Thermostat not working? Check these before you call

A dead thermostat looks like a dead furnace or AC. Before you pay for a service call, run through these checks. They take about ten minutes, and they fix more "no heat" and "no cool" calls than you'd think.

General guidance only. Your thermostat's manual comes first. If you smell gas or a carbon monoxide alarm is sounding, leave the house and call 911 or your gas utility from outside.

If the screen is blank

Thermostat maker Honeywell Home gives this order of checks for a blank display (Honeywell Home):

1. Replace the batteries if your thermostat uses them. Many do, even ones that are also wired.
2. If it's wired to the system, try batteries anyway. If it comes on with batteries, the thermostat is probably fine and the HVAC system isn't sending it power.
3. Check power to the system:
4. The breaker for the furnace or air handler in your electrical panel
5. The furnace power switch, which often looks like a light switch on or near the furnace
6. The furnace door safety switch: if the front panel isn't seated firmly, a switch cuts power. Make sure the door is on snug.
7. If the system has power and the thermostat only works on batteries, or won't come on at all, Honeywell says you may need a technician.

One more to check on AC systems: many air handlers have a condensate safety switch that shuts the system off if the drain line backs up. If there's water in the drain pan or around the unit, that may be why. A technician can clear the line.

If the screen works but nothing happens

• Check the mode. Heat, Cool, Auto or Off. It's easy to bump to Off or the wrong mode when the seasons change.
• Check the set point. For heat, set it a few degrees above the room temperature. For cool, a few degrees below.
• Check the schedule. A programmed schedule or "away" setting can override what you just set. Look for a "hold" or "permanent hold" option.
• Wait a few minutes. Many systems have a built-in delay of several minutes before restarting, especially heat pumps and AC compressors, to protect the equipment.
• Fan set to On vs. Auto. If the fan setting is On, the blower runs all the time, even when the system isn't heating or cooling. That can feel like cold air blowing in winter. Auto runs the fan only during a heating or cooling cycle.

Smart thermostats

• Check the app for an error message or an "offline" notice.
• Wi-Fi changes (a new router or password) can disconnect a smart thermostat.
• Power problems are common on smart thermostats installed without a common wire ("C-wire"). Symptoms include a screen that dies, reboots, or a low-power warning. A technician can add one.
• Installed it yourself recently? If problems started right after you swapped the thermostat, a wiring mistake is the likeliest cause, especially on heat pumps, which use extra wires. See heat pump repair.

Is the thermostat in a bad spot?

A thermostat in direct sun, over a lamp, near a supply vent, or on an outside wall can read the wrong temperature. That can make the system run too long or too short. If your system short cycles (turns on and off every few minutes), location is one possible cause. See furnace repair and AC repair for others.

When to call a technician

• The system has power and the thermostat still won't turn it on
• A breaker trips again after you reset it. Don't keep resetting it.
• The furnace or air handler hums, clicks or smells hot but doesn't start
• There's water around the indoor unit
• You'd rather not touch wiring. Thermostat wiring is low-voltage, but mistakes can damage the system's control board.

Tell us what's going on and where you are in Danville or Pittsylvania County. We'll connect you with a local HVAC company. It's free.