Heat Pump Service and Defrost Cycles

It is 18°F outside, cold air is coming from the registers, and the outdoor unit is locked in a block of ice. That does not mean the heat pump is finished. It means the defrost sequence failed. Heat pump service comes down to knowing how the reversing valve, defrost board and backup heat work together, and what normal looks like in winter. This guide covers those parts, plus charge checks and the most common calls.

What You Will Be Able to Do

• Explain how a reversing valve shifts and how brands differ.
• Follow a defrost sequence and find where it fails.
• Tell auxiliary heat from emergency heat.
• Find a home's balance point.
• Verify charge without misreading heating-mode numbers.
• Replace a burned-out compressor correctly.

The Reversing Valve

The reversing valve is a four-port valve near the compressor. One port connects to the compressor discharge, one to the suction side, one to the outdoor coil and one to the indoor coil. Refrigerant pressure pushes a slide inside it. The small solenoid only opens a pilot port, so the valve needs pressure difference to shift. A valve test on a system that is off or very low on charge tells you nothing.

Always check the wiring diagram. A wrong thermostat setting makes the system heat when asked to cool.

Stuck mid-stroke. The slide jams halfway, and discharge gas reaches both coils. Suction rises, discharge falls, capacity drops sharply and the air is lukewarm. A firm tap on the valve body while the compressor runs may free it once. Expect it to stick again, and plan on replacing the valve.

The Accumulator

An accumulator sits on the suction side of the compressor. It catches liquid refrigerant before it reaches the compressor. Liquid does not compress, and a flooded compressor can break valves. In cold weather, the outdoor coil may not boil all the liquid, so this part is important. It feeds vapor to the compressor along with a small amount of oil through a metering hole.

Metering Devices

Heat pumps have a metering device for each direction of flow. Common types are TXVs, EEVs and fixed pistons. Check valves or bypass paths let refrigerant pass the idle one.

Auxiliary and Emergency Heat

Backup heat is usually electric strips in the air handler. A 5 kW bank adds about 17,000 BTU/h. Sequencers bring banks on one at a time so they do not hit the panel at once. Dual-fuel systems use a gas furnace instead. Emergency heat costs far more to run, so warn the customer.

The Defrost Cycle

In heating, the outdoor coil runs colder than the air. Below about 45°F, moisture can freeze on it. Ice insulates the coil and blocks air, so suction pressure falls and capacity drops.

When Defrost Starts

Time and temperature boards use an accumulated runtime interval, usually 30, 60 or 90 minutes, set by a jumper. A coil sensor must also read cold, about 32°F or lower. Defrost ends when the coil reaches about 55 to 70°F, or at a time limit, often 10 minutes.

Demand defrost boards compare outdoor air temperature with coil temperature. When the difference grows, ice is building, so they start a defrost only when needed.

The defrost sensor is clipped to the outdoor coil, usually low on the coil where ice builds most.

The Sequence

1. The valve shifts to cooling, so hot gas enters the outdoor coil.
2. The outdoor fan stops, so the coil heats up quickly.
3. Indoor backup heat comes on, so cold air does not reach the home.
4. Ice melts for 5 to 10 minutes.
5. The board ends defrost, the valve shifts back and the fan restarts.
6. Backup heat drops off if it is not needed.

Customers may hear a loud whoomp and see white vapor rise from the unit every 30 to 90 minutes in cold weather. That is normal. Tell them during installation so they do not call.

Defrost Faults

Warning: Do not use a hammer or tool on an iced coil. You can puncture a tube. Use the defrost cycle, warm water if the manufacturer allows it, or backup heat while the unit thaws.

Balance Point

The balance point is the outdoor temperature where the heat pump's capacity equals the building's heat loss. Above it, the heat pump can carry the home alone. Below it, auxiliary heat is needed.

Worked example: a home loses 30,000 BTU/h at 30°F. A heat pump delivers 30,000 BTU/h at that same temperature. Its balance point is 30°F. At 20°F, the home needs 36,000 BTU/h and the heat pump delivers 26,000 BTU/h, so backup heat must supply about 10,000 BTU/h.

A standard heat pump in a mid-climate home often has a balance point around 32 to 38°F. An outdoor thermostat or smart control can lock backup heat out above a set temperature.

Cold-Climate Heat Pumps

Cold-climate heat pumps use variable-speed inverter compressors and often enhanced vapor injection. These keep capacity up at low temperatures, with many models still running well below 0°F. Their balance points can be 5 to 15°F or lower. Their inverter drives mean pressures move around, so follow the manufacturer's service mode.

What Normal Looks Like in Heating

Supply air of 85 to 105°F feels cool on your hand, because skin is warmer than the air at that point. If the air is in that range, explain it. Do not add refrigerant.

A discharge line far above normal in mild winter weather points to low charge or a restriction.

Checking Charge

Heating-mode numbers are hard to read. Superheat often runs high in heating even on a correct charge, so high superheat alone does not mean the system is low.

1. If it is warm enough, usually 65°F and rising, run the system in cooling mode and use the cooling chart. Charge a TXV or EEV system by subcooling. Charge a piston system by superheat.
2. If it is too cold, check heating subcooling at the liquid line leaving the indoor coil.
3. Compare compressor amps with the nameplate.
4. After a full recovery, weigh in the charge from the manufacturer's charge card. This is the correct way to set charge in winter.

Do not add refrigerant to a heat pump in heating mode based on superheat alone.

EPA Section 608 Type II covers these systems.

Common Service Calls

Low airflow in heating acts the opposite way from cooling. The indoor coil is the condenser, so head pressure climbs.

Replacing a Burned-Out Compressor

A burnout spreads acid and carbon through the system. The accumulator holds that contaminated oil, so leaving it behind can kill the new compressor.

1. Recover the refrigerant, and treat the oil as acidic.
2. Replace the compressor.
3. Replace the accumulator and the filter-drier.
4. Check the metering devices and reversing valve for debris.
5. Pressure test with nitrogen, then evacuate to 500 microns.
6. Weigh in the charge.
7. Test the oil or the drier for acid after a period of running, and change the drier again if needed.

Before You Leave a Heat Pump Call

1. Confirm emergency heat is off.
2. Run the system through at least one defrost, or force one if the manual allows it.
3. Confirm backup heat energizes in defrost.
4. Confirm the thermostat's O/B setting.
5. Make sure the unit is clear of snow and drains well.

Common Mistakes

Check Your Understanding

It is 25°F. A TXV system shows 18°F superheat and 10°F subcooling at the indoor coil outlet. Does it need refrigerant? Not based on that. Superheat can run high in heating, and subcooling is in range.

A time and temperature board is set to 60 minutes. The coil sensor reads 45°F. Will the board start defrost? No. The sensor has to read cold, around 32°F or below.

The outdoor fan keeps running during defrost, and the system trips on high pressure. Where do you look? The defrost board's fan output, because the fan should stop during defrost.

A Rheem unit heats when set to cool. The thermostat is on O. What do you change? Change the setup to B, since Rheem energizes the valve in heating.

Key Takeaways

• The reversing valve needs pressure difference to shift, and brands use different conventions.
• Defrost shifts the valve, stops the outdoor fan and brings on backup heat.
• Aux runs with the compressor, and emergency heat locks it out.
• The balance point is where capacity equals heat loss.
• High superheat in heating does not prove low charge, so use subcooling, the manufacturer's data or a weigh-in.
• After a burnout, replace the accumulator and filter-drier along with the compressor.