Heat Pump Service and Defrost Cycles

It is 18 degrees outside, the homeowner called because "the outside unit is running but cold air is blowing out of the vents," and you pull into the driveway to find the condenser locked up in a block of ice. This is not a broken heat pump. This is a heat pump that failed its defrost cycle, and the difference between a quick fix and a three-hour diagnostic is whether you understand how the reversing valve, defrost board, and auxiliary heat strips dance together to keep the outdoor coil clean.

A heat pump is not a furnace. It is an air conditioner that runs backward, and everything strange about it - the odd sounds every 40 minutes, the burst of white vapor off the outdoor unit, the electric bill that spikes below freezing - makes sense once you see the refrigeration cycle in both directions.

The Refrigeration Cycle in Both Directions

Start with cooling mode, the one you already know:

• Compressor pumps low-pressure vapor into high-pressure, high-temperature vapor.
• Outdoor coil (the condenser in cooling) rejects heat to outdoor air, vapor condenses to liquid.
• Metering device (TXV, EEV, or fixed orifice/piston) drops pressure.
• Indoor coil (the evaporator in cooling) absorbs heat from indoor air, liquid boils to vapor.
• Back to the compressor.

Now flip the reversing valve and run heat mode:

• Compressor pumps low-pressure vapor into high-pressure, high-temperature vapor.
• Indoor coil (now the condenser in heating) rejects heat to indoor air, vapor condenses to liquid. This is your heat source.
• Metering device drops pressure.
• Outdoor coil (now the evaporator in heating) absorbs heat from outdoor air, liquid boils to vapor.
• Back to the compressor.

The compressor never reverses direction. The refrigerant never changes flavor. Only the direction of flow through the indoor/outdoor coils changes, and the only part that does the changing is the 4-way reversing valve.

How a 4-Way Reversing Valve Works

Bolted to the side of most heat pumps, the reversing valve is a fat brass body with four copper stubs: one to the compressor discharge, one to the compressor suction (via the accumulator), one to the outdoor coil, one to the indoor coil. Inside are a main slide and a pilot tube assembly.

The slide is pushed back and forth by refrigerant pressure, not by the solenoid. The solenoid on top is a tiny pilot valve: when energized, it opens a port that vents one end of the slide to the low-pressure side, letting high-pressure discharge gas on the opposite end shove the slide over. No pressure differential, no slide movement - which is why a reversing valve test on a system that is off or low on charge tells you nothing.

Convention on most brands (Carrier, Trane, Lennox, Goodman, York):

• Solenoid energized = cooling mode (O terminal drives the valve on a call for cool).
• Solenoid de-energized = heating mode (fail-safe rest position).

Rheem and Ruud flip the convention in many models - the solenoid is energized in heating and the thermostat's B terminal drives the valve. Always look at the manufacturer's wiring diagram printed inside the service panel before assuming. A mismatched thermostat wire (O for Carrier/Trane, B for Rheem/Ruud) running to the wrong terminal will give you a system that tries to heat in summer and cool in winter.

Stuck mid-stroke is the worst failure mode. The slide jams halfway and both the indoor and outdoor coils get a fraction of discharge gas, the suction pressure climbs, the discharge pressure drops, superheat goes insane, and you get lukewarm air on whatever call you are on. Tapping the valve body firmly with a rubber mallet while the compressor is running will often free it once. After that, expect the call back.

The Accumulator

Downstream of the reversing valve's suction stub is a tall vertical can called the accumulator. In heat mode, if outdoor temperatures drop and the outdoor evaporator cannot fully boil the liquid refrigerant, you get liquid floodback toward the compressor. Liquid does not compress. A flooded compressor breaks valves or hydraulically locks up.

The accumulator catches that liquid, lets it boil off slowly through a small bleed hole at the bottom, and feeds only vapor (plus a trickle of oil) to the compressor. Heat pumps have accumulators; straight cool condensers often do not. If you replace a compressor on a heat pump and leave the old accumulator, you just filled the new compressor with the old compressor's acid and debris. Replace the accumulator and filter-drier every time you open a burned-out heat pump.

TXV vs Fixed Orifice

Most modern heat pumps ship with a thermostatic expansion valve (TXV) or an electronic expansion valve (EEV) on both the indoor and outdoor sides. A TXV modulates refrigerant flow to maintain a constant superheat at the evaporator, so the system self-adjusts to load changes and charge variations within a narrow band.

