Heat pump not heating? What's normal, what's broken, and how to fix it
A heat pump blowing cool air is often not broken at all — it may be running a normal defrost cycle, or you may just be feeling its naturally gentler warm air. When it's truly not heating, the cause is usually a thermostat, a dirty filter, a frozen unit, low refrigerant, or a stuck reversing valve.
If your home is getting dangerously cold, call (888) 810-2291 for a licensed local contractor — and switch the thermostat to emergency heat to stay warm while you wait.
On this page
Is it actually broken? Defrost mode and lukewarm air
Before assuming a fault, rule out the two things that look like failures but aren't. First, defrost mode: in cold, damp weather frost builds on the outdoor coil, so the unit briefly reverses to melt it — sending steam outside and cool air from the vents for 5 to 15 minutes before normal heating resumes. Second, lukewarm air is normal: a heat pump's supply air runs about 90–100°F, versus 120–140°F from a gas furnace, so it feels cool even when it's working. The simple test is whether the air is warmer than the room (working) or truly cold for long stretches (a real fault). The full mechanism is on how a heat pump works.
What should you check first?
Four free checks rule out most heat-pump "no heat" calls. Work them in order before booking a visit:
- Thermostat. Set it to HEAT — not COOL or constant-fan — and a few degrees above room temperature. Constant-fan blows unheated air and mimics a failure.
- Power. Check the outdoor unit's 240V breaker, its disconnect box, and the indoor air-handler switch (it looks like a light switch). Reset a tripped breaker once.
- Air filter. Pull it and hold it to a light. A clogged filter chokes airflow and leaves the vents blowing lukewarm — swap it and wait 30 minutes.
- Outdoor unit. Clear 2–3 feet around it and gently brush off snow or debris. Never chip ice off the coil — you'll puncture it.
Checks done, still cold?
Enter your ZIP — we'll connect you to a licensed local contractor for the diagnosis. The call is free, 24/7.
In a hurry? Call (888) 810-2291 now.
Licensed contractors serve . One call routes you to one for .
☏ Call (888) 810-2291Availability is subject to provider participation, location, technician availability, and demand.
What causes a heat pump to stop heating?
Once the normal behaviors and the free checks are ruled out, the fault is one of these — the first two you can often handle, the rest are technician work:
Thermostat set wrong or uncalibratedDIY · $0–$300
The simplest cause: the thermostat is on COOL or constant-fan, or it has drifted out of calibration so the room never matches the setting. Confirm it's on HEAT, and recalibrate per the manual if the reading is off. Recalibration costs nothing; a replacement thermostat is $100–$300.
Dirty filter or restricted airflowDIY / PRO · $5–$40+
A clogged filter or blocked ducts starve the system, so it blows lukewarm or shuts down to protect itself. Swap the filter (a $5–$40 DIY fix) and open any closed vents; hidden leaky ducts need a technician to find and seal.
Frozen or iced-over outdoor unitDIY / PRO · $150–$600
A light frost that clears during defrost is normal, but thick ice that keeps returning blocks heat transfer and the house goes cold. Clear snow and debris (never chip the ice); if it re-ices, the defrost control or refrigerant is the culprit. A defrost board or sensor repair runs $150–$600. The freeze mechanism is covered in depth on our frozen coil guide.
Low refrigerant (a leak)PRO · EPA 608 · $300–$1,600
Low refrigerant leaves the system unable to move enough heat, and a low charge always means a leak — never normal use. The tells are weak heat and persistent icing. A find-and-fix with recharge runs about $300–$1,600, and handling refrigerant is EPA Section 608–licensed work. Confirm the symptoms on our low refrigerant signs page.
Stuck reversing valvePRO · $600–$1,200
This is the heat-pump-specific fault: if it blows cold in HEAT but works fine in COOL, the reversing valve that flips the refrigerant's direction is stuck. That contrast is the tell that separates a heat-pump problem from a generic one. A reversing-valve repair or replacement runs $600–$1,200. What the valve does is on our how a heat pump works page.
