Set your thermostat to Heat mode and raise the setpoint at least 3-4°F above the current room temperature. That single step resolves about 30% of “no heat” calls before any tools are involved.
If the heat still won’t turn on, work through the checklist below in order. Start at the thermostat, move to the indoor unit, then the outdoor unit.
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Step 1: Thermostat checks (2 minutes, do these first)
1. Mode = Heat. Not Auto, not Cool, not Off. Auto mode will switch to cooling on a warm afternoon and won’t call for heat again until the room drops far enough.
2. Setpoint above current room temp by 3-4°F. If the room reads 68°F, set to 72°F. A 1°F difference rarely generates a strong enough demand signal to start the system.
3. Confirm the thermostat is calling for heat. Most thermostats show a flame icon or “Heat On” on the display. On Nest: the screen turns orange when calling. On ecobee: the status line reads “Heating.” You can also check the wiring diagnostic screen on most smart thermostats, where the W terminal (heat call) should show as active.
4. Replace the batteries. Dead thermostat batteries are a leading winter no-heat cause. Some thermostats show a low-battery warning. Others just go blank or freeze mid-call. Replace them now: typically 2x AA or 2x AAA. Takes 90 seconds.
5. Wait out a compressor delay. Heat pumps and most smart thermostats impose a 5-minute delay after a mode switch to prevent short-cycling. Nest shows “Heating in 5:00” and counts down. Ecobee briefly shows “Calibrating” after a mode change. If you see either message, wait before assuming there’s a fault.
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Step 2: Air handler and indoor unit
6. Pull the air filter. A clogged filter blocks airflow and trips the high-limit switch on a furnace, which shuts down the heating cycle as a protective measure. Hold the filter up to a light. If you cannot see light through it, replace it before continuing. A fresh filter costs $5-20 and fixes more “no heat” calls than people expect.
7. Check the air handler breaker. Find the air handler’s dedicated breaker in your main panel, typically a 15-20A single-pole breaker. If it’s tripped, reset it once. If it trips again immediately, stop. Do not reset a repeatedly tripping breaker.
Line voltage warning: If the breaker trips a second time, do not open the air handler’s electrical compartment to inspect wiring. 120V/240V work at the air handler requires a licensed HVAC technician.
8. Check the indoor unit service switch. Most air handlers have a dedicated power disconnect on the unit itself. It looks like a standard light switch and is usually mounted on or near the air handler cabinet. Confirm it’s in the On position.
9. Read the furnace or air handler status light. Most units have a small LED inside or visible through a sight glass in the access panel. It blinks error codes. Count the blink pattern and match it to the legend on the label inside the access panel door. Common codes on gas furnaces:
- 3 blinks: pressure switch fault (check drain line or inducer motor)
- 4 blinks: high-limit switch tripped (usually a dirty filter or blocked return)
- 5 blinks: flame sensor fault (requires cleaning or replacement by a tech)
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Gas safety. Read before opening the furnace cabinet.
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If you open the furnace access panel and smell gas: close the panel immediately. Leave the house. Call your gas utility’s 24-hour emergency line. Do not flip any light switches, do not use a cell phone inside the house, and do not attempt to relight a pilot. Gas diagnosis and repair require a licensed technician.
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Step 3: Outdoor unit (heat pumps only)
Skip this section if you have a gas furnace with no outdoor unit.
10. Check the outdoor unit breaker. Heat pump outdoor units run on 240V and typically have a dedicated 30-50A double-pole breaker. Check the main panel. Reset once if tripped. If it trips again, stop and call.
11. Check the outdoor disconnect box. There’s usually a pull-out fuse block or lockable switch mounted within a few feet of the outdoor unit. Pull the block out and inspect the fuses visually. A blown fuse often has a visible gap or dark interior.
12. Is the outdoor unit running? In Heat mode, the outdoor unit should be running: fan spinning, refrigerant moving (you’ll hear a low hum or flow sound). A unit that is completely silent while the thermostat is calling for heat points to an electrical fault, a refrigerant low-pressure lockout, or a failed compressor. None of those are thermostat problems.
13. Ice on the outdoor unit. A light frost coating on the coil during cold weather is normal. Thick ice covering the coil or the refrigerant lines indicates a defrost system failure or low refrigerant charge. Do not chip ice off the coil. The defrost cycle handles normal frost automatically.
14. AUX heat showing on the thermostat. If the thermostat displays “AUX” or “Aux Heat” and warm air is coming out of the vents, the system is working correctly. At low outdoor temps, the heat pump hands off to electric resistance strips for backup heat. That is normal heat pump behavior, not a fault.
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Step 4: Brand-specific gotchas
Nest on a heat pump. If Nest was recently installed on a heat pump and blows cold air in Heat mode, check the O/B wire setting. In Nest’s installer menu: Settings > Equipment > Heat Pump > O/B. Most systems use O (the reversing valve energizes in Cool mode). Rheem and Ruud systems use B (the valve energizes in Heat mode). Setting this wrong inverts cooling and heating.
Ecobee compressor lockout threshold. Ecobee’s Thresholds setting includes a “Compressor Min Outdoor Temperature,” which defaults to 35°F. Below that temperature, ecobee locks out the compressor and calls for AUX heat instead. If the backup strips are not wired, undersized, or offline, heating stops below that threshold. Adjust in: Main Menu > Settings > Installation Settings > Thresholds > Compressor Min Outdoor Temp.
Honeywell/Resideo T6/T9/T10 series. After install, confirm the equipment type was configured correctly in Installer Setup. Access it by holding Menu and the down arrow simultaneously. Navigate to System Type and confirm it’s set to Heat Only or Heat Pump as appropriate. A misconfigured system type causes incorrect staging or no call at all.
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When to stop and call a pro
- Gas smell at any point during diagnosis
- Breaker trips a second time after reset
- Outdoor unit is completely silent with power confirmed and thermostat calling for heat
- Thick ice on outdoor unit coil or refrigerant lines
- Furnace error code that indicates a combustion or refrigerant fault
- Any wiring inspection at the 240V disconnect or inside the air handler electrical compartment
The low-voltage control wiring (24V AC) at the thermostat is safe to inspect. The line-voltage side of the air handler or the outdoor unit is not.
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Related
Sources & verification
✓ Verified against a manufacturer or standards source
- A clogged air filter restricts airflow and can trip the furnace high-limit switch, shutting down the heating cycle — a dirty filter is one of the most common causes of heat not turning on — standard HVAC service call diagnostic documentation
- Heat pump low-pressure lockout: if outdoor temp drops below compressor lockout threshold and AUX/backup strips are not wired or sized correctly, thermostat will call for heat with no output — manufacturer service documentation
⚠ Provisional — industry-average estimate, not a direct manufacturer/spec citation
- Specific lockout temps and error codes vary by equipment brand and age