Older or builder-grade heat pumps use a fixed orifice - a bullet piston with a precision bore. A fixed orifice does not modulate. Charge is critical: too much and the evaporator floods, too little and the compressor runs hot with high superheat. Charging a fixed orifice heat pump means reading the manufacturer's charging chart in cooling mode using superheat, because superheat on the indoor coil is set by the piston and the load, not by a valve.

Rule of thumb:

• TXV / EEV system: charge by subcool (8-12 deg F typical) in cooling mode.
• Fixed orifice / piston system: charge by superheat in cooling mode per the manufacturer's chart.
• Never try to charge a heat pump in heat mode as the primary method unless you have the manufacturer's heat-mode chart. Charge in cooling whenever outdoor temperature allows (65 deg F and rising).

Auxiliary Heat Strips

A heat pump alone cannot always keep up with a house's heat loss on the coldest nights. That is what the electric resistance heat strips in the air handler are for.

Typical strip banks:

• 5 kW single bank (~17,000 BTU/hr at 240V).
• 10 kW (two 5 kW banks).
• 15 kW, 20 kW (three or four banks) in larger or colder-climate units.

Each bank is wired through its own sequencer (time-delay relay) or contactor so they do not all hit the meter at once. On the thermostat side:

• W1 - first-stage auxiliary heat call.
• W2 - second-stage auxiliary.
• E - emergency heat (often jumpered to W1 in most smart thermostats).

When the thermostat senses the indoor temperature is drooping below setpoint while the compressor is already running heat, it closes W1 and staggers on the strips. During a defrost cycle, the defrost board energizes the strips regardless of the thermostat call to temper the cold air blowing out of the registers while the outdoor coil is being de-iced. If you do not energize strips during defrost, the customer feels 55 deg F air out of the vents and calls you.

Emergency heat switch at the thermostat locks out the compressor completely and uses only strips. Use it when the compressor is dead or the outdoor unit is iced solid and you need to keep the house warm until you can return with parts. Warn the customer the electric bill will spike.

The Defrost Cycle

In heat mode the outdoor coil runs colder than outdoor air (that is how it absorbs heat). Below about 45 deg F outdoor, surface moisture condenses and freezes on the coil fins. Ice insulates the coil, reducing heat transfer, dropping suction pressure, and eventually bricking the unit. The defrost cycle reverses the system to melt the ice.

Three defrost-board strategies:

1. Time and temperature (classic). A defrost thermostat clamped to the outdoor coil reads coil temp. When coil temp is below ~32 deg F and a time interval has elapsed (30, 60, or 90 minutes selectable via board jumper), defrost initiates. Terminates on coil temp reaching ~57 deg F or a 10-minute timeout.
2. Demand defrost (modern). Microprocessor monitors the temperature differential between outdoor air and coil. When the delta grows past a threshold (ice loading the coil), defrost initiates. Smarter, fewer unnecessary defrosts, better efficiency.
3. Pressure differential (commercial). Reads suction pressure drop across the coil or across the air stream to detect ice load. Rare in residential.

Defrost Sequence of Operation

When the defrost board decides it is time:

1. Reversing valve energizes (or de-energizes on Rheem/Ruud) to flip the system to cooling mode. Outdoor coil is now the condenser, indoor coil is the evaporator.
2. Outdoor fan shuts off. The coil needs to stay hot to melt ice; a running fan would drag cold outdoor air across it.
3. Auxiliary heat strips energize to temper the supply air. Without strips, the indoor air handler is pumping chilled air into the house while the outdoor coil de-ices.
4. Defrost runs typically 5 to 10 minutes, terminated on coil sensor reaching ~57 deg F or a timeout (usually 10 minutes).
5. Reversing valve returns to heating, fan comes back on, strips drop off (or cycle per W1 call), system resumes heat.

The telltale sign from outside: a burst of white vapor or light steam off the top of the condenser and a solid "whoomp" from the reversing valve switching. Customers call and report "smoke" or "it made a loud noise." That is normal defrost. Educate the customer during install.

Balance Point

The balance point is the outdoor temperature at which the heat pump's output exactly equals the house's heat loss. Above the balance point, the pump alone can hold setpoint. Below it, strips supplement. Below about 5-10 deg F below balance point, most standard heat pumps have dropped to about 50-60 percent of their rated capacity and strips are doing the bulk of the work.

Typical balance point for a standard heat pump on a standard-insulation 2000 sq ft house in a mid-climate market: 32 to 38 deg F. The colder the climate, the higher the balance point, the more strips run, the higher the electric bill.