Undersized system or extreme coldDIY / PRO · Varies
Below its balance point — often 25–35°F for older units — a heat pump can't pull enough heat from the air and leans on backup heat. That's a comfort limit, not always a fault. If it's simply very cold out, nothing needs fixing; if the system chronically can't keep up, it may be undersized or aging. See heat pump vs furnace for the balance-point math.
What is emergency heat, and should you use it?
Emergency heat — sometimes labeled auxiliary or "aux" heat — is a backup, usually electric resistance strips, that runs when the heat pump can't keep up. Use it for short-term support, such as when the outdoor unit is down, but not as everyday heat: it draws far more electricity and will spike your bill. If your system keeps switching to aux heat on its own, that's a sign the heat pump is struggling, not a fix — worth a diagnosis.
Get a licensed heat-pump tech on the phone
One call routes you to a licensed local contractor for a heat pump that won't heat — 24/7, nationwide. The call is free.
☏ Call (888) 810-2291Availability is subject to provider participation, location, technician availability, and demand.
Which cause is it? Match your clue
The pattern usually narrows it fast — and two of the most common patterns are actually normal. Find the clue that matches:
| What you're seeing | Likeliest cause | DIY or pro |
|---|---|---|
| Cold air in HEAT, fine in COOL | Stuck reversing valve | PRO |
| Steam + cool air for 5–15 min | Normal defrost cycle | Normal |
| Lukewarm air, warmer than room | Normal heat-pump air | Normal |
| Thick ice that keeps returning | Defrost or refrigerant | PRO |
| Weak heat, house won't warm | Filter, refrigerant, or capacity | DIY / PRO |
The clue narrows the suspects; a technician's gauge readings confirm anything on the refrigerant or reversing-valve side.
What does a heat pump repair cost?
DIY checks are free-to-$40. When it's a technician repair, here's the national range for each — the reversing valve and refrigerant leaks are the pricier end, with the actual quote coming from the contractor:
| Repair | Typical installed range |
|---|---|
| Thermostat replacement | $100–$300 |
| Defrost board / sensor | $150–$600 |
| Reversing valve | $600–$1,200 |
| Refrigerant leak find + recharge | $300–$1,600 |
Full pricing is on HVAC repair cost, and the visit fee itself on HVAC service call cost.
When should you call — and is it repair or replace?
Call if the free checks didn't restore heat, if it blows cold in heat only (a reversing valve), if ice keeps returning, or if the breakers trip. Describe the symptoms to shorten the visit; the diagnostic fee of $75–$200 is often credited toward the repair. And if the unit is aging and chronically can't keep up, weigh the repair against replacement using the age-and-cost math on repair or replace. What happens on the visit is covered in what to expect from a service call.
Frequently asked questions
Why does my heat pump blow cold air when the heat is on?
Most often it's a normal defrost cycle — the unit briefly reverses to melt frost off the outdoor coil, sending cool air from the vents for 5–15 minutes. If the cold air lasts far longer, suspect a stuck reversing valve or low refrigerant.
Is lukewarm air from a heat pump normal?
Yes. A heat pump's supply air runs about 90–100°F, versus 120–140°F from a gas furnace, so it feels cooler even when the system is working perfectly. As long as the air is warmer than the room, it's heating.
At what temperature does a heat pump stop working?
Older units start leaning on backup heat around 25–35°F, their balance point. Modern cold-climate heat pumps keep producing useful heat well below zero. Losing efficiency in deep cold is expected, not a failure.
How do I reset my heat pump?
Turn the thermostat off, flip the breakers for both the indoor and outdoor units off for one minute, then back on, and wait a few minutes before switching the thermostat back to HEAT.
Should a heat pump run constantly in winter?
Long run times are normal — a heat pump delivers steady, lower-temperature heat over longer cycles. Running nonstop while the house still won't warm up is not normal and points to a fault or a capacity limit.
When should I use emergency heat?
Only as a short-term backup — for example, when the outdoor unit is down. Emergency heat is electric resistance heat that costs far more to run, so frequent use means the heat pump itself needs attention.
Why does my heat pump cool fine but not heat?
That's the classic sign of a stuck reversing valve, the part that switches the system between heating and cooling. It's a technician repair, typically $600–$1,200.