Heat Pump Not Heating? 10 Causes, From Free Fixes to Service Calls

Here’s the part that catches almost everyone the first winter.

You put your hand over a supply register, the air feels lukewarm at best, and your gut says the thing is dead. It usually isn’t. A gas furnace dumps air into the ducts at roughly 120°F to 125°F, hot enough to feel like heat. A heat pump moves a much larger volume of gentler air, typically in the 90°F to 110°F range coming off the coil and often 85°F to 95°F by the time it reaches a register at the far end of a duct run. Your skin is about 91°F. So heat pump air can be warming your house and still feel cool on your palm.

That single fact explains a large share of “not heating” complaints. The rest are real, and this guide walks through them in the order a good technician would: cheapest and most likely first, expensive and rare last.

First: is it actually broken?

Run this test before anything else, because it costs nothing and settles the question.

Set the thermostat three degrees above room temperature so the system definitely calls for heat. Let it run for fifteen minutes straight. Then hold a simple cooking thermometer in the airflow at a supply register near the air handler, and take a second reading at a return grille. The difference between the two is your temperature rise.

A healthy air-source heat pump in mild weather usually shows a rise of about 20°F to 30°F over return air, landing somewhere near 85°F to 95°F at the register. In very cold weather that number drops, because the outdoor coil has less heat to harvest. That’s physics, not failure. If you’re seeing a genuine rise and the house is slowly gaining temperature, your system is working — it’s just working the way heat pumps work. Our explainer on how heat pumps work covers why the output changes with outdoor temperature.

Now the flip side. If the temperature rise is close to zero, or the supply air is colder than the room, or the house has been losing ground for hours, something is genuinely wrong. Keep reading.

A heat pump doesn’t make hot air. It makes a lot of warm air. Judge it by the room temperature at the end of the hour, not by how it feels on your hand.

— The single most useful reframe for a first-winter heat pump owner

The 60-second triage table

Find your symptom in the left column. This is the fastest route from “something’s wrong” to “here’s who fixes it.”

Causes 1–4: the free fixes

Do these four in order. They cover the majority of no-heat calls, and none of them require a tool more complicated than a screwdriver.

1. Cause 1: The thermostat is in the wrong mode

   Confirm the mode is set to HEAT, not COOL, not OFF, not AUTO-changeover with a setpoint the system will never reach. Then confirm the fan is on AUTO, not ON. Then set the target at least three degrees above the current room reading so there’s a real call for heat. Also check that a schedule or a vacation hold isn’t overriding you — smart thermostats will happily sit at 62°F all day if someone set that up in July. Finally, if the thermostat screen is blank, replace the batteries.

2. Cause 2: A tripped breaker or an outdoor disconnect that’s off

   A heat pump usually has two or three circuits: one for the outdoor unit, one for the air handler, and often a separate double-pole breaker for electric backup heat. Open the panel and look for a breaker sitting between ON and OFF, or one that feels loose. Reset it by pushing firmly to OFF first, then back to ON. Then walk outside. There’s a gray box on the wall near the outdoor unit — the disconnect. Someone doing yard work or a previous service call may have left it pulled or switched off. If a breaker trips again within a few hours, stop resetting it and call a technician. Repeated tripping means something is drawing too much current, and that’s a fire risk, not an inconvenience.

3. Cause 3: A filthy air filter

   This is the number one cause of “it runs but barely heats.” A loaded filter strangles airflow, which drops your heat output, freezes coils, and eventually trips safety limits. Pull the filter and hold it up to a light. If you can’t see light through it, replace it. Standard 1-inch filters generally need replacing every one to three months; thick media filters run six to twelve months. Ductless mini-split heads have washable filters behind the front panel that want rinsing every two to four weeks in heavy use. Our maintenance checklist has the full schedule.

4. Cause 4: The outdoor unit is blocked

   In heating mode the outdoor unit is your heat source, and it needs to breathe. Walk out and look. Leaves packed against the coil, a snowdrift over the top, shrubs grown in tight, a tarp someone threw over it in November, a dryer vent blowing lint at it — any of these starve it. Clear at least 18 to 24 inches on all sides and several feet above. Pull debris out by hand or with a soft brush, with the power off at the disconnect. Never cover a heat pump for winter; unlike a summer-only air conditioner, it runs all season.

Cause 5: it’s just in defrost

This one gets misread as a breakdown more than any other normal behavior.

