When the thermostat stops working, it looks like the whole system is broken. Often it's a power or settings problem you can fix in a few minutes.
If the Screen Is Blank
- Replace the batteries. Many thermostats run on AA or AAA batteries even when they're wired in. A low-battery warning is easy to miss.
- Check the breaker and the furnace switch. Most thermostats get their power from the furnace or air handler. If the furnace is off, the thermostat goes dark.
- Make sure the furnace door is on tight. The door presses a safety switch. If it's loose, the furnace and the thermostat both lose power.
- Look for a tripped float switch. Many systems have a small float switch in the condensate drain pan or line. When the drain clogs and water rises, the switch cuts power to the thermostat to prevent a flood. A blank screen plus water in the drain pan points right at this.
If the Screen Works but There's No Heat
- Check the mode and setpoint. It should be on HEAT, and the set temperature should be a few degrees above the room temperature.
- Check the schedule. A programmable or smart thermostat may be holding an away or sleep temperature. Use a temporary hold to test.
- Wait out the delay. Many thermostats and heat pumps have a built-in delay of up to 5 minutes before they start, to protect the compressor.
Smart Thermostat Problems
Smart thermostats need steady power, usually from a C-wire (common wire). Without one, some models steal power from the heating circuit and drop offline, reboot, or cycle the system on and off. If your smart thermostat keeps losing Wi-Fi or rebooting, a missing C-wire is the likely cause. We can add one or install a power adapter.
If the Temperature Reading Seems Wrong
- Location matters. A thermostat in direct sun, above a lamp, near a kitchen or on an outside wall reads wrong.
- Dust inside older thermostats throws off the sensor. A gentle cleaning can help.
- Mercury and dial thermostats from the 1980s and earlier lose accuracy over time. Replacing one is a quick upgrade.
If you've worked through this list and still have no heat, the problem may be a blown low-voltage fuse on the furnace control board or a wiring fault. Those are quick fixes for a technician.
