Heat Pump Is Not Heating? What Usually Causes It

Settings and airflow come before component faults

If the fan is blowing but your rooms still feel chilly, we'd check the thermostat first because continuous fan operation can circulate air while the heat pump isn't actively heating. That setting can explain why you feel cool airflow even with heating selected.

Carrier names incorrect thermostat settings, a tripped circuit breaker and a dirty air filter as its most common causes. Settings and filter condition belong first because they're homeowner checks. Constant cold air or a frozen outdoor unit brings icing, refrigerant and reversing-valve problems into consideration.

Lukewarm air can still be heating your home

A heat pump delivers a steady stream of warm air that can feel surprisingly mild if you're used to a gas furnace. In heating operation, an air-source heat pump brings heat from outdoors into your home, with refrigerant carrying that heat through the system.

Carrier's US heating guidance (opens in a new tab) gives a typical supply-air range of about 85–100°F, compared with roughly 120–140°F from a gas furnace. Carrier also gives a narrower estimate of 90–100°F when describing normal lukewarm output.

Trane's heating guidance (opens in a new tab) gives a different usual range, 85–93°F. It explains that this air can feel cool against an average body temperature of 98.6°F.

Those estimates describe expected warmth, not a universal pass-or-fail temperature for every heat pump. Keep the room's heating complaint separate from how the register feels: lukewarm output can be normal, while a system that can't keep up with heating demand still needs attention.

Three thermostat settings worth checking

Look at the operating mode, requested temperature and fan setting separately. Each answers a different question: whether you've selected heating, whether you're asking for a temperature above the room's current temperature, and whether the fan is running continuously.

Select Heat if the thermostat is still set to Cool. Then compare the heating setpoint with the displayed room temperature and set it above the current room temperature, as described in Lennox's thermostat checks (opens in a new tab).

Set the fan to Auto instead of On. Trane's fan-setting guidance (opens in a new tab) explains that On keeps air moving even when the heat pump isn't actively heating. Heat mode and the fan setting do different jobs, so check both.

After correcting the settings, check whether the cool-air complaint has cleared. If it hasn't, you can seek professional help at this point. You don't have to work through every remaining check before calling.

A dirty filter can interrupt heating

Find and inspect the air filter, usually in or near the indoor unit. A clogged filter restricts airflow and can leave you with lukewarm air. In more severe cases, airflow can become blocked or the system can shut down.

If it's dirty, clean or replace it according to the manufacturer's recommendations. Carrier's filter guidance (opens in a new tab) ties that work to the manufacturer's directions, so follow your filter's instructions for the actual cleaning or replacement.

The filter matters even when the equipment has stopped altogether. Trane explains that restricted airflow can make a heat pump struggle to heat, overheat and shut down. That makes filter condition a useful check before treating a stopped system as a failed component.

A brief cool spell may be defrost

During cold weather, heat pumps periodically enter defrost mode to remove frost from the outdoor coil. That normal process can temporarily send cooler air indoors, so a brief change in register temperature has a different meaning from cold air that continues.

For a Trane example, if your thermostat displays Defrost, Trane's instructions (opens in a new tab) say to wait for the cycle to finish. Heating should resume once the unit leaves that mode.

Carrier describes a typical defrost cycle as lasting about 5–10 minutes. Watch for frost clearing and heating returning as the cycle ends. The distinction that matters is whether the outdoor unit sheds its frost or stays heavily iced over.

Carrier example with frost and persistent ice

Visually inspect the outdoor coil and the copper tubing between the unit and your house for frost or ice. Ice buildup interferes with the system's ability to transfer outdoor heat into your home, making this observation useful when the equipment runs but the house stays cold.

For a worked example, Carrier's US consumer guidance for Carrier heat pumps (opens in a new tab) describes light frost generally less than 1/4 inch thick as normal when it clears during defrost. It identifies ice thicker than 1/4 inch, or ice remaining after defrost, as abnormal.

In that Carrier example, persistent ice can point to restricted airflow, low refrigerant or a defrost-system problem. Keep this an observation check and have an HVAC professional inspect excessive ice, heavy ice remaining after a cycle, or defrost that happens unusually often. Leave the remedy for a frozen outdoor coil to the technician.

What the remaining symptoms can mean

A unit that isn't operating can have a power problem, while one that runs without meeting heating demand can have a heat-transfer problem. A tripped breaker can cut power, and electrical faults or failed components can also prevent operation. Electrical component testing and repair require a qualified professional.

Low refrigerant can keep the system from meeting heating demand and can accompany a frozen coil. If you suspect this on a Carrier system, contact a Carrier dealer. Carrier's Canadian service guidance (opens in a new tab) assigns charge checks, leak repairs and recharging to the dealer, who has the equipment for that work.

Another useful clue is that the heat pump worked properly in cooling mode but now won't heat. The reversing valve changes the direction of refrigerant flow, and a stuck or failed valve can cause that pattern. For Carrier equipment, its US guidance calls for a local Carrier expert (opens in a new tab) to handle this possible fault.

What to tell the HVAC technician

If the basic checks haven't restored heating, book an HVAC service visit. You can also call without doing the checks if you're uncomfortable carrying them out. For Carrier equipment, the recommended destination is a qualified Carrier HVAC dealer.

Leave internal component repairs to the technician. Lennox's repair warning (opens in a new tab) explains that attempting those repairs yourself can be dangerous, can void your warranty and can increase repair costs. Technicians have the tools to test electrical connections and inspect motors and capacitors safely.

When you contact the technician, describe what you observed: whether the unit runs, whether the air stays cold or changes briefly during defrost, what the thermostat showed, the filter's condition and whether outdoor ice remained. If cooling worked before the heating complaint began, mention that too.