Search guides

Fix a thermostat that will not turn on heat

We'll check the mode, temperature, batteries, wiring, and furnace—or tell you when to call a pro.

Troubleshooting · Home appliances
10–30 min
What you'll need
  • Fresh batteries (AA or AAA, check your thermostat)
  • Replacement furnace filter (if dirty)
  • Screwdriver (for thermostat cover, if needed)
Step 1 of 6

Quick triage — pick your path

Get started

Choose the option that matches what you see. You can jump straight to that section.

Show full guide

Steps

Goal: Confirm the thermostat is set correctly, then rule out batteries, power, and furnace.

  • Confirm the mode is Heat and the set temperature is above room temperature.
  • Good: Settings are correct. Proceed to Check batteries.
  • Bad: Wrong mode or temp—fix and retest.

Check settings and batteries

Goal: Rule out mode, temperature, and dead batteries.

  • Replace the thermostat batteries with fresh ones. Confirm the display lights up and responds.
  • Good: Batteries are good. Proceed to Check furnace.
  • Bad: Display still blank—try new batteries; if still blank, call a technician.

Check furnace

Goal: Rule out furnace power and filter.

  • Check the furnace circuit breaker. Reset if tripped. If it trips again, call a technician.
  • Replace the furnace filter if dirty.
  • Check that the furnace has power (switch on, door closed).
  • Good: Breaker on, filter clean, furnace powered. Proceed to Thermostat click.
  • Bad: Breaker trips or filter severely clogged—call a technician.

Thermostat click

Goal: Confirm the thermostat sends the heat signal.

  • Raise the set temperature above room temp. Listen for a click from the thermostat—that is the relay closing.
  • If no click, the thermostat may not be sending the signal. Turn off power at the furnace breaker. Remove the thermostat cover. Check that the wire on the W terminal is secure. Do not touch wiring if unsure—call a technician.
  • If the thermostat clicks but the furnace does not start, the fault is at the furnace—call a technician.

When to get help

Call an HVAC technician if:

  • The mode and temperature are correct, batteries are new, the furnace breaker is on, the filter is clean, and the heat still does not run.
  • The thermostat is blank or unresponsive.
  • You see damaged wiring.
  • The furnace makes unusual noises.
  • You smell gas—evacuate and call 911 or your gas utility from outside.

Do not work on high-voltage wiring or gas equipment yourself.

Verification

  • The thermostat display shows Heat mode and the set temperature.
  • The furnace runs when the set temp is above room temp.
  • Warm air flows from the vents.
  • No unusual noises or gas smell.

Escalation ladder

Work from the device outward. Stop when the problem is fixed.

  1. Mode and temperature Confirm Heat mode and set temp above room temp.
  2. Batteries Replace thermostat batteries.
  3. Furnace power and filter Check furnace breaker and filter.
  4. Thermostat wiring Check wires at thermostat (power off). W terminal for heat.
  5. Call a pro Wiring fault, furnace not responding, or unsure—call an HVAC technician.

What to capture if you need help

Before calling support or posting for help, have these ready. It speeds everything up.

  • Thermostat mode and set temperature
  • Battery condition
  • Furnace breaker and filter status
  • Whether thermostat clicks when calling for heat
  • Steps already tried

Is the thermostat set to Heat with temp above room temp?

Mode must be Heat. Set temperature must be above room temp to call for heat.

Confirm the mode is Heat (not Off, Cool, or Auto). Confirm the set temperature is above the current room temperature. Good: Heat mode, temp above room. Bad: wrong mode or temp—fix and retest.

Yes No

You can change your answer later.

Are the batteries good?

Dead batteries can cause blank screen or wrong commands.

Replace the thermostat batteries with fresh ones. Good: display lights up and responds. Bad: still blank or wrong—may be thermostat fault; try batteries first, then call a pro.

Yes No

You can change your answer later.

Is the furnace breaker on and filter clean?

Tripped breaker or clogged filter can prevent heat.

Check the furnace circuit breaker—reset if tripped. Replace the furnace filter if dirty. Good: breaker on, filter clean. Bad: breaker trips again or filter severely clogged—call a pro.

Yes No

You can change your answer later.

Do you hear the thermostat click when calling for heat?

The relay closes when heat is requested. You should hear a click.

Raise the set temperature above room temp. Listen for a click from the thermostat—that is the relay closing. No click: thermostat may not be sending signal—check W terminal wiring (power off) or call a pro. Click: furnace should respond—if not, furnace fault; call a pro.

Yes No

You can change your answer later.

Does the furnace start when the thermostat clicks?

Furnace should start—igniter glows (gas) or elements engage (electric).

When the thermostat clicks, the furnace should start. Gas: igniter glows, burner lights. Electric: elements engage. Starts: heat should flow. Does not start: furnace fault—call a pro. If you smell gas, evacuate and call 911 or gas utility from outside.
Question

Does the furnace start when the thermostat clicks?

Done

Call a technician

Call an HVAC technician if: mode and temp are correct, batteries are new, furnace breaker is on, filter is clean, and heat still does not run; the thermostat is blank or unresponsive; you see damaged wiring; or you smell gas. Do not work on high-voltage wiring or gas equipment yourself.

Done

Reviewed by Blackbox Atlas

Frequently asked questions

Why would a thermostat not turn on the heat?
Common causes: mode set to Off or Cool, set temperature below room temp, dead batteries, tripped furnace breaker, clogged furnace filter, or faulty wiring. Check mode, temp, and batteries first, then power and filter.
Can I fix a thermostat that will not turn on heat myself?
Yes, for mode, temperature, batteries, and filter. Do not touch thermostat or furnace wiring unless you have experience. Call an HVAC technician for wiring issues, furnace faults, or if you are unsure.
When should I call a technician for a thermostat heat issue?
Call a technician if the mode and temperature are correct, batteries are new, the furnace breaker is on, and the heat still does not run. Call if the thermostat is blank or unresponsive, you see damaged wiring, or the furnace makes unusual noises.

Rate this guide

Was this helpful?

Continue to