, the thermostat can't get it to relent. –  Fiasco Labs Aug 10 '13 at 5:36

Check the thermostat O (orange) terminal with a multimeter to ground. Most brands energize (apply 24V AC on the terminal) in cooling mode. A bunch of things could be going on such as a wire loose, bad thermostat, bad control board in heat pump or bad reversing valve but the first thing to check is the thermostat then you can work your way back to the heatpump.

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