When your heater won’t turn on but AC works, check power, thermostat heat mode, safety switches, and airflow before calling a pro.
If the air conditioner cools fine but the house stays cold in heat mode, you already know something: the system has power and the blower can move air. That narrows the hunt. The heat side uses extra parts—ignition, gas flow or heat strips, and safety switches—that the cooling side does not. This guide walks you through clear checks you can do in minutes, when to stop for safety, and how to talk to a technician with confidence.
Heater Won’t Turn On But AC Works: Quick Checks
Run through these quick wins first. Most take less time than a coffee break and solve many no-heat calls.
- Set the thermostat to Heat and raise the setpoint 3–5°F. Make sure “Auto” runs the fan only when heating starts; “On” runs the fan all the time and can mask a heat failure.
- Confirm power: furnace or air handler switch on, breaker on, and any GFCI or service switch not tripped.
- Replace the air filter if it looks dirty or older than 1–3 months. A starved furnace can trip a limit switch and shut the burners.
- Check the front panel door on the furnace. A jarred door switch kills power to the burner circuit.
- Look for a wet condensate pan or a tripped float switch on high-efficiency furnaces and some air handlers. A full pan stops heat to prevent water damage.
- Gas only: confirm the gas cock is in line with the pipe. No gas, no flame.
- Heat pump: switch to “Emergency Heat.” If strips warm the house, the outdoor unit likely needs service.
Fast Symptom Map
Use this table to translate what you see into a likely path.
| Symptom | Likely Cause | What To Try |
|---|---|---|
| Fan runs, air is cold | Burner or heat strip not lighting | Thermostat mode, ignitor, gas, or strip breaker |
| Short starts, then shutdown | Flame sensor or pressure switch opens | New filter, clear vents, call for sensor check |
| No fan in Heat, fan OK in Cool | Thermostat wiring or heat relay | Reseat thermostat, check Heat call wire (W) |
| Clicks but no burner | Ignitor cracked or gas off | Gas valve handle, pro ignitor test |
| Nothing at all in Heat | Furnace switch, door switch, or fuse | Reset switch, close door, replace low-voltage fuse |
| Outdoor unit runs, still cool air | Heat pump valve stuck | Set to Emergency Heat and schedule service |
| Water near furnace | Condensate blockage | Empty pan, clear drain, float switch reset |
| Burner lights then quits | Dirty flame sensor | Call a tech to clean/replace |
How Cooling And Heating Share Parts—And Where They Split
Central AC is a split system with an outdoor condenser and an indoor coil and blower. The blower and thermostat serve both modes, while cooling adds a compressor outdoors and heating adds burners or electric heat. The shared blower is why air still moves when heat fails. The extra heat-side parts explain why cooling can run while heat stalls. You can read a simple overview of split systems on the U.S. Department of Energy’s central air conditioning page.
Thermostat And Power Checks That Rule Out Simple Stuff
Thermostat Steps
Pop the faceplate off, if it’s a slide-off style, and check for fresh batteries. Re-seat the plate to the base so the pins make contact. Verify Heat mode, then bump the setpoint up. If your fan runs in Cool but not in Heat, the W call from the thermostat may not be landing. Many models label the heat call wire “W” or “W1.”
Power Path
Find the light switch near the furnace and leave it on. Check the service panel for a tripped breaker. Some air handlers include a fuse on the low-voltage board; if it pops, nothing responds in Heat. Also close the furnace door fully so the safety switch engages.
Safety First Around Combustion
Gas furnaces make heat by burning fuel. That process sits behind multiple safeties: pressure switch to prove venting, flame sensor to prove a flame, and limit switches to prevent overheating. If any one opens, the board stops the heat cycle. Because combustion brings carbon monoxide risk, keep CO alarms powered and never bypass a safety. The CDC explains why CO alarms matter on its carbon monoxide basics page.
Gas Furnace: Heat Fails While AC Still Works
Dirty Flame Sensor
A flame sensor is a small metal rod in the burner flame. If it’s coated, the control board reads “no flame” and shuts gas within seconds. Cooling still runs because it doesn’t rely on that signal. Cleaning needs a gentle touch and should be done with the burner cool; many homeowners leave this to a pro.
