Heat Pump Not Heating: How to Tell Normal From Broken

Before you book anything, answer three questions in order — is the air actually cold or just cooler than you expected, is the outdoor unit in a defrost cycle, and is the thermostat sitting in emergency heat. Those three account for most winter calls that end with no parts fitted. If the house genuinely will not hold temperature after that, the usual real faults are a restricted filter, a buried outdoor unit, a failed defrost board or sensor, a stuck reversing valve, or a low refrigerant charge, and only the first two are homeowner territory.

First: is it actually broken?

A heat pump does not heat the way a furnace heats, and almost every winter surprise traces back to that. A furnace burns fuel and delivers a short blast of genuinely hot air. A heat pump moves heat and delivers a long, steady stream of mildly warm air — commonly in the 90s Fahrenheit, roughly the temperature of your own skin. Hold your hand to the register and it feels cool even while the house is warming. That is not a fault, and no amount of servicing changes it.

The second difference is capacity. A furnace produces the same output at 45°F as at 20°F; a heat pump does not, because colder air holds less heat to collect, so output falls exactly when demand rises. Every heat pump has a balance point — the outdoor temperature at which its output stops matching the house’s heat loss. Above it the heat pump carries the house alone; below it, something has to make up the difference, which is what auxiliary heat is for. A system running continuously on the coldest morning of the year, sitting a degree below setpoint, is a heat pump at its balance point rather than a failed one.

Third, heat pumps run long and slow by design. Forty minutes at a stretch in January is correct; starting and stopping every few minutes is short-cycling, a different problem. If a heat pump is new to you, our heat pump vs AC explainer is worth reading alongside this page.

The defrost cycle: ice and steam are normal

This is the most common false alarm in heat pump ownership, and the panic response — attacking the ice — causes real damage.

In heating mode the outdoor coil is the cold side of the system, running below outdoor air temperature. When the air is cold and damp, moisture condenses on it and freezes. Frost insulates the coil, and an insulated coil cannot collect heat. So the system periodically reverses itself, running briefly in cooling mode to send hot refrigerant gas out and melt the frost off.

Here is what that looks like from outside, in the manufacturer’s own words. Rheem’s Outdoor Heat Pump, Two Stage R-410A Split System, User’s Information/Installation Instructions (form 7083670) describes it: “During cold weather heating operation, the outdoor unit will develop a coating of snow and ice on the heat transfer coil. This is normal, and the unit will periodically defrost itself. At the beginning of the defrost cycle, both the outdoor condenser fan and compressor will de-energize. After approximately 30 seconds, the compressor will energize and begin to heat the outdoor coil causing the ice and snow to melt. After the ice and snow have melted, some steam may rise from the outdoor unit as the warm coil causes some melted frost to evaporate.”

Match that to what you saw. The outdoor fan stops while the compressor keeps running. A cloud that looks like smoke rises off the unit. The indoor air goes tepid for a few minutes. All three are the manual describing correct operation.

How long, and how often

Defrost is set at installation rather than fixed. The same Rheem manual describes a defrost cycle timer with three interval settings — 30, 60 and 90 minutes — chosen for the local climate, all units shipping on a 30-minute default, with 90 minutes recommended for the dry climate of southern Arizona and 30 for the moist climate of Seattle. Damp air means more frost, so more frequent defrosts.

The interval is the gap between defrosts; the cycle itself is short, and on that control the board forces defrost to end after ten minutes regardless. So the working expectation is a short interruption of minutes, not hours, at intervals of half an hour to an hour and a half in cold damp weather, and rarely at all in dry cold.

What is not normal

Carrier draws the line usefully: “During winter, it’s normal for a little frost to develop on the outdoor coil, but a coating of ice indicates an issue.” Treat these as faults worth a call:

• Ice that never clears — still there hours later or the next morning.
• Ice encasing the whole unit as a solid block, fan grille included, rather than frosting the coil face.
• Ice on top of the unit or on the fan blades. Frost belongs on the coil; ice above it usually means water is arriving from a gutter, downspout or roof valley and refreezing.
• Constant defrosting — more time in defrost than in heating suggests a mis-set control, low charge, or a failed sensor.
• The unit standing in a frozen puddle. Defrost water has to drain away, or the unit re-freezes its own meltwater and never gets clear.

Never chip, hammer or scrape the ice

The outdoor coil is thin aluminium fin over thin copper tube carrying refrigerant under pressure. A screwdriver, hammer or stiff broom handle will go through it. That converts a defrost fault — often just a control board — into a coil replacement plus a full recovery and recharge, and it releases refrigerant under pressure at close range. Do not pour hot water over it either: it refreezes in the base pan and around the fan, and on a running unit you are pouring water into live electrical components. If a unit is iced solid, shut the system off at the thermostat, let it thaw, and call.

AUX and EM HEAT are not the same thing

Confusing these two words is the most expensive mistake a heat pump owner can make in a winter.

AUX — auxiliary or supplemental heat — is automatic. When the heat pump alone cannot keep up, during defrost, or when you jump the setpoint several degrees at once, the thermostat brings in a bank of electric resistance elements while the heat pump keeps running. AUX lighting up on a cold morning is the system working as designed.

