What causes the air conditioner to run more in the morning than during the day?

Question

It's a mercury thermostat, but I've seen this at the last two apartments that I lived at, and am really curious as to why this happens.

I keep it set at around 78 degrees, and it works as it should, kicks off right at 78 during the day.

But when i wake up every morning, it's in the low 70's, as if somebody bumped it down to 71-72 over night. If I don't touch it, as the day warms up it slowly gets back to how it was set originally (to shut off at 78 degrees).

Every morning. I'm just wondering why this happens. I'm guessing something to do with it being a mercury based thermostat instead of a digital?

Answer 1

Yes it is likely just a function of the fact it is a mercury Tstat. That is unless the sun is shining on it in the morning or something of the like.

The run time is too long when it is cooler outside. So the temp is dropping too low before it shuts off. There is a lag time from the air temperature to when the thermostat itself cools down.

In heat, it has a built in device called a "heat anticipator" that corrects for it in heat by warming the Tstat as it runs so it shuts off early to give it time to sense the actual room (air) temperature before running again.

There is no such device in cool. So it can run too long in the morning when there is not a lot of heat coming in the room.    

And yes, a digital would sense much faster and more accurately.

Good Luck.

Answer 2

Most likely the outside temperature is dropping below the level in which your thermostat is set so really your AC unit doesn't kick on at all during the night because it is colder than what you want it to be.  Also Mercury thermostats are not nearly as accurate as today's digital thermostats so a little variance in temperature will be normal.

Answer 3

Try putting a small level on it.Check it's accuracy also with another thermometer near it.Maybe there is a warm draft near it in the morning that is tricking it.Check them all.good luck.