My apartment has a wall-mounted AC unit with a digital temperature setting, which works great, but it has a consistent problem where its thermostat appears to read much differently than the actual room temperature, usually instead reading what appears to be the outside temperature. This means that when it is hotter outside than inside when it is cooling, once it reaches the set temperature it turns on again after what appears to be a very short while (~5 minutes), and when it is cooler outside than it is inside (such as during the night), it will read low, and will allow the room to become much hotter than what the temperature is set to.

When this happens, it you run its fan for a minute or so it will then "realize" the actual room temperature, the thermostat reading will significantly change accordingly, and then it will cool as usual. This is a problem though, particularly at night when it's inconvenient to manually adjust the fan settings, because it allows the room to get much hotter than what it's set to (sometimes +10 degrees F). I can't compensate by setting the temperature lower than actual desired temperature, because once it does kick on, it will "read" correctly, and then cool for much longer (and to much colder) than I would like it to be.

Because using the fan causes it to get an accurate reading, my hunch is an airflow problem - I've cleaned the filter, the front grill, the thermometer bulb, and the radiator fins inside, and it still happens. Would anyone have any insight on why this would be? Do you think getting a separate floor fan to increase the room circulation (and therefore hopefully improve the airflow from the room through the unit) would help? Or is it time to get in touch with a professional to investigate the unit?

The air conditioner model is a General Electric AJCQ08ACCM2, and it's rated for 8000 BTUs. The area that it's cooling is ~200-250 square feet.

  • It sounds like outside air may be leaking into the evaporator side of the unit and heating up the thermostat. Can you check if there is any leakage? The inside vs. outside should be well sealed and insulated. – jwh20 May 27 at 18:52
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    Does direct sun hit the thermostat area? This can happen; it's indicating accurately for its location, but the system needs to "stir the air" before it realizes the rest of the building is not at that temp. Some smarter 'stats intermittently circulate the fan for exactly that reason. Obviously an old mechanical Honeywell is incapable of that... – Harper - Reinstate Monica May 27 at 20:31
  • I don't current have an opportunity to open the unit again, but will check the seal between chambers when I next can. Since it's a wall-mounted unit, the thermostat is internal to the package - I don't think sunlight is the issue, as that wouldn't explain why it reads cold at nighttime. – Mejwell May 28 at 1:15

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