Thermostat Blank or Not Responding? Honeywell, Ecobee, Nest

When the thermostat screen goes blank or stops responding, the furnace or air conditioner stops too, because the thermostat is the switch that tells them to run. The good news: most of the time the thermostat itself is fine and it simply isn't getting power. Here's how to work out which, for the three brands we see most in Denver homes.

Quick diagnosis: what the thermostat is telling you

Step-by-step checks

1. Batteries

Many Honeywell models run on AA or AAA cells, and some wired models use batteries as backup. Honeywell Home's support guidance for a blank display starts here: replace the batteries. The plain Nest Thermostat (the one without "Learning" in its name) takes two AAA alkaline cells, and Google recommends replacing both with fresh 1.5-volt alkalines when the screen won't wake.

2. Power to the system

Wired thermostats get power from a transformer inside the furnace or air handler. If the furnace has no power, the thermostat goes dark. Check the furnace service switch, the breaker labeled furnace or air handler, and that the blower door is fully latched so its door switch is pressed. Honeywell lists exactly these three: circuit breakers, the furnace power switch, and the furnace door kill switch.

3. The condensate overflow switch

Many systems have a float switch in the drain pan or condensate line that shuts the system down if water is backing up, and on some it does so by cutting thermostat power. In summer that's usually the AC drain. On a high-efficiency furnace it can happen in winter too. If you see water in a pan or around the unit, empty what you safely can and call. Don't bend or hold the float switch to restart the system.

4. Seat the faceplate

Pull the thermostat body straight off its wall plate and press it back until it clicks. Google's first step for a dark Nest Learning Thermostat is exactly that: remove it, make sure no wire is blocking it, and push it back on.

5. Brand-specific tests

Honeywell (wired models): if batteries bring the screen back but the system has power, the thermostat isn't getting 24 volts. Honeywell's guidance at that point is to call a technician.

Google Nest: a Nest Learning Thermostat has a rechargeable battery that the system keeps topped up. If the display won't light, Google suggests charging it through the USB port on the back using a wall charger. If it lights up on USB but dies again on the wall, the problem is power from the HVAC system. Google also notes a clogged filter can make the system overheat and shut off, which stops the thermostat from charging.

ecobee: ecobee thermostats don't run on batteries; they need continuous power. Homes without a C wire use the Power Extender Kit (PEK) that ecobee includes. ecobee's installation guide says to check that the PEK and terminal connections are secure if the thermostat won't turn on, and to switch off the heating and cooling system first.

6. Check the settings

A screen that lights up but doesn't start anything is usually a setting: mode on Off or Cool, a vacation or eco hold, a schedule, or a setpoint below room temperature. Air conditioners and heat pumps also have a built-in delay of a few minutes after shutting off to protect the compressor, so give it five minutes before deciding it's broken.

7. Try the old thermostat

Google's last step is a smart one for any brand: if you kept your old thermostat, put it back. If the system runs on the old one, the new thermostat or its setup is the problem. If not, the problem is in the HVAC equipment.

Colorado specifics

• Heat pumps and dual-fuel systems. Smart thermostats have outdoor temperature lockouts and auxiliary heat settings. If they're set wrong, a heat pump can seem to "not respond" during a cold snap. Our heat pump defrost guide explains what's normal in winter, and heat pump repair covers the rest.
• Strong sun. At Denver's elevation, direct afternoon sun is intense. A thermostat on a sunlit wall can read several degrees high and hold off the heat. Close the blind or ask about relocating it.
• Drafts from the wall. Cold air from an exterior wall cavity can blow through the wire hole behind the thermostat and make it read low. Stuffing a little insulation into the hole (with power off) fixes it.
• Storm outages. Spring and summer thunderstorms cause brief outages. Smart thermostats usually reboot on their own; ecobee may show a calibrating message for a while.
• Mini-splits. Ductless systems usually use their own remotes or controllers, not a wall thermostat. See ductless mini-split service if one isn't responding.

When to call

• The thermostat works on batteries or USB but not from the system.
• A fuse on the furnace control board has blown, or the breaker trips again.
• There's water in the drain pan or a float switch has tripped.
• You'd need to add a C wire or rewire to fix it.
• The settings are right but the furnace, AC or heat pump still won't start.

MrFixer Heating & Cooling offers thermostat troubleshooting, smart thermostat installation and furnace repair in every part of the Denver metro. Reach us at (720) 466-7983, or use our emergency service if the house is getting cold. If the furnace itself is the problem, see furnace won't turn on; for cooling, see AC not cooling.

Frequently asked questions

Why is my thermostat screen blank?

Usually dead batteries or no power from the HVAC system: a tripped breaker, the furnace service switch off, an unlatched furnace door, or a tripped condensate float switch. Less often, a blown low-voltage fuse or a failed thermostat.

How do I reset a Honeywell thermostat that's blank?

Replace the batteries first. On wired models, try batteries; if it only works on batteries, check the breaker, furnace switch and furnace door. If the system has power and the thermostat still only runs on batteries, call a technician.

Why won't my Nest thermostat turn on?

The battery may be drained because the system isn't supplying enough power. Reseat the display, check the breaker, then charge it from a wall USB charger. If it works on USB but dies on the wall, have the wiring and furnace checked.

Does an ecobee need a C wire?

It needs continuous power. If your home has no C wire, ecobee supplies a Power Extender Kit that installs at the furnace. A loose PEK or C-wire connection is a common cause of an ecobee going blank or rebooting.

Can a clogged furnace filter make the thermostat stop working?

Indirectly, yes. A clogged filter can make the furnace overheat and shut off. Google notes this can stop a Nest from charging. Replace the filter and see whether the problem clears.

Is a blank thermostat an emergency?

Not by itself, but with no thermostat the furnace won't run. In freezing weather, call for service if the basic checks don't fix it. If it's hot, smells burnt or the breaker keeps tripping, turn the breaker off and call right away.