# a/c doesn't turn on when temperature is reached

I just changed my thermostat and it does the same thing [as the old thermostat did?].

I get up in a.m. thermostat is set at 77 F. When inside room temp reading on the thermostat reaches 77, a/c doesn't turn on. I raise setting to 78 and when ambient hits 78 nothing happens. I then turn temp set point down to 77 and a/c kicks on. Then it runs normal 'till next morning.

What could it be?

• What happens in the morning with set point at 77 F if you wait for the temp to reach 78 F? Does it turn on? Maybe the measured temperature has to exceed the set point for the thermostat to signal "turn on". – Jim Stewart May 18 at 17:44

## Because 77F is the desired temperature

Let's suppose you have an envelope for the \$800 rent due this month. Add money til it's \$800. Not til it's \$801, because \$800 is just right.

The system is viewing temperature the same way. Temperature == 77F, just right, nothing to do.

## Anyway, the system should have hysteresis

You don't even want the thermostat hitting 77.1 degrees, going "Too hot, run A/C" and running it for 20 seconds until it's 76.9 degrees. That would result in something called short cycling which is rather bad for air conditioning units.

Not knowing your system*, the thermostat doesn't want to risk a short cycle, so it will intentionally add some hysteresis to the cycle. For instance it may fire off at 79F, and run until 75F. That assures the unit isn't running too often.

Regardless, the command temperature is the satiety temperature; ideally if actual temperature == command temperature, it should not start a heat or cool cycle.

If 77F is the "unbearable" temperature at which you want the A/C to come on, then set your thermostat to your "comfort" temperature e.g. 74 or 75. Adjust to suit.

*many systems are "oversized" in the sense that in normal conditions they have more than plenty of capacity; however on very hot days that also means they have enough capacity.

Never hear of tstat adding multiple degrees to set point. Time delay between last on off, yes.

But otherwise comment spot on. If you set you tstat to "I command you to run until reach 77 degrees" then ac doest have need to turn on tI'll above that