AC never stops running: is it the thermostat?

Is it the AC running, or just the fan?

Start with the fan setting, because it is the easiest thing to miss. Most thermostats offer "Auto" and "On" for the fan. "Auto" runs the blower only while the system is cooling or heating. "On" runs it all the time, so air keeps coming from the vents between cooling cycles. That air is not cooled, and it can feel as if the air conditioner never shuts off.

Sensi's support page for a system that runs constantly starts here too. It says to set the thermostat mode to Off and the fan to Auto, and notes that with the fan on, the system can sound as if it is heating or cooling when it is not. It also says to make sure the Circulating Fan feature and any humidity control features are not active. A circulating fan mode runs the blower part of each hour, and some dehumidifying features, such as overcooling to dehumidify, can keep cooling past the setpoint on purpose.

Put your hand near a supply vent. If the air is room temperature while the fan keeps running, you are most likely hearing the fan, not the cooling. Set the fan to "Auto" and listen for the blower to stop once the current cycle ends. The full comparison is in fan "On" vs "Auto", and a fan that runs with the system set to Off has its own page: fan runs when the system is off.

Is the setpoint lower than the house can reach?

A thermostat stops cooling only when the room reaches the setpoint, plus whatever small band it allows. Google's Nest example: with a 70°F target and a 1°F maintenance band, the thermostat cools the room to 69°F before it stops. If the room never gets there, the call for cooling never ends.

On a very hot day an air conditioner may not be able to pull the house down to a low setpoint at all. The thermostat keeps asking and the system keeps running. Look at the screen: compare the current temperature with the setpoint. If they are several degrees apart and the gap is not closing, the setpoint is the reason it keeps running. Raising it a few degrees lets the system finish a cycle.

Also confirm the setpoint you see is the one you chose. A schedule or a hold can set a lower cooling target than you expect; see thermostat is not following its schedule.

Is it just a very hot day?

Long runs in peak heat are normal. Honeywell's installation guides for the T4 Pro and T6 Pro Smart rate the cycle rate at a 50% load and state that the system cycles less often when the load is less than or greater than 50%. On the hottest afternoons the load is high, so the air conditioner runs longer each time and rests less between runs.

What matters is whether the room is holding near the setpoint. If the thermostat shows the room within about a degree of the target and the runs shorten as the evening cools, the system is keeping up. Honeywell says its thermostats are designed to control temperature to plus or minus 1°F.

Is the thermostat reading warmer than the room?

A thermostat that reads high keeps cooling a room that is already cool. Sensi says that in nearly all cases where a digital thermostat does not read accurately, the cause is a poor location or heating and cooling equipment of the wrong size for the home. Google's list of poor spots for a Nest Temperature Sensor includes direct sunlight, drafts, vents and mounting too high or too low, with chest height as the ideal.

Carrier makes the same point about thermostats installed near heat sources, drafts or direct sunlight: they misread the actual room temperature. For a constantly running air conditioner, the usual suspects are afternoon sun on the thermostat, a lamp or appliance beside it, or an outside wall that warms up. Compare the thermostat reading with a thermometer in the middle of the room. If the thermostat reads several degrees warmer, see thermostat shows the wrong room temperature, which covers offsets by model.

Could a setting make the runs longer?

The settings that control how often the system starts can also stretch each run. A slower cycle rate or a wider temperature differential lets the room drift further before cooling begins, so each run is longer. Sensi describes its slower cycle rate as allowing a wider temperature swing. That makes runs longer, but it does not make the system run without stopping. If the air conditioner truly never shuts off, the cycles per hour setting is not the cause.

What if it keeps running with the thermostat set to Off?

Then the thermostat settings are not the cause. Sensi says that if the system still runs a few minutes after you set the mode to Off and the fan to Auto, there may be a wiring fault. Carrier says a thermostat stuck in the on position makes the system run without stopping, whatever the indoor temperature. Both are jobs for a licensed HVAC technician, not something to fix from the screen.

What should you check now?

Is the air from the vents cool while it runs?

• Yes

  Is the room temperature on the screen several degrees above the setpoint and not dropping?

  • Yes

    Is the setpoint unusually low, or is it the hottest part of the day?

    • Yes

      The system is working toward a target it cannot reach yet. Raise the setpoint a few degrees and watch whether it starts to cycle.

    • No

      The thermostat is asking for cooling and the room is not responding. That points past the thermostat; check the signs below.

  • No

    The thermostat may be reading warmer than the room. Check its location and compare with a thermometer.

• No

  The fan is probably set to "On". Set it to "Auto" so the blower stops between cycles.

If the problem is the reverse, a system that starts and stops every few minutes, see AC turns on and off frequently.