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AC works but there is no heat in the house

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Cooling fine and no heat usually points to the heating side of the system, not the whole thing. Start by working out what kind of system you own.

If the AC cools well but the house will not warm up, the cooling half is healthy and the heating half is not. What that means depends on your equipment. A heat pump uses one system for both jobs, while an AC paired with a gas furnace has two separate heat sources.

First, find out what you own

Look at the outdoor unit's label. A heat pump says so. A plain air conditioner does not. If you have an AC outside and a furnace in the basement, closet or attic, heat comes from the furnace. So the AC working tells you nothing about it.

Check the easy things first

Most of these take two minutes and cost nothing.

  • Set the thermostat to Heat and raise the setpoint at least five degrees above the room temperature.
  • Check the thermostat batteries if the display is dim or blank.
  • Replace a clogged air filter. A blocked filter can shut a furnace down on its safety limit.
  • Confirm the furnace switch near the unit is on, and that the breaker has not tripped. Reset a breaker once only.

If you have a heat pump

A heat pump that cools but will not heat often has a reversing valve that is stuck in cooling mode, or a mode problem at the thermostat. In cold weather, ice on the outdoor coil can also block heating. A short, steamy defrost cycle is normal. A coil that stays encased in ice is not. Also note whether the strip heat or backup heat comes on at all.

If you have an AC and a gas furnace

Look at whether the furnace tries to start. A furnace that clicks, hums or runs the blower with no flame points to ignition, a flame sensor or a gas supply problem. Do not take panels off or work on the gas line. If you smell gas or a carbon monoxide detector sounds, leave the house and call the gas company or 911 from outside.

Do not try these

  • Do not open the outdoor unit or test any capacitor.
  • Do not add refrigerant. EPA Section 608 certification is required by federal law.
  • Do not run space heaters all night as a workaround.

Our advice: do not wait for a freeze

Heating failures get urgent on the first cold night. Run through the checks above, write down your system type and what the unit does when it calls for heat, then book a licensed HVAC technician before the cold arrives. A tune-up call now beats an emergency call later.

Frequently asked questions

Why does my AC work but not my heat?

The cooling side is fine and the heating side is not. On a heat pump that often means a stuck reversing valve or a thermostat mode issue. With a furnace, it usually means an ignition, sensor or gas supply fault.

Can a dirty filter stop the heat from working?

Yes. A badly clogged filter restricts airflow and can make a furnace shut off on its safety limit. Replacing it is a safe first check.

When should I call a technician?

Call a licensed HVAC technician if the basic checks do not restore heat, if the outdoor coil stays iced over, or if the furnace will not start. If you smell gas or a CO alarm sounds, leave and call from outside.

Sources

What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.

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