Cracked Or Weak Hot Surface Ignitor
Hot surface ignitors get brittle with age. A cracked element won’t glow, so the burner never lights. You may hear clicks and smell no gas. A tech can ohm-test the part and match the correct replacement.
Closed Gas Valve Or Empty Tank
Hand valves can get bumped during filter changes. On a straight-through style, the handle should line up with the pipe. For LP tanks, check the gauge; for natural gas, call the utility if you suspect a supply issue.
Tripped Rollout Or Limit Switch
Poor airflow or a blocked flue can overheat the heat exchanger. The limit switch opens and the board locks out heat until the cause clears and the switch resets. Many furnaces need a manual reset by a technician after a rollout trip.
Condensate Float Switch On High-Efficiency Units
Condensing furnaces make water. A clogged drain or full pan raises a float switch that kills heat to prevent a spill. Clear the trap and hose, empty the pan, and the switch will allow a new cycle.
Heat Pump: Cooling Fine, Heat Not Engaging
Heat pumps move heat with a reversing valve. In heat mode, the outdoor unit becomes the evaporator for outside air. If the valve sticks or a control fault keeps it in the cooling path, you’ll get fan airflow with no warmth. Switch to Emergency Heat so the electric strips carry the load and book service.
Outdoor Unit Locked Out
Many heat pumps protect themselves with defrost and low-temperature logic. A sensor fault or low charge can trigger a lockout. The indoor blower will still run if the thermostat calls for heat. A pro can read fault codes and check pressures.
Auxiliary Heat Off Or Tripped
Electric strip heaters often have their own breaker or reset. If strips don’t energize, heat output drops fast in cold weather. Check the sub-panel near the air handler and any high-limit reset on the element cabinet.
Vent And Airflow Checks That Save Service Calls
Filters are the big one, but vents matter too. Open at least a few supply registers in each room and clear return grilles. Look for crushed flex duct near the air handler. On 90%+ furnaces, check the intake and exhaust pipes outdoors; leaves, snow, or a bird screen can block flow and stop the pressure switch from closing.
Diagnostic Flow You Can Follow
Move step by step. If you hit a safety issue or smell gas, stop and call right away.
| Step | Check | Action If Failed |
|---|---|---|
| 1 | Thermostat on Heat, setpoint above room | Set mode to Heat, raise setpoint |
| 2 | Fan Auto vs On | Use Auto to avoid false “air only” feel |
| 3 | Furnace/air handler switch on | Turn on service switch |
| 4 | Main breaker and any fuses | Reset breaker, replace low-voltage fuse |
| 5 | Filter clean and doors shut | Change filter, seat doors |
| 6 | Condensate pan and drain clear | Empty pan, clear trap, reset float |
| 7 | Gas cock open (gas units) | Open valve; if unsure, call utility |
| 8 | Heat pump Emergency Heat test | If strips heat, schedule outdoor unit service |
When To Call A Technician
Stop and call for help if you smell gas, hear repeated ignition clicks with no light-off, see water pooling, or the furnace locks out and flashes a fault code. Share what you checked, any code you saw, filter age, and whether Emergency Heat produced warmth. That cuts time and avoids repeat visits.
Prevent Moves That Keep Heat Ready
Change Filters On Time
Match filter size to the cabinet and keep spares on a shelf. Many homes do well with a 1–3 month swap. A clean filter protects the blower and helps limit switches stay closed.
Clear Vents And Drains
Vacuum returns, open supply registers, and sweep the area around the furnace. Pour a cup of warm water and a drop of soap into the condensate trap at the start of the season.
Test Thermostat Before First Cold Snap
Run Heat for ten minutes on a mild day. You’ll find weak ignitors or bad strips when a fix is easier to schedule.
Schedule Annual Service
A tune-up checks combustion, safeties, and wiring. Pros clean burners, verify gas pressure, and test heat rise. This keeps heat steady without wasting energy.