Heat Pump Blowing Cold Air in Heat Mode: What's Normal and What's Not

Start by working out whether the customer is describing a fault or normal heat pump behavior. Then measure.

What's normal

• Supply air feels lukewarm. Heat pump supply air is cooler than a gas furnace's, and it drops as outdoor temperature falls. Air in the 80s or 90s °F can feel cool on the skin because it's below body temperature.
• Defrost cycles. In cold, humid weather the outdoor coil frosts. The system periodically switches to cooling mode to melt it, so the outdoor fan stops and you hear a whoosh and see steam. Many systems turn on auxiliary heat during defrost to temper the supply air. If aux heat isn't wired or configured for defrost, the customer feels cool air for a few minutes.

Measure first

• Supply and return temperatures and the temperature rise.
• Outdoor temperature.
• Whether the outdoor unit is running, iced, or in defrost.
• Whether aux heat is energized (amps on the heat strips).

A system that's working in heat mode should show a clear temperature rise across the indoor coil. A rise near zero with the compressor running points to the problems below.

1. Reversing valve or O/B setup

• Check the thermostat setting: most brands energize the reversing valve in cooling (O), while some, notably Rheem/Ruud, energize it in heating (B). A mismatch swaps heating and cooling.
• Check the valve: with the system in heat, confirm the solenoid gets 24 V when it should (or doesn't when it shouldn't). Feel or measure line temperatures: in heating, the large vapor line to the indoor coil should be hot. A valve stuck midway gives poor capacity in both modes.

2. Low refrigerant charge

An undercharged heat pump loses heating capacity, can ice the outdoor coil heavily and can trip on low pressure. Diagnose it with the manufacturer's heating charge chart. Many manufacturers want the charge checked or weighed in cooling mode, so follow the data plate and installation instructions.

3. Outdoor coil iced over

A coil that stays iced points to a defrost problem: a failed defrost sensor or thermostat, a defrost board fault, a reversing valve that doesn't shift, or an outdoor fan relay that doesn't drop out. Also check for water dripping on the unit from a gutter, or snow blocking airflow.

4. Auxiliary or emergency heat not working

Below the balance point, a heat pump alone can't keep up, and aux heat makes up the difference. If the heat strips aren't working (tripped breakers, failed sequencers or elements, a thermostat not configured for aux), supply air feels cool and the house loses ground on cold days.

5. Low airflow

A dirty filter or coil lowers airflow, raises head pressure in heating and can trip the high-pressure switch. Measure static pressure.

FAQ

What supply temperature should a heat pump put out in heat mode?

It varies with outdoor temperature and equipment. Supply air in roughly the 85–105 °F range is common in moderate weather, and it falls as it gets colder outside. Check against the manufacturer's performance data.

Why does a heat pump blow cold air for a few minutes and then warm up?

That's usually a defrost cycle, or the warm-up after one. If aux heat isn't set to come on during defrost, the indoor air feels cool until defrost ends.

Is O or B the right setting?

Use whatever the equipment manufacturer specifies. Most brands use O (energized in cooling); Rheem and Ruud have traditionally used B (energized in heating). Wiring the wrong one reverses heating and cooling.

Sources

• HVAC School: technical tips and diagnostics