Know your system · Heating & cooling

Thermostat screen blank? Check these in order

Short version: a blank thermostat is usually a power problem, not a dead thermostat. Work from easiest to hardest: fresh batteries, then the breaker, then the furnace power switch, then the condensate float switch. If none of those fix it, have a pro take a look.

1. Batteries

If your model takes them, dead batteries are the most common cause and the easiest fix - Lennox calls fresh batteries "often the complete solution" for battery-powered models. Pop the faceplate off and look at the wall base. If a C (common) wire is connected along with R, those two wires power the thermostat and batteries are not required, so skip to the breaker. If there is no C wire, swap in a fresh set. Replacing them once a year (tie it to another annual task, like smoke-detector batteries) helps reduce the chance of a battery-related blank screen.

2. The breaker

A thermostat powered through R and C gets its power from the furnace, so it loses its display when the furnace’s circuit loses power. Only check a clearly labeled furnace breaker at a dry, safely accessible panel. Do not touch a wet, damaged, hot, buzzing, sparking or burning-smelling panel. For smoke or fire, get out and call 911. Otherwise leave those warning signs to an electrician. Do not reset any breaker more than once. To reset a clearly identified tripped breaker, move it fully to "OFF", then "ON". If it trips again, leave it off and call a pro. A breaker that trips again needs to be checked out for your safety. If you are not comfortable, it is okay to call a pro instead. If the panel's labels are guesswork, do not guess at the breaker. Call a pro.

3. The furnace power switch

Most furnaces and air handlers (the indoor unit of a heat pump or air conditioner) have a switch on or near the unit that looks exactly like a light switch. They get flipped accidentally - by a broom handle, a storage box, a helpful guest. An accidentally switched-off system can explain a blank hard-wired thermostat. If it is a clearly identified power switch and there are no leaks, smells or other warning signs, check its position.

4. The float switch

Here is the one most people never suspect: if the condensate drain line backs up, standing water can trip an installed float switch. Depending on how it is wired, it may stop cooling or cut thermostat power. Not every system has one, and not every shutdown makes the display go dark. Water near the unit is a reason to stop cooling and call for service, not proof that the float switch caused the blank screen. Do not bypass the float switch. If you are comfortable clearing the clog yourself, first turn the system off at the thermostat. The safest way is a shop vac at the drain’s end, where the line pours out. Do not use Drano or similar drain cleaners: they are very dangerous and can blind the homeowner or the pro if handled incorrectly. Blowing out or snaking the line can damage the PVC, depending on how fragile it is and whether the joints were glued correctly. If you are not comfortable, call a pro. While you are there, a dirty filter can contribute to the coil icing and overflow cycle that fills the pan.

5. Where to go from here

If the furnace still will not heat the home, then it is recommended to have a pro evaluate the situation and mention what you have done so far. This will help get the problem solved quicker and get the home heating again.