Heat Pump Not Heating: Diagnostics
"It runs but it's blowing cold" is one of the most common winter heat-pump calls — and one of the easiest to misdiagnose. This is the flow that isolates the fault in order, from the thermostat call through the reversing valve, charge, and defrost, so you replace the right part the first time.
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In This Guide
- → Step 1: Confirm the Call and Measure the Split
- → Step 2: Is the Compressor Actually Running?
- → Step 3: Rule Out Aux and Emergency Heat
- → Step 4: Read the Reversing Valve
- → Step 5: Check the Charge in Heating Mode
- → Step 6: Defrost, Ice, and Airflow
- → Symptom-to-Cause Quick Table
- → Worked Example: Cold Air at 30°F
Step 1: Confirm the Call and Measure the Split
Before you pull a panel, stand at a supply register and take a temperature reading, then grab return air at the filter grille. A heat pump running on the compressor alone delivers a temperature rise of roughly 15 to 30°F, with supply air commonly landing in the 90 to 105°F range on a mild day. That's cooler than a gas furnace — a heat pump feels "lukewarm" to homeowners even when it's working perfectly, which generates a lot of false "no heat" calls. Confirm the complaint is real before you chase it.
Then confirm the thermostat is genuinely in HEAT and calling, the setpoint is above room temperature, and — critically — that it isn't stuck in a fan-only or emergency-heat state a homeowner flipped. A surprising number of "heat pump not heating" tickets are a thermostat in the wrong mode or a tripped 24V circuit.
What the split tells you
Use the airside relationship Qs = 1.08 × CFM × ΔT. A 3-ton system moving ~1,200 CFM at a 20°F rise is delivering about 1.08 × 1200 × 20 = 25,920 BTU/hr of sensible heat — right around a ton and a half of the rated capacity, normal for a heat pump in cold weather. A near-zero split with the compressor running means the refrigerant side isn't producing heat: valve, charge, or compressor.
Step 2: Is the Compressor Actually Running?
"The unit is running" almost always means "the outdoor fan is spinning." Those are not the same thing. Clamp the compressor common (or the run leg at the contactor) and confirm it's pulling amps in the range of its nameplate RLA. If the fan runs but the compressor is dead, no heat is being pumped indoors no matter what the valve or charge is doing.
When the compressor won't start or drops out, work the usual electrical suspects in order:
- Contactor: pitted or welded contacts, or a coil not being pulled in by the 24V circuit. Check for 24V across the coil and line voltage across the load side.
- Run capacitor: a weak or failed dual-run cap is one of the single most common failures. Read microfarads and replace if more than ~6% off the rated value (e.g. a 45/5 reading 39 MFD on the herm side).
- Low-pressure lockout: a low-charge system in cold weather can trip the LP switch and lock the compressor out. Don't mistake a protective lockout for a dead compressor.
- Defrost board / control: the board sequences the compressor, valve, and outdoor fan. A failed board can leave the compressor off or stuck in a defrost hold.
Field tip: A compressor that hums, draws locked-rotor amps for a moment, then trips is usually a failed start component or a mechanical seizure — not a refrigerant problem. Prove the capacitor and check for a hard-start kit before you condemn the compressor.
Step 3: Rule Out Aux and Emergency Heat
Most heat pumps carry supplemental heat — electric resistance strips or, in a dual-fuel setup, a gas furnace. Flipping the thermostat to emergency heat is a fast, powerful split test: it locks out the compressor and runs the backup heat only.
Emergency heat works
The strips or furnace are fine and the low-voltage supply to them is good. Your fault is on the heat-pump side — valve, charge, compressor, or the O circuit. Focus there.
Emergency heat also fails
You may have two problems, or a shared one: a tripped sequencer, an open heat-strip fuse or limit, a failed W output, or a blown low-voltage transformer feeding both.
Remember the homeowner impact: if the heat pump is down but the aux heat runs, the house stays warm — expensively. Note it on the ticket, because a system leaning on resistance heat all winter is what drives the eye-watering electric bill that generated the call. When aux is staging early, it's also worth revisiting the balance point to confirm the backup is only picking up load below the design crossover, not covering for a weak compressor.
Step 4: Read the Reversing Valve
If the compressor runs and aux heat is fine but supply air is still cold, the classic cause is a heat pump stuck in cooling while calling heat. The four-way reversing valve reroutes discharge gas; when it fails to shift, the system quietly runs as an air conditioner in January. Read the four valve ports by touch or with a clamp thermocouple:
- Single top tube: always hot — that's compressor discharge feeding the valve.
- Center bottom tube: always the coolest — common suction back to the compressor.
- Two outer tubes: one to the indoor coil, one to the outdoor coil. In heating, the indoor-coil port should be the hot one.
If the outdoor-coil port is hot while the thermostat calls heat, the slide never moved. Two things to check, in order: the O signal and the coil. On most residential systems the valve energizes in cooling (24V on the O terminal), so in heating the coil should read 0V. If you measure 24V at the coil during a heat call, something is holding it in cooling — a shorted O wire, a mis-set thermostat, or a defrost board stuck energizing O.
The internal-leak signature
A valve can also fail by leaking internally — a worn slide seat lets hot discharge gas bypass straight into the suction port. The tell is a common suction line that runs unusually warm and mediocre performance in both modes. If the discharge-to-suction temperature drop across the valve is under about 25 to 30°F, suspect an internally leaking valve. For the full coil, signal, and pressure workflow, see reversing valve troubleshooting.
