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A few months ago, my wife and I started sleeping with our bedroom doors closed to keep out our cat. We would wake up in the middle of the night to find our bedroom was very hot and it felt really hard to breathe. We got an Awair some time later and noticed the CO2 was shooting up to about 1500ppm within 2 hours.

To solve this, I began using the thermostat's "Fan" function set to ON to get ventilation continually throughout the night. My understanding is that this only blows air, but is separate from heating/cooling.

The problem is that even when we set our AC to heat or turn that system off, the fan appears to be cooling the house. Despite it never getting below 70°F outside, this fan function is getting the house down to 64°F within a few hours. Then, because I have the heat on to prevent it getting so low, the heat starts running.

I am running heating+cooling all night just to prevent from suffocating. Obviously our energy costs are out of control.

Am I misunderstanding something? Is it possible that thermostat is miswired such that the fan is hooked up to the AC itself? And how would I even begin to diagnose this?

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    What make and model are your thermostat and your air-handler, and can you post photos of the control (thermostat) wiring at each end + the wiring diagram for your system? – ThreePhaseEel Aug 17 at 17:08
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Sounds like it's miswired... But maybe your unit has limitations. Pictures would be great.

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As a temporary workaround, shut off the A/C breaker at night. This is a large double circuit breaker marked typically 20, 30 or 40. A nearby hand-written map will show which breaker is for what circuit. After you shut off an A/C unit, wait 10 minutes before turning it back on, to avoid "short cycling" and breaking the compressor.

Let's see... Basic thermostat wiring looks like

  • W is the white wire, and calls for heat
  • Y is the yellow wire, and calls for air conditioning
  • G is the green wire, and calls for fan

The thermostat connects any of them to the R wire, Red, which is the supply. It sounds like R is right, but double check it.

Closely inspect both ends of the thermostat wire to make sure they all go where they are supposed to.

Did you upgrade to a smart 'stat?

Most smart thermostats need a C (common) wire (which you put on any which wire you can, but preferred is blue). Sometimes, their thermostat was wired with no spare wires, so they poach one of the other wires for that. There are two strategies used to make up for this, which are of interest here.

  • They poach the G (fan) wire to use for common, which if done properly would eliminate the "continuous fan" switch functionality. However they have mis-wired it somehow, so the G terminal on the 'stat connects to the Y wire.
  • They make up for the missing wires with multiplexer boxes at each end. Those multiplexer boxes might fail.

Sanity check your conclusions

There is a large machine situated somewhere outside your house. If your A/C is actually on, it will be making a lot of noise, and it will be blowing hotter-than-ambient air. Double check your actual nighttime air temp, It can be cooler than you think, especially in desertine environments like the American southwest.

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