Heat Pump Blows Cold Air In Heat Mode Decision Tree

Why this matters

Most "blowing cold in heat mode" calls land in three buckets: defrost cycle running (normal but mistaken for failure), reversing valve stuck in cool position (real failure), or no compressor running with the air handler in fan-only because the system flipped to emergency heat with the strips not energized. The buckets sort cleanly with three observations and a thermometer. Get the wrong bucket and you either replace a reversing valve that didn't need it, or you tell the customer "that's normal" when the system is actually broken.

Symptom presentation

Customer complaint: "Heat doesn't work" or "Cold air from the vents in heat mode." System is running - air is moving - but supply temperature is at or below room temperature. May be intermittent (defrost), continuous (reversing valve), or only on cold mornings (control logic, charge migration). Verify the thermostat is actually set to Heat (not Auto with cool active) before going further.

Quick checks (under 2 min)

Step 1: Outside, observe the outdoor unit:

• Compressor running, fan running, frost on coil? Probably in defrost cycle. Watch for 2 to 10 minutes; system should exit defrost and return to heating.
• Compressor running, fan running, no frost, condenser exhaust feels warm? System is in cool mode despite thermostat call for heat - reversing valve issue.
• Compressor running, fan running, condenser exhaust feels cold? System is in heat mode, problem is downstream (charge, indoor airflow, indoor coil).
• Compressor not running? Different decision tree - go check 24 VAC at the contactor.

Step 2: Indoor: measure supply air temperature with a digital thermometer at a register near the air handler. Compare to room air temperature.

• Supply at room temp +/- 5 F = no heat delivery, system effectively running fan only.
• Supply at room temp - 10 F or colder = system is cooling, reversing valve issue.
• Supply at room temp + 15 to 30 F = system is heating, customer expectation problem (heat pumps deliver lower supply temperatures than gas furnaces; 90 to 105 F at the register is normal).

Step 3: At the thermostat, check if "Auxiliary" or "Emergency Heat" indicators are on. Emergency heat = compressor disabled, strips only. Auxiliary heat = compressor + strips. Neither = compressor only.

Isolation tree

Branch 1: Defrost cycle. Normal behavior. Heat pumps reverse to cool mode briefly (5 to 10 minutes typical) to melt frost from the outdoor coil. During defrost: outdoor fan stops, indoor fan continues, electric strips should energize to temper supply air. Customer-side education plus a check that strips are actually firing during defrost (clamp the strip leg at the air handler, expect 20 to 40 amps depending on strip kW). If strips don't fire during defrost, the strip relay, sequencer, or the defrost board's strip command is failed.

Branch 2: Reversing valve stuck in cool position. Confirmed by:

• Outdoor unit exhaust warm in "heat" mode (when it should be sucking heat from outside, exhaust is cold).
• Indoor coil cold to the touch.
• Suction pressure lower than expected for outdoor ambient, discharge pressure lower than expected for indoor temp. Causes and fixes:
• Stuck solenoid. Energize the reversing valve solenoid (typically 24 VAC, B or O signal depending on system convention). If valve doesn't shift, tap the body with a wrench while energized to free a stuck pilot.
• Failed solenoid coil. Read resistance across the coil; open or shorted coil = replace coil (no system pumpdown required).
• Failed pilot valve internal to the reversing valve. Confirm with temperature checks at each of the four ports (per OEM service manual). If pilot is failed, replace reversing valve (pumpdown, braze under nitrogen, evacuate to 500 microns, recharge by weight).

Branch 3: O / B signal not being sent by thermostat. Honeywell convention: O = energized in cool. Trane convention: B = energized in heat. Mismatch between thermostat config and equipment will leave the reversing valve in cool position during heat call. Check thermostat installer settings, correct to match the outdoor unit's convention (read the data plate or outdoor board label).

Branch 4: Heat pump on but suction pressure dropping toward zero. Low charge, restricted indoor airflow, or a clogged liquid line filter drier. In heat mode the evaporator is outside, the condenser is inside, so a low-charge symptom presents as cold indoor supply because the indoor coil isn't getting enough hot refrigerant. Charge by subcool at the outdoor liquid line (in heat mode, this is the inlet side of the outdoor coil). Confirm indoor airflow first - dirty filter, plugged coil, undersized return all starve the system the same way.

Branch 5: Aux strips not staging in. On a call for second-stage heat (room temperature significantly below setpoint, or outdoor temp below balance point), the strips should energize. If they don't:

• W signal not being sent by thermostat (check thermostat staging settings).
• Sequencer or strip relay failed (read 24 VAC on coil, 240 VAC on load side).
• Open thermal limit on the strip element.
• Strip element burned out (clamp meter shows 0 amps when W energized; resistance check the strip).

Branch 6: Refrigerant migration on cold mornings. On below-freezing starts, refrigerant migrates to the cold outdoor coil overnight. The system needs to pump the charge back before it can heat. Crankcase heater failure makes this worse and you'll see prolonged "cold blow" on morning startups. Confirm with crankcase heater amp draw (typically 0.2 to 0.4 amps); replace heater if open.

Confirming the diagnosis

After repair, run a complete heat cycle:

• Outdoor exhaust cold (heat being extracted from outside air).
• Indoor supply 30 to 45 F above return at steady state.
• Defrost cycle initiates and exits cleanly within OEM-spec interval.
• Aux strips stage in on second-stage call and energize during defrost.

Repair / remediation

• Stuck reversing valve solenoid: tap or replace coil (no pumpdown for coil only).
• Failed reversing valve body: pumpdown, braze new valve under nitrogen purge at 2 to 5 psig, replace drier, evacuate, recharge by weight per data plate.
• Wrong O/B thermostat config: 5-minute fix at the thermostat installer menu.
• Failed sequencer or strip relay: match voltage and amp ratings; install new.
• Crankcase heater: match wattage, mount per OEM.

References

• AHRI Standard 210/240 - Performance Rating of Unitary Air-Conditioning Equipment
• ASHRAE Handbook - HVAC Systems and Equipment, heat pump chapter
• ACCA Manual S - Equipment Selection (balance-point calculation)
• OEM service manuals - reversing valve test procedures per model