When your heat pump runs in cold, damp weather, moisture from the outdoor air freezes onto the outdoor coil. Frost blocks airflow, and blocked airflow kills heating capacity, so the system periodically clears it. It does this by briefly running in reverse — sending hot refrigerant out to the outdoor coil to melt the frost off.

From the outside, a defrost cycle looks alarming. The outdoor fan stops. The compressor keeps running and may get noticeably louder. A cloud of steam rolls off the unit, which people regularly report to fire departments. Water streams onto the pad. Inside, because the system is temporarily pulling heat from your house, the supply air turns cool for a few minutes — which is exactly why most units fire the backup heat strips during defrost to cover the gap.

A normal defrost runs somewhere between about 5 and 15 minutes and ends when the coil sensor reads warm enough or a timer expires, depending on the control. Older systems use time-and-temperature defrost on a fixed interval, typically every 30, 60, or 90 minutes of run time. Newer systems use demand defrost, which compares coil temperature against outdoor air temperature and only defrosts when frost is actually there. Demand defrost is easier on the compressor and wastes less energy.

Cause 6: frozen outdoor coil

A light dusting of frost is expected. A coil buried under a solid layer of ice, or a unit with ice on top of the cabinet or built up around the base, is not. When ice fully blocks the coil, the system can’t extract heat from outdoor air at all, so heating output collapses even though the equipment sounds like it’s running normally.

The usual causes, roughly in order: the defrost control board or coil sensor has failed, so defrost never triggers; meltwater can’t drain out of the base pan and refreezes into an ice dam that grows back into the coil; the unit sits too low or too close to a wall for water to escape; the charge is low, which drives coil temperature below normal and produces frost faster than defrost can clear it; or the fan motor has failed, so there’s no airflow across the coil.

You can safely thaw a frozen unit yourself as a stopgap. Turn the system to OFF at the thermostat, or use the fan-only setting if your air handler has one, and let the ice melt. Some units have an emergency heat mode that keeps the house warm from the backup elements while the outdoor coil sits idle and thaws. Warm — not boiling — water from a hose will speed it up if it’s above freezing outside and the water can actually drain away.

Thawing gets you through the night. It doesn’t fix why the ice formed. If the coil re-ices within a day, you need a technician to test the defrost board, the sensor, and the charge.

Cause 7: auxiliary heat isn’t engaging

Most ducted heat pumps in the U.S. include supplemental heat — usually electric resistance strips in the air handler, sometimes a gas furnace in a dual-fuel setup. When outdoor temperatures fall below what the heat pump alone can cover, or during defrost, that backup is supposed to kick in. If it doesn’t, you get a house that holds 68°F in November and slides to 60°F in January while the outdoor unit runs nonstop.

Two checks are yours. First, the thermostat: some setups require aux heat to be enabled, and an aggressively configured “lockout” temperature can prevent strips from ever running. Second, the breaker. Electric strips draw serious current and sit on their own double-pole 240V breaker, often labeled Air Handler, AHU, or Heat Strips. That breaker trips more often than people expect, especially the first cold snap of the season. Reset it once. If it trips again, stop.

Past that, it’s a service call. Common professional findings are a loose W2 wire at the thermostat, a failed sequencer that staggers the elements, a burned contactor, a blown high-limit or thermal cutout, or a control board that isn’t sending the signal. A technician can also confirm your outdoor thermostat and balance-point settings make sense for your climate — badly set lockouts either waste money running strips too early or leave you cold by locking them out too aggressively.

Cause 8: low refrigerant from a leak

Let’s be direct about this one, because the internet is full of bad advice.

A heat pump is a sealed system. Refrigerant is not consumed, it doesn’t evaporate over time, and it doesn’t need topping up on a schedule the way a car needs oil. If your system is low on refrigerant, refrigerant escaped. There is a leak. Anyone who offers to “top you up” without finding that leak is selling you a repair that will fail again, usually within a season, while venting refrigerant into the atmosphere.

Signs of a low charge in heating mode: the system runs continuously and never satisfies the thermostat, output feels weaker than it used to, the outdoor coil frosts unusually fast or unevenly, and you may see frost or ice on the small copper line at the outdoor unit. Sometimes there’s an oily residue near a joint or a service valve, since refrigerant oil travels with the leak.

