Heat pump not heating?

The house stays cold even though the system is running. The usual causes are thermostat settings, a clogged filter, or an outdoor unit iced over. Check those first; refrigerant and electrical parts are for a pro.

Not sure which one it is? Snap your heat pump and we’ll narrow it down.

Is cold air blowing out of the vents?

That can be normal defrost mode or a sign of a fault. Heat pump blowing cold air

Likely causes

  1. Thermostat settings or dead batteries

    Most commonEasy DIY

    Set to Cool or Off, a blank or dim display, or a set temperature at or below the room temperature. Heat pump thermostats also have an emergency or aux heat setting that's easy to bump.

    Part: $5–15 (batteries)

  2. Clogged air filter

    Most commonEasy DIY

    Weak airflow from the vents, a filter caked with dust, or the system shutting off early or running constantly without warming the house.

    Part: $10–40 (filter)

  3. Ice on the outdoor unit

    CommonCall a pro

    A thick coat of ice that doesn't clear within an hour or two, or ice covering the fan and coil. A thin frost that melts on its own during a defrost cycle is normal.

    Part: $150–700 (installed)

  4. Blocked outdoor unit or tripped breaker

    CommonEasy DIY

    Leaves, snow or debris packed around the outdoor unit, or an outdoor unit that is silent while the indoor fan runs. Outdoor units have their own breaker and disconnect.

  5. Low refrigerant, failed reversing valve or compressor

    Less commonCall a pro

    The system runs but only lukewarm air comes out, the outdoor unit hums without the fan or compressor starting, or there's hissing and oily residue on the lines.

    Part: $100–2,500 (a recharge at the low end, a compressor at the top)

Quick checks

Turn the system off at the thermostat and the breaker before touching anything on the outdoor unit, and never reach into the fan. Don't chip ice off the coil or fins; you can puncture the refrigerant line. Refrigerant and electrical work need a licensed technician. If you smell burning or see smoke or sparks, switch the breaker off and call a pro. If a carbon monoxide alarm sounds (for homes with a gas backup furnace), leave the house and call 911 from outside.

  1. Set the thermostat to Heat and a few degrees above room temperature, and replace its batteries if the screen is dim or blank.
  2. Check the air filter and replace it if it's grey and clogged.
  3. Look at the outdoor unit: clear leaves, snow and debris from around it, but don't chip at ice or poke the coil.
  4. Confirm the breaker hasn't tripped, for both the indoor unit and the outdoor unit. Reset a tripped breaker once only.
  5. Open every supply vent and clear anything blocking them.
  6. If the thermostat shows Em Heat, someone switched it to backup heat by hand, so switch it back to Heat. If it shows Aux, the backup is helping on its own, which is normal in very cold weather; call a pro if Aux stays on in mild weather.
Sharpen the read

Diagnose your heat pump in seconds, for free

No account, no phone number. Fixology’s AI reads your photo and tells you what’s likely wrong, how worried to be, and whether it’s a DIY fix.

Snap one of these:

  • the thermostat display, including any Em Heat or Aux setting
  • the outdoor unit, especially any ice on the coil or fan
  • the model label on the side of the unit

Add a note or more photos next. Nobody calls you.

When to call a pro

The house still isn't warming after the filter, thermostat and breaker checks, the outdoor unit stays iced over, you hear grinding or hissing, or Aux heat stays on in mild weather. For lifespan and upkeep, see the heat pump guide.

Typical cost: $150–650 for most repairs including the service call (about $400 on average). A diagnostic visit typically runs $75–250, often credited if you go ahead. A compressor replacement can reach $2,500.

Sources: Fixr: How Much Does Heat Pump Repair Cost in 2026? (updated January 21, 2026)

After a scan, Fixology can also help you find pros near you.

Related problems

  • Heat pump blowing cold air?

    Heat pump

    Cool air from the vents is normal for a few minutes during defrost; if it persists, look at the thermostat, filter or a refrigerant fault.

  • Furnace won't heat?

    FurnaceFall

    The furnace runs but the house stays cold, which is usually a clogged filter, a thermostat setting, or an ignition fault.

  • Washer won't drain?

    Washer

    Water is left in the drum after the cycle, which is usually a clogged pump filter or a kinked drain hose.

  • Fridge not cooling?

    Fridge

    The fridge section is warm or warming. The cause depends on whether the freezer is still cold, and the first fix is often free.

Fixology helps you understand your home — it isn't a substitute for a licensed pro on serious gas, electrical, or structural work.