Heat Pump Cools Fine but No Heat After Reversal: Decision Tree

Why this matters

A heat pump that cools normally but blows neutral or cool air in heat mode has a reversing-valve or reversal-control fault, not a refrigerant fault, because the refrigerant circuit that worked in cooling is the same circuit. The split between "cools fine, no heat" and "no heat at all" is the diagnostic gift: it tells you the compressor, charge, and metering are healthy and the problem is in the reversal command, the valve solenoid, or the valve body itself. Techs lose hours chasing charge on these calls. This tree isolates the reversing valve, the O/B thermostat wiring, the defrost board's valve output, and a valve stuck mid-stroke, in the order that wastes the least time and refrigerant.

Symptom presentation

The clean version: in cooling the system makes cold air and meets setpoint. Switched to heat (or on a cold morning), the compressor runs but supply air is room-neutral or only slightly warm, suction and discharge pressures look like cooling (low suction, high discharge on the wrong sides), and indoor temperature does not rise. Often the only "heat" is from electric strips or gas backup masking the failure.

Variants that change the tree:

• Air is warm at first then goes cold mid-cycle: valve slipping toward cooling under pressure, or a stuck defrost.
• No heat and compressor will not run: control or safety lockout, a different tree.
• Heats fine until it goes into defrost and never recovers: defrost-termination or valve fault on the defrost stroke.
• Brand-new install never heated: O/B wiring polarity set wrong at the thermostat.

Quick checks before gauges

Confirm the thermostat O/B output. Most heat pumps energize the reversing-valve solenoid in cooling (O) or in heating (B) depending on manufacturer. A thermostat configured for the wrong O/B convention leaves the valve in cooling position during a heat call. Meter 24 VAC at the valve solenoid in heat mode and compare to what the manufacturer expects for that valve (Carrier/Bryant energize in cooling; many others vary). This single check resolves a large share of "never heated since install" calls.

Feel the valve body and the four tube connections with the unit in a heat call. A correctly switched valve has the hot discharge feeding the indoor coil; a valve stuck in cooling sends discharge outdoors. The pipe temperatures tell you valve position without gauges.

Isolation tree

Branch 1: reversal command (control side).

• No 24 VAC at the solenoid when heat is called, and the manufacturer expects it energized: trace from thermostat O/B through the defrost board valve terminal. Open wire, miswired O/B, or a defrost board with a dead valve output.
• 24 VAC present at the solenoid but the valve does not shift: solenoid coil open (measure resistance, compare to spec) or pilot port plugged.

Branch 2: solenoid and pilot valve.

• Solenoid coil reads open or far off spec ohms: replace the coil only; the valve body may be fine.
• Coil good and energized, valve still in cooling: pilot bleed ports gummed or the pilot piston stuck. A light tap on the valve body while switching sometimes frees a sticky pilot, confirming a mechanical hang rather than a control fault.

Branch 3: valve body stuck or leaking internally.

• Valve shifts but heat is weak and discharge line and suction line are abnormally close in temperature: internal valve bypass, the slide is not seating, hot gas leaks across to suction. Confirm with a temperature gradient across the valve ports; a leaking valve runs the permanent suction line warm.
• Sufficient pressure differential is required to complete the shift. Very low charge or a compressor that lost differential can leave a healthy valve mid-stroke. Verify charge by subcool in cooling before condemning the valve.

Branch 4: defrost interaction.

• Heats fine then loses heat after a defrost: defrost board not de-energizing the valve correctly, or the valve hanging on the defrost stroke. Force a defrost (jumper the board test pins per the manual) and watch the valve and the defrost termination thermostat.

Confirming the diagnosis

After repair, run a full heat call and verify the indoor coil becomes the condenser: high-side pressure on the indoor coil, rising supply air temperature, and a normal heating temperature split (typically 20 to 30 F rise across the indoor coil at moderate outdoor temperature). Then force one defrost and confirm the valve shifts and recovers heat cleanly. Log solenoid voltage, valve port temperatures, and the heating split. A valve replacement is justified only when the coil, charge, and control command are all confirmed and the body still bypasses.

References

• ASHRAE Handbook HVAC Systems and Equipment, Chapter on Unitary Air Conditioners and Heat Pumps - reversing-valve operation and O/B logic.
• AHRI 210/240-2023 - heat pump heating capacity rating conditions.
• OEM heat pump service manuals (Carrier/Bryant 38/25 series, Trane/American Standard) - O/B convention and defrost board valve output wiring.
• ANSI/AHRI 1230 (where applicable) - VRF and multi-split reversing operation.