None of the repair is DIY. Handling refrigerant legally requires EPA Section 608 certification. Beyond the law, the diagnosis itself needs equipment most homeowners don’t own: electronic leak detectors, nitrogen pressure testing, a micron gauge, and a recovery machine. Charging by feel destroys systems — overcharging is as damaging as undercharging.

Cost varies enormously with where the leak is. A leaking service valve or a fitting in an accessible spot might run a few hundred dollars including recovery and recharge. A leak inside an indoor coil, or one in a buried line set, can pass $1,500 once parts and labor are counted. If you’re on an older R-410A system past twelve years and facing a four-figure leak repair, ask the contractor to price replacement in the same visit — the math often favors new equipment, especially given that the federal 25C tax credit no longer applies.

Causes 9–10: capacitor, contactor, reversing valve

These are the three failures that turn a working system into a dead one, and all three are technician territory.

Failed run capacitor

The capacitor is a cylindrical component that gives the compressor and outdoor fan motor the electrical kick they need to start and keep spinning. It’s one of the most common failure points in any HVAC system, and heat and age both wear it out. The classic symptom: the outdoor unit hums but the fan doesn’t turn, or the fan spins if you nudge it and then keeps going. Sometimes the unit tries to start, clicks, and shuts down repeatedly. A visibly bulged or leaking capacitor top is a giveaway. Replacement is quick and among the cheapest repairs on the list — typically $120 to $300 installed. Capacitors store a dangerous charge even with power off, which is exactly why this is not a homeowner job despite the part costing very little online.

Failed contactor

The contactor is the heavy-duty relay that switches high voltage to the compressor when the thermostat calls. Contacts pit and burn over the years, and insects — ants especially — get inside and weld them. A failing contactor produces chattering, buzzing, an outdoor unit that won’t start, or occasionally one that won’t stop because the contacts have welded closed. Replacement typically runs $150 to $400. If your outdoor unit has been running when the thermostat is satisfied, shut it off at the disconnect and call someone; a welded contactor can run the compressor to destruction.

Failed reversing valve

The reversing valve is what makes a heat pump a heat pump. It flips refrigerant flow so the same equipment can heat in winter and cool in summer. When it fails or sticks, you get the most confusing symptom of all: the system blows cold air in heating mode, continuously, no matter what the thermostat says. Some units cool fine and refuse to heat. Sometimes the valve is fine and the solenoid coil that shifts it has burned out, which is a much cheaper fix. Full valve replacement means recovering the refrigerant, brazing in a new valve, evacuating, and recharging — typically $350 to $800, and more on larger or harder-to-access systems. On an aging system, that number starts a replace-or-repair conversation.

What each repair typically costs

Use this as a sanity check on a quote, not a price list. Published 2026 contractor data puts the average heat pump repair near $400, with most jobs landing between roughly $150 and $650. Anything far above that range should come with an explanation you understand.

Two things move these numbers more than anything else. First, warranty: most manufacturers cover parts for five to ten years if the system was registered, which can cut a capacitor or valve job down to labor only. Dig out your paperwork before you approve a repair. Second, refrigerant type: R-410A pricing has climbed as the industry moves to A2L refrigerants, so a big recharge on older equipment costs more than it did a few years ago.

6 mistakes that make things worse

When to stop and call someone

You’ve done the free checks and the system still won’t heat. Here’s the line.

Call a licensed technician if: the outdoor unit is completely silent while the indoor blower runs; a breaker trips more than once; the coil ices over and stays iced; you smell burning or hot electrical insulation; the unit is making a grinding, screeching, or metallic banging noise; refrigerant is involved in any way; or the system blows cold air in heat mode with everything set correctly. Noise diagnosis in particular is a useful early warning — our guide to what heat pump noises mean matches sounds to causes.

When you call, save yourself money by having answers ready. What are the outdoor temperature and the thermostat setting? What’s the temperature at a supply register versus a return? Is the outdoor fan turning? Is there ice, and where? Which breakers did you reset, and did they hold? What’s the model and serial number from the data plate on the outdoor unit? A technician who arrives with that information already in hand spends the visit fixing rather than interviewing, and diagnostic fees are usually billed by time.

One last thing worth knowing: a heat pump that limps along for years often isn’t broken so much as badly matched to the house. If yours has never really kept up, it’s worth reading whether a heat pump suits your home and our sizing guide before you spend serious money on repairs.

Frequently asked questions