When the thermostat says heat but the furnace won't start, first find out whether the furnace is getting the signal. A thermostat doesn't make heat. It only tells the furnace to start. A dead battery, a blank screen, a schedule or a setting can stop that signal. If the signal is going out and the furnace still does nothing, the problem is past the thermostat.
Safety first: If a CO alarm goes off, leave the home now and call 911 from outside. If you smell gas, leave and, from outside, call 911 and Enbridge Gas's emergency line at 1-866-763-5427. On propane, call 911 and your propane supplier. Don't stop to check the thermostat.
How can I tell if my thermostat is calling for heat?
Look at the display first. Many thermostats show "Heat On", a flame icon or a similar symbol when they're asking the furnace for heat. On some models, a flashing symbol means the thermostat is waiting a few minutes before it starts. Some thermostats also click softly when they switch on. Then listen at the furnace. Shortly after a real call for heat, many gas furnaces start a small fan.
Three signs to check, in order
- The display: set the mode to heat and the temperature a few degrees above the room. Wait for "Heat On" or a flame icon.
- The thermostat: listen for a soft click. Not every model makes one.
- The furnace: within a few minutes, listen for a fan starting or any sound at all.
If the display never shows a call for heat, focus on the thermostat. If it does and the furnace stays silent, the signal or the furnace is the problem.
Why is my thermostat screen blank or dim?
It depends on how your thermostat is powered. A battery thermostat goes blank or dim when its batteries run low. Many hardwired and smart thermostats have no batteries at all and get their power from the furnace. For those, a blank screen can mean the furnace itself has lost power.
Battery thermostats
- Replace the batteries with the type listed in the manual or inside the cover.
- Watch for a low-battery icon. Some models stop calling for heat before the screen goes fully blank.
Hardwired and smart thermostats
- Check that the thermostat is pushed firmly onto its wall plate. Many snap or click into place, with no gap.
- If it's seated and still blank, check the furnace's power. Our furnace not working checklist covers the furnace switch, breaker and door in order.
- If the screen keeps going dark or restarting, mention it when you call. Some smart thermostats struggle without a steady power supply from the furnace.
Don't remove wires, swap them or connect terminals together to test the furnace.
Can a schedule, hold, eco or away mode stop the heat?
Yes. A programmable or smart thermostat follows whatever temperature its schedule or mode says right now, not the number you last chose. Eco, away and vacation modes often use much lower temperatures. A permanent hold keeps one temperature until you cancel it. Check which setting is active before deciding the furnace has failed.
What to look for on the screen or app
- Words like "Hold", "Eco", "Away", "Vacation" or "Schedule" next to the temperature
- A lower temperature than you expected
- An away mode that turned on because the app thinks no one is home
To test, set a temporary hold a few degrees above room temperature and wait a few minutes.
What if my Wi-Fi or thermostat app is down?
An app or Wi-Fi outage usually doesn't stop the heat. Many smart thermostats keep following their saved settings when they go offline. You just can't change them from your phone. Go to the thermostat and adjust it there. If the thermostat itself won't respond, treat it like a blank or frozen screen.
If the app says offline
- Change the temperature at the thermostat itself.
- Once Wi-Fi is back, check that the mode and schedule are still what you expect.
Winter storms can knock out power in Barrie and the nearby towns. When power comes back, the furnace and the thermostat both restart. Give them a few minutes before deciding something is wrong.
Could the heat, cool or auto setting be the problem?
It can. In cool or off mode, the thermostat won't call for heat at all. In auto mode, the thermostat switches between heating and cooling on its own, using two target temperatures that usually have to stay a few degrees apart. If the heat target is set low, auto mode may never start the furnace. Many thermostats also wait until the room is slightly below the set temperature before calling.
Settings to check
- Mode: choose heat rather than auto while you test.
- Heat target: make sure the heat number, not the cool number, is above the room temperature.
- Swing or differential: this setting controls how far the room can drop before heat starts. A large value means a longer wait. Leave advanced settings alone if you're not sure what they do.
What if the problem started after a new thermostat was installed?
Think about what changed. A new thermostat has to be set up for the right type of equipment, like a gas furnace rather than a heat pump or electric heat. If it's set up for the wrong type, it may call for the wrong thing or nothing at all. If you had the same problem with the old thermostat, the furnace is the more likely cause.
Before you call
- Check the setup guide for an equipment type setting, and see whether it matches a furnace.
- Make sure the thermostat is seated on its wall plate.
- Don't rewire it, move wires to other terminals or bridge terminals to force heat.
- Have photos of the original wiring ready if you took any.
How do I know the problem is past the thermostat?
If the display shows a call for heat, the settings are right and the furnace still won't start, look past the thermostat. A furnace that stays completely silent points to power, a door switch or a lockout. A fan that hums with no flame points to ignition. Either way, the thermostat has done its job.
Where to go next
- Furnace silent: work through the power and door checks in our guide for a furnace that won't turn on at night. Those checks apply any time of day.
- Fan runs, no flame: see why a furnace won't ignite.
- House getting cold: call 24/7 if it's below freezing outside or anyone in the home is vulnerable to cold.
Our emergency furnace repair page explains what happens when you call. Furnace Repair Barrie answers 24/7 at (249) 209-2216.