One more nuance unique to the valve: it's pressure-operated, not electrically operated. The coil only shifts a pilot pin; discharge-to-suction pressure difference drives the main slide. A weak compressor that can't build head may leave the slide floating even with a perfect coil — which is exactly why charge and compressor health come next.
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Step 5: Check the Charge in Heating Mode
If the valve is shifting correctly and the compressor is healthy but the system is still short on heat, put your gauges on. In heating, the roles flip: the indoor coil is the condenser and the outdoor coil is the evaporator. That changes which pressure you read for saturation, but the fundamental math is the same:
Superheat
High superheat with low subcooling points to undercharge or a restriction starving the coil.
Subcooling
Low subcooling generally means undercharged; high subcooling means overcharged or a downstream restriction.
Charge verification in heating is trickier than in cooling — most manufacturers want you to charge in cooling mode or to weigh in per the nameplate, because ambient swings make heating-mode targets move. As a field sanity check, a badly undercharged heat pump in heat shows low subcooling, elevated superheat, low suction pressure, and a big temperature drop in capacity. Bubbles in the sight glass, if equipped, confirm it. Find and fix the leak — don't just top it off.
Compression check: the compression ratio (absolute discharge ÷ absolute suction) should typically land around 2.5:1 to 3.5:1. A valve-worn compressor shows a weak spread — low discharge, high suction — and can't build the head pressure the reversing valve needs to shift or the system needs to make heat.
Run your numbers through the superheat/subcooling calculator to confirm the charge picture before you recover, add, or condemn anything.
Step 6: Defrost, Ice, and Airflow
In heating, the outdoor coil runs below freezing and accumulates frost. A working defrost cycle periodically reverses the valve to shed it. When defrost fails, the coil ices over solid, airflow across it collapses, and heating capacity falls off a cliff. Two failure modes matter:
- Coil iced, no defrost: a failed defrost board, an open defrost sensor, or a stuck reversing valve leaves the coil buried in ice. You'll see a solid block of frost and poor-to-no heat.
- Stuck in defrost: the unit keeps reversing to cooling to melt frost that isn't there, blowing cold indoors. A faulty sensor or board is usually behind it.
Finally, don't forget the airside. Low indoor airflow — a clogged filter, a slipping blower, or crushed ductwork — starves the indoor coil (now the condenser) and drags down capacity. Confirm a clean filter and total external static pressure at or below 0.50 iwc for most residential systems; readings above ~0.80 iwc signal a serious airflow restriction.
Cold-weather reality check
All air-source heat pumps lose capacity as the outdoor temperature drops. Below the system's balance point, the compressor simply can't meet the load and aux heat is supposed to carry the difference. If the "problem" is only a mildly cool supply at 10°F outside on a standard-efficiency unit, the equipment may be behaving exactly as designed — verify against the manufacturer's low-temperature capacity data before condemning anything.
Symptom-to-Cause Quick Table
Use this as a fast field reference once you've confirmed the complaint is real:
| Symptom | Likely Cause | First Check |
|---|---|---|
| Cold supply, compressor + fan running | Reversing valve stuck in cooling | Port temps + O signal at coil |
| Fan runs, compressor off | Contactor, capacitor, LP lockout | 24V at contactor, MFD reading |
| Weak heat, low suction, low subcooling | Undercharge / refrigerant leak | Superheat & subcooling, leak search |
| Outdoor coil iced solid | Failed defrost (board/sensor/valve) | Force defrost, check board & sensor |
| Periodic cold blasts on heat | Stuck / frequent defrost cycles | Defrost sensor & termination logic |
| Mediocre heat & cool, warm suction | Internally leaking reversing valve | Discharge-to-suction ΔT across valve |
| High static, low airflow, weak heat | Dirty filter / duct restriction | Filter + total external static pressure |
Worked Example: Cold Air at 30°F
The call:
R-410A split heat pump, O-orientation, homeowner reports weak heat and a high electric bill. 30°F outside. Outdoor unit running.
Step 1 — Measure the split: Return 68°F, supply 78°F. Only a 10°F rise — low for a heat pump that should be pushing 90°F+ supply. The complaint is real.
Step 2 — Compressor: Clamp shows the compressor pulling near nameplate RLA. It's running, so not an electrical no-start.
Step 3 — Valve ports: Indoor-coil port is hot, outdoor port cold — the valve is in heating. Coil reads 0V in heat as expected. Valve is doing its job.
Step 4 — Gauges: Suction pressure low, subcooling at 3°F (target ~10°F), superheat elevated. Sight glass flashing bubbles. Classic undercharge.
Step 5 — Leak search: Electronic detector hits at a rubbed line-set joint against the outdoor unit's corner. Repair, evacuate to 500 microns, and weigh in the nameplate charge.
Result: undercharge, not the valve
With charge restored, subcooling came up to target, suction normalized, and supply air climbed to 96°F. The reversing valve was never at fault — the low-subcooling reading pointed straight to charge, and the aux strips explained the electric bill while the compressor limped along undercharged.
Don't just top it off
Adding refrigerant without finding the leak is a callback waiting to happen and, on systems using A2L refrigerants like R-454B, a safety issue. Any refrigerant recovery, evacuation, or charging requires EPA 608 certification, and A2L work adds ventilation, no-ignition, and leak-detection requirements. Fix the leak, then charge.
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