EM HEAT is manual and different in kind: it switches the heat pump off and heats on the resistance elements alone. Rheem’s manual is exact: “With the system switch set to EM. HT. the outdoor unit will be locked off, and supplemental heat (typically electric resistance heating) will be used as a source of heat. Sustained use of electric resistance heat in place of the heat pump will result in an increase in electric utility costs.”

Resistance heat runs at a coefficient of performance of exactly 1.0 — one unit in, one unit out, no multiplication. Here is what that is worth in money. Prices are EIA figures; the per-million-Btu conversions are our arithmetic on them.

California residential electricity 33.25¢/kWh (EIA Electric Power Monthly Table 5.6.A, May 2026 data, released 2026-07-23); California residential gas $2.00–$2.21/therm (EIA, release 2026-07-31). Per-million-Btu figures are our derivation.

Switching to EM HEAT and forgetting roughly triples what your heating costs. Someone who flips it during a December cold snap and leaves it until March gets a bill that looks like a system failure. If your bill has jumped and nothing else changed, check that switch first. Emergency heat has one legitimate use: keeping the house warm while you wait for a repair.

Now troubleshoot

1. Thermostat mode and setpoint. HEAT, not AUTO or EM HEAT; setpoint above room temperature; no stale hold. A blank display is its own problem — start at thermostat not working.
2. Filter and airflow. A restricted filter starves the indoor coil and drags heating capacity down. The Department of Energy’s 5–15% figure for routinely replacing or cleaning a filter is a cooling figure, for air-conditioner energy use; the same restricted airflow is what drags heating down. Replacement costs under $30; see how and how often.
3. Outdoor unit blocked, buried or iced. Clear snow, leaves and debris from around and above it, and check nothing is dumping water onto it.
4. Reversing valve. The component that flips the system between heating and cooling. Stuck, it delivers cooling when you ask for heat, or nothing while the compressor runs. The tell is a system that cools perfectly in summer and does nothing in winter, sometimes with a loud clunk as it tries to shift.
5. Defrost board or sensor. If defrost never initiates the coil ices over and stays iced; if it never terminates the system sits in cooling mode blowing cold air indoors.
6. Low refrigerant charge. A leak shows up worse in heating than cooling, and it damages the compressor: Copeland’s bulletin AE4-1365 R6 states that loss of charge “will result in overheating and recycling of the motor overload protector,” and prolonged operation that way “could result in oil pump out and eventual bearing failure.” See the signs of a leak.
7. A tripped breaker on the aux heat strips. Resistance banks sit on their own high-amperage circuits, so if one trips the heat pump still runs but the backup is gone. Reset a breaker once; the Electrical Safety Foundation International states that equipment which “repeatedly blows a fuse or trips a circuit breaker could indicate a defect that may cause a fire or electrical shock.”

Where to stop

Do not open the electrical compartment to poke at a capacitor. A capacitor holds a dangerous charge after power is off; OSHA’s regulation at 29 CFR 1910.333(b)(2)(ii)(C) requires stored energy be released before work and the part treated as energized while that is done. A homeowner should not discharge one — our page on a fan that hums but won’t spin carries the full warning.

Refrigerant is not homeowner territory either. EPA Section 608 prohibits venting it, and the sales restriction means you cannot legally buy most modern refrigerants. Recent systems may be charged with R-454B or R-32, which are A2L refrigerants — mildly flammable: if you suspect a leak, kill ignition sources, ventilate and keep people away. A2L equipment also carries a refrigerant sensor, and when that trips the system runs the blower and locks out operation, so on a new system, “the fan runs but there’s no heat” can be a safety feature rather than a fault. Say so when you call. And note that owning an R-410A heat pump is not a reason to replace it: servicing and recharging installed R-410A equipment remains legal with no end date.

What each repair tends to cost

No government body or standards organisation publishes installed residential HVAC repair prices. These are contractor-reported market estimates, given wide because that is the width.

The reversing valve is the one to think hard about. Replacing it means recovering the charge, brazing in a new valve, fitting a fresh liquid-line filter drier, evacuating and recharging — compressor-class labour. On an out-of-warranty heat pump past ten years old the quote lands close enough to a replacement quote that you are really making a replace-or-repair decision. Watch for the warranty trap that catches compressor claims: the part may be covered while labour, refrigerant and the drier are not.

Should you add a backup heat source?

If your heat pump keeps up except on a handful of the coldest mornings, usually no. The case changes if you are below your balance point often, because then the strips become a routine expense at roughly $97 per million Btu. A dual-fuel setup pairs the heat pump with a gas furnace and lets a thermostat pick whichever is cheaper, handing the coldest stretches to gas at roughly $22 — at the cost of owning two appliances.

What to do tonight

1. Look at the thermostat. HEAT, not EM HEAT. Setpoint above room temperature. No stale hold.
2. Go and look at the outdoor unit. Fan stopped with a cloud rising off it means defrost — give it ten minutes and check again. Frosted but running normally is also normal.
3. Pull the filter. If light will not pass through it, replace it and give the system an hour.
4. If the house is genuinely cold and none of that applies, switch to EM HEAT to get through the night, book a licensed pro, and switch it back off the moment the repair is done.
5. If a breaker has tripped, reset it once. If it trips again, leave it off.

Worth having open when you call: heat pump vs furnace if the quote is big enough to raise the replacement question, and heat pump replacement costs if it already has. If your backup is a gas furnace and that is the half that failed, start at a furnace blowing cold air.

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