Heat Pump Not Heating? Troubleshooting Before You Call a Pro

If your heat pump is running but the house is getting colder, the fix is often something you can check in ten minutes: a thermostat set wrong, a clogged filter, or an outdoor unit buried in ice or snow. Before you pay for a service call, work through the basics below. A heat pump that briefly blows cool air during a defrost cycle is usually normal; a unit that never delivers warm air points to a real problem worth diagnosing.

First, know what “normal” feels like

Heat pumps do not heat like a furnace. A gas furnace blasts air in the 120F to 140F range, so it feels hot on your hand. A heat pump moves heat instead of burning fuel, so its supply air is usually 90F to 105F: warmer than your skin, but not hot. Many “not heating” complaints are really “not as hot as my old furnace,” which is expected behavior, not a fault.

You should also expect the outdoor unit to pause and steam a few times an hour in cold, damp weather. That is the defrost cycle clearing frost off the outdoor coil. During defrost the system may blow cooler air for a minute or two, and electric auxiliary heat often kicks in to cover the gap. Frost on the outdoor coil in winter is normal. A solid block of ice that never clears is not.

The ten-minute checklist before you call anyone

Run through these in order. Each is free, safe, and fixes a large share of “no heat” calls.

1. Check the thermostat mode and setpoint. It should be on Heat, with the target several degrees above room temperature. If it reads “Emergency Heat” or “Aux,” someone may have switched it manually. See our guide to heat pump thermostat settings for what each mode does.
2. Confirm the fan setting. Fan set to “On” runs the blower constantly, so between heating cycles you feel room-temperature air and think it is blowing cold. Set the fan to “Auto.”
3. Replace or clean the air filter. A clogged filter starves airflow, trips safety limits, and can freeze the indoor coil. If you cannot remember the last change, change it now.
4. Look at the outdoor unit. Clear snow, leaves, and ice away from it. It needs open airflow on all sides and clearance above. A unit packed in snow cannot pull heat from outside air.
5. Check your breakers. Heat pumps often run on two circuits: one for the outdoor condenser and one for the indoor air handler or backup strips. A tripped breaker on either one can leave you cold.
6. Reset the system. Turn the thermostat off, wait five minutes, then turn it back to Heat. This lets the compressor’s internal timers settle.

Heat pump blowing cold air: what it usually means

“Blowing cold air” has a few common causes, and they range from harmless to a real repair.

It is in defrost (harmless)

If the cool air lasts a minute or two and the outdoor unit is steaming, that is defrost doing its job. Wait it out.

The fan is set to “On” (harmless)

As above, constant fan means you feel unconditioned air between cycles. Switch to Auto and the problem disappears.

The reversing valve is stuck (real repair)

A heat pump switches between heating and cooling with a reversing valve. If it sticks, your system may be running in cooling mode while you are asking for heat. That is a technician’s job.

Low refrigerant charge (real repair)

If the refrigerant charge is low, usually from a leak, the system cannot move enough heat and airflow feels weak and cool. Refrigerant is not “used up” in normal operation, so low charge means a leak that needs finding and sealing, not a simple top-off.

The outdoor unit is iced over

A thin layer of frost is normal and defrost handles it. A thick, persistent block of ice is a warning sign. Common causes include a failed defrost control, a stuck outdoor fan, low refrigerant, or blocked airflow around the unit. As a short-term step you can switch the thermostat to defrost or run the fan-only mode to help melt light ice, and clear away anything blocking airflow. Never chip at ice with a tool or pour hot water over the coil; you can puncture the coil or bend fins and turn a small problem into a major one.

When only lukewarm air comes out

If air is moving but never gets properly warm, the issue is often the balance between the heat pump and its backup. In very cold weather the heat pump alone may not keep up, and the system leans on auxiliary heat. If the backup strips or the dual-fuel furnace are not engaging, you get lukewarm air on the coldest days.

Sizing matters here too. An undersized system, or one paired with the wrong backup, can struggle in a cold snap even when nothing is broken. If your home has always felt underheated, the real answer is a load calculation, not a part swap. Read what size heat pump you need and why a Manual J load calc beats a rule of thumb. Cold-climate performance also depends on the model itself: our guide on whether heat pumps work in cold climates covers what ratings to look for.

Lukewarm air on the coldest day of the year is usually a backup-heat or sizing story, not a broken compressor.

Common causes at a glance

* Reset a tripped breaker only once. If it trips again immediately, leave it off and get it inspected, because a repeated trip signals an electrical fault.

When to stop troubleshooting and call a pro

Some symptoms mean the DIY phase is over. Call a licensed technician if you notice any of these:

• Burning, electrical, or hot-plastic smells from the vents or unit.
• Repeated breaker trips after a single reset.
• Ice that reforms within hours of clearing it.
• Hissing or bubbling near the outdoor unit, a sign of a refrigerant leak.
• Loud grinding, buzzing, or a compressor that hums but will not start.
• No heat at all after you have checked the thermostat, filter, and breakers.

Knowing what a repair is likely to cost helps you judge a quote and decide whether to fix or replace. See our breakdown of common heat pump repairs and prices, and if the system is aging, weigh it against how long heat pumps last before you sink money into an old unit. Getting on a regular maintenance schedule prevents many of these calls in the first place.

The bottom line

Most “heat pump not heating” problems trace back to four things: the thermostat, the filter, airflow around the outdoor unit, and the breakers. Work through the checklist, give the system time to recover, and reset the expectation that heat pump air is warm rather than furnace-hot. If you have cleared the basics and still get cold or lukewarm air, or you smell something burning or hear grinding, that is your cue to bring in a professional. Spending ten minutes on the simple checks first means you either fix it for free or walk into the service call knowing exactly what to describe.

Frequently asked questions