Cold-Climate Heat Pumps

A new generation of cold-climate heat pumps (Mitsubishi Hyper-Heat, Bosch IDS, Carrier Greenspeed, Trane XV20i, Daikin Aurora) uses variable-speed inverter compressors and enhanced vapor injection (EVI) to stay above 75 percent of rated capacity down to 5 deg F and continue to operate - at reduced output - down to minus 15 deg F or colder. Balance points of 5 to 15 deg F are common. In cold climates these pumps displace natural gas as the economical heating choice, and federal / utility rebates often subsidize them.

Verifying Refrigerant Charge in Heat Mode

You cannot use standard cooling-mode delta-T or superheat charging tables in heat mode. What you can do:

• TXV/EEV systems: check subcool at the liquid line leaving the indoor coil (which is the condenser in heat mode). Target typically 8-12 deg F per the manufacturer's chart for the outdoor temperature.
• Discharge line temperature: with an outdoor temperature between 30 and 50 deg F, discharge line temperature should sit 170 to 230 deg F. Above 230 deg F signals low charge, restriction, or overworked compressor.
• Saturated suction temperature: should track about 20 to 25 deg F below outdoor ambient in heat mode. If saturated suction is 15 deg F below outdoor and falling, you are icing the coil.
• Current draw: compare compressor amps against RLA on the nameplate. Low amps with low suction = low charge. High amps with high head = overcharge or dirty coils.

If the outdoor temp allows (65 deg F and rising), run the system in cooling (switch thermostat mode) and charge to the cooling chart. That is always more accurate.

Typical Service Calls

Things that go wrong and how to read them:

• No heat, outdoor fan running, compressor running, cold air at registers. Reversing valve stuck in cooling position, or thermostat wired to the wrong O/B convention. Check voltage at the reversing valve solenoid against the wiring diagram.
• Outdoor unit iced solid, no defrost action. Defrost board failed, defrost thermostat open, or outdoor fan stuck on during defrost. Manually force defrost via the test pins on the board.
• Warm indoor temp swings, aux running constantly at 45 deg F outdoor. Low charge dropping capacity, or strip sequencer stuck closed. Check subcool, check strip amps without a W1 call.
• Short cycling on high pressure during defrost. Outdoor fan running during defrost (should be off), or strips not energized. Check defrost board outputs.
• Strips burning but house not warming. Compressor off (check contactor, capacitor, overload), thermostat locked in E-heat, or outdoor disconnect pulled. Walk the cabinet.
• Reversing valve buzzes but will not shift. Low charge (not enough pressure differential to move slide), solenoid coil open, or slide stuck. Check charge first, then solenoid continuity.

Day 1 Checklist

• [ ] Before touching wiring on any heat pump, pull the outdoor disconnect and lock it out. Verify zero volts at the contactor with a meter, not a non-contact tester.
• [ ] Identify the reversing valve convention for this brand (Carrier/Trane O-energized for cooling, Rheem/Ruud O-energized for heating). Confirm at the thermostat O/B terminal.
• [ ] Confirm emergency heat switch is off before running a heat-mode diagnostic, otherwise the compressor is locked out and you will chase a phantom.
• [ ] If customer reports ice on the outdoor unit, check defrost operation first - force a defrost via the defrost board test pins and watch the sequence.
• [ ] In heat mode, do not trust cooling-mode superheat charging charts. Use subcool, discharge line temp, and manufacturer heat-mode charts.
• [ ] Verify strip heat energizes during defrost (amp clamp on each strip's contactor). No strips = customer complaint of cold air during defrost.
• [ ] Before leaving, cycle the system through at least one full defrost and confirm heating resumes cleanly.

Expert Tips

• "Reversing valve convention changes by brand." O energizes for cooling on Carrier/Trane/Lennox. O energizes for heating on Rheem/Ruud. Read the diagram every time.
• "The defrost board is the brain. The reversing valve is the muscle. The strips are the painkiller." When the sequence is broken, trace it in that order.
• "Ice on the top of the coil means defrost is working. Ice on the bottom or around the base means defrost is failing." Location tells you the failure mode.
• "Subcool on TXV, superheat on piston." Memorize which system you have before you reach for gauges.
• "Strips during defrost are not optional." Without them, the customer calls about cold air and you lose the renewal.
• "Check the heat pump in cooling whenever possible." You have better charging references in cooling than in heat mode, period.