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Routine

Thermostat blank or not controlling the AC

A blank or unresponsive thermostat is usually a power problem. Dead batteries are the simplest cause. Otherwise a tripped breaker, float switch or blown fuse at the air handler is cutting power to the thermostat circuit. If the screen works but the AC doesn't respond, check the mode and schedule, then the wiring or the equipment itself.

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Round smart thermostat with a lit display mounted on an interior wall

Most fixes are on this list

Batteries, settings and a tripped float switch account for a lot of thermostat calls. Work through these in order.

  1. Replace the batteries.

    The most common cause of a blank screen.

  2. Set the mode to Cool and the temperature a few degrees below the room, and cancel any hold or schedule.

    Rules out settings.

  3. Check the air handler's breaker and look for water at the float switch.

    Either one can cut power to the thermostat.

  4. If the problem started right after a new thermostat was installed, note that for the technician.

    Wiring compatibility is a common cause with smart thermostats.

If it's still not working

Pick a cause to see how likely it is and who can deal with it.

Choose a likely cause

Common

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Dead batteries

Many thermostats run their display on batteries even when they're wired. Weak batteries cause a blank screen or erratic behavior.

Common

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No power from the air handler

Most thermostats get low-voltage power from the air handler. A tripped breaker, a float switch shut off by a clogged drain, or a blown control fuse cuts that power, and the thermostat goes blank.

AC Maintenance

Common

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Settings, schedules or smart-thermostat modes

Wrong mode, a hold or schedule, an eco or away setting, or a setpoint above room temperature all look like a broken thermostat.

Occasional

Technician fix

Loose, broken or incompatible wiring

A wire pulled loose behind the wall plate, a damaged cable, or a smart thermostat installed without the power wire it needs can make the thermostat lose power or fail to call for cooling.

Occasional

Technician fix

Failed thermostat or poor location

Thermostats do fail. One mounted in sun, near a supply vent or on an outside wall can also control the AC badly even when it works.

What the thermostat actually controls

A thermostat doesn't power anything itself. It switches a low-voltage signal that starts each part of the system.

How a split air-conditioning system worksSchematic: inside the house, an air handler with an evaporator coil and blower cools indoor air; a thermostat on the wall controls it; a condensate drain carries water away from the coil. Two copper refrigerant lines, the line set, run through the wall to the outdoor condenser, where the compressor and condenser coil release the heat outside.INSIDEOUTSIDEAIR HANDLERCONDENSER1235
Parts of the diagram

Battery-powered models go blank when the batteries die. Many smart thermostats need a common (C) wire for steady power and can act up or reboot without one.

Its transformer supplies the thermostat's power, usually through a small fuse on the control board. A blank thermostat with fresh batteries often means no power is arriving from here.

More on air handler

On many systems the float switch is wired into the thermostat circuit. When the drain backs up it cuts the signal, and the thermostat can look dead.

More on condensate drain

When cooling is called for, the signal closes a contactor out here. If the thermostat says Cool and only the indoor fan runs, the fault is past the thermostat.

More on outdoor unit

When to stop and call

Turn the system off and call a technician if:

  • Scorched or melted wires behind the thermostat.
  • The thermostat or its wall plate is hot to the touch.
  • The breaker for the air handler trips again after one reset.

Where a thermostat should be

A thermostat measures the air right where it hangs, then decides for the whole house. If that spot isn't typical, say next to a supply vent, in afternoon sun, or near a lamp, a TV or the kitchen, it reads warmer or cooler than the rooms you live in, and the system runs too long or shuts off too soon.

A better spot is an inside wall in a room you use, away from vents, windows, doorways and heat-producing appliances. Moving one means moving its wiring too, so it's worth doing alongside other work rather than on its own. If the trouble started right after a new thermostat went in, the first thing to check is the wiring, including whether the new model needs a common (C) wire.

What our visit looks like

If the screen shows an error message, a photo of it sent through Text a Tech before the visit helps the technician come prepared. On site, the technician checks for low-voltage power at the air handler and at the thermostat, tests each control wire, and confirms the equipment responds when cooling is called for directly. Then the wiring, fuse or thermostat is repaired or replaced, or the thermostat is moved to a better spot. Free service call with repair.

Questions

Why did my thermostat screen go blank?

Most often dead batteries. If new batteries don't fix it, the thermostat may have lost power from the air handler because of a tripped breaker, a float switch shut off by a clogged drain, or a blown control fuse. The last of those needs a technician to find the short that blew it.

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