Heat Pump Defrost Cycle Reference

Why this matters

In heat mode, the outdoor coil acts as the evaporator - pulling heat from cold outdoor air, the coil temperature drops below outdoor dew point, and ice accumulates. Without periodic defrost, the coil ices over and capacity drops to near zero. The defrost cycle is critical to heat pump operation; understanding what triggers it and how it sequences is the difference between diagnosing "heat pump not working in winter" correctly vs replacing a board that wasn't bad.

Why ice forms

The outdoor coil in heat mode is colder than outdoor air. When outdoor temperature is 30-40 °F with 70% RH, water vapor condenses on the coil. Below 32 °F coil surface temperature, it freezes. Ice insulates the coil from the airstream → heat transfer drops → capacity drops further.

Below outdoor temperature of about 40 °F, defrost cycles become regular. Below 20 °F, defrost is needed every 30-60 minutes. Below 5 °F (or even lower for cold-climate hyper-heat units), defrost frequency can outpace heating output and the system practically stops working for resistive backup.

Two main defrost-initiation methods

Time-temperature (legacy, 1970s-2010s):

• Initiated by a timer (every 30, 60, 90, or 120 min selectable on the defrost board) AND a defrost thermostat reaching a low set-point
• Board accumulates compressor run time; at the set interval, checks defrost sensor. If sensor reads below ~30 °F, initiates defrost.
• Simple, reliable, but defrosts even when not needed (e.g., 35 °F outdoor air dry, no ice formed - still defrosts at the timer interval, wasting energy and dropping supply air temp).

Demand defrost (modern, 2010+):

• Uses multiple sensors: outdoor coil temperature, outdoor ambient, compressor run time, sometimes pressure differential
• Initiates only when actual icing conditions are detected
• More efficient - only defrosts when needed
• Industry-standard on most modern equipment (Carrier Infinity, Trane XV, Bryant Evolution, Lennox SignatureStat)

The defrost cycle sequence (reverse-cycle defrost)

The most common defrost method temporarily switches the heat pump back into cooling mode (sending hot refrigerant to the outdoor coil to melt the ice). Step by step:

1. Defrost board initiates - sensors satisfied, conditions met.
2. Reversing valve solenoid energizes (or de-energizes, depending on O vs B convention) - refrigerant flow reverses.
3. Outdoor fan stops - without airflow, the now-condenser coil heats up quickly (the outdoor air would just blow heat away).
4. Auxiliary heat strip (or backup heat) energizes - supply air to the home is now cold (the indoor coil is now an evaporator pulling heat from indoor air). Backup electric strips or gas furnace warms the supply air.
5. Hot refrigerant flows through outdoor coil - coil temperature rises, ice melts. Customer sees steam / mist coming off the unit, sometimes pools of water around base.
6. Termination occurs when:

• Coil sensor reaches termination temperature (typically 60-90 °F outdoor coil)
• OR maximum defrost timer expires (typical 8-12 minutes hard limit)
• OR pressure-based termination on advanced boards

1. Reversing valve de-energizes (or energizes) - back to heat mode
2. Aux strip de-energizes - supply air gradually returns to warm
3. Outdoor fan restarts after a brief delay (purges any drips)

Total defrost: 4-12 minutes typically. Customer sees: outdoor unit fan stops, sometimes "whoosh" of refrigerant through reversing valve, steam/mist from coil, then fan restarts and unit returns to heat mode.

What a normal defrost looks like to the customer

• Outdoor fan runs for 30-90 minutes (heating)
• Outdoor fan stops, defrost begins
• 4-10 minutes of: outdoor unit quiet (no fan), steam visible, water dripping/draining from coil
• Outdoor fan restarts, normal heating resumes

If a customer thinks the unit is broken because the outdoor fan stopped - it's defrost. Normal.

Common defrost-related problems

No defrost ever (coil ices completely):

• Defrost board failed
• Defrost sensor (coil thermistor) open / disconnected
• Time/temp wrong - frequency set too high or temp set too low
• Customer hears unit running but no heat coming out → coil is iced over

Constant defrost (unit goes in/out of defrost repeatedly):

• Defrost sensor shorted (always reads cold)
• Board failed in defrