AC constantly running and overcooling my house

Question

My AC just will not stop running. The temperature outside has been fluctuating from super cold to around 70 degrees, so we’ve been setting it to 73 with the cool air on. The problem is, when we wake up the thermostat says that it’s 73 in the house, is still running, and our independent thermometers say it’s like 65 in our house. I have a baby who can’t be too cold or too hot so we’re having to wake up in the middle of the night to adjust the temperature. We don’t have this problem when it’s set on heat, but it just gets too hot sometimes for us to have it on.

Answer 1

To summarize the stated problem:

• Thermostat set to COOL - 73F.
• House temperature is significantly lower than 73F.
• Air conditioning appears to still be running.

There are two very different possible problems:

Thermostat Broken

The thermostat is broken in some way. Maybe a relay sticks so that once it turns on the air conditioning at 74F, it never turns the air conditioning off. Maybe something else. Replace the thermostat.

Fan set to ON instead of AUTO

Most thermostats used with air conditioning and/or forced air furnace have a "Fan" setting independent of the COOL vs. HEAT setting. This can generally be set to ON or AUTO.

When the fan is set to AUTO, the fan runs whenever the air conditioning is running and most of the time when the furnace is running (the fan is then under control of the furnace itself, which will often not run the fan during ignition/warmup but will run the fan during cooldown).

When the fan is set to ON, the fan runs ALL THE TIME (except... * see below). If your fan is set to ON then it will be blowing even when there is no call for COOL or HEAT.

If your fan is set to ON then the blower fan will run but your air conditioning compressor (the big box outside) will not run except when the temperature rises above the set COOL temperature. But if you don't look outside at the compressor you have no easy way to know that the compressor isn't running. It can seem like "the air conditioner is running non-stop".

So check your thermostat. If the fan is set to ON, change it to AUTO and that should solve the problem. If fan is set to AUTO then there is likely something wrong with the thermostat.

There is an additional complication in your specific situation. If the temperature inside drops to 65F, it really doesn't matter whether the fan is running or not, your house is going to feel cold. Meaning turn on the heat.

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* Anecdote time.

I am a computer consultant, not an HVAC technician. Years ago one of my biggest customers had people in either office or warehouse almost continuously from 4am Monday morning until 5pm Friday afternoon. Warehouse from 4am Monday morning until 5pm in the afternoon and Tuesday through Friday starting at midnight. Office from 8am until 5pm but often much later. For comfort reasons, they preferred to have air circulating all the time, whether in heating or cooling mode and whether the furnace/air conditioner was running or not. 72F all the time. Air blowing all the time. Yes, that cost a bit extra but they wanted to do that to keep the employees comfortable. In other words, "Fan" set to "ON".

Everything was running fine until they replaced the entire system, including the thermostat. Night shift complained that air often wasn't blowing during the shift. HVAC company came out. Checked everything. Couldn't figure it out.

Finally, I decided to take a look at it. After all, a modern thermostat is a small computer. This was 16+ years ago so it wasn't WiFi or super-smart by today's standards, but still microprocessor controlled. I read through the Chinglish manual and figured it out. The thermostat, like many, required dividing the day into 4 periods of operation, two occupied and two unoccupied. The HVAC technicians had set the thermostat to the same temperature for all periods, with typical times (9am, 5pm, 11pm, 6am or something like that). They were always on site during "occupied" times. It turned out that during "unoccupied" times, the fan would always run as "AUTO", no matter how the switch was set. Which makes sense if the building is truly unoccupied. But this building was basically always occupied.

I fixed the problem by setting the times so that each "unoccupied" period was 15 minutes long (the minimum). So the fan would run 23.5 hours a day, more if the thermostat called for heat or cool during unoccupied times.