Heat Pump Not Heating in Cold Weather: A Field Technician's Diagnosis Guide

The Four Most Common Heat Pump Heating Failures

When a heat pump fails to heat adequately in cold weather, there are four likely causes — and they present differently. Getting to the right one fast matters on a cold call.

1. Reversing valve not shifting to heating mode
2. Defrost cycle locked on or cycling too frequently
3. Low refrigerant charge (reads differently in heating mode than cooling)
4. Auxiliary/emergency heat sequencing issue

Diagnosing a Reversing Valve Problem

The reversing valve shifts the refrigerant flow direction between heating and cooling modes. When it fails to shift, the unit essentially runs in cooling mode regardless of the thermostat call.

Quick field test without gauges: In heating mode, feel both refrigerant lines at the outdoor unit. The suction line (large, insulated) should be warm — not hot, distinctly warm. The liquid line (small, uninsulated) should be cool. If both lines are the same temperature, the valve isn't shifting.

To confirm: disconnect the reversing valve solenoid wire and reconnect it rapidly several times while the unit runs. This sometimes dislodges a mechanically stuck valve. If it shifts intermittently, the valve is failing mechanically. If it never shifts, test the solenoid coil — should read 24–28 ohms.

Also verify you're getting 24V to the solenoid on a Y1 call. Some thermostat wiring jobs omit the O/B wire or configure it incorrectly, which means the valve never gets the shift signal.

Defrost Cycle Problems

A unit stuck in defrost means the defrost termination sensor isn't reading the coil correctly, or the defrost board has failed. Test the coil temperature sensor resistance against the manufacturer's NTC thermistor chart — most residential systems use a 10K NTC sensor at 77°F. A sensor reading colder than actual coil temperature will keep the board in defrost indefinitely.

If defrost cycles too frequently, check the defrost initiation timer settings. Some boards allow field adjustment; others are fixed. Extended cold spells can also cause legitimate over-frosting if the system is undersized for the load at design conditions.

Low Charge in Heating Mode

Low refrigerant reads differently in heating mode than cooling mode. In heating mode, the outdoor coil is the evaporator. Low charge means low suction pressure — which means the outdoor coil gets colder than ambient and frosts aggressively.

With gauges: in heating mode at 35°F outdoor ambient, typical suction saturation temperature should be around 20–25°F on R-410A. If you're seeing saturation temperatures 10–15°F below ambient, you're likely short on charge. Subcooling at the liquid line service valve is your most reliable indicator — target 8–12°F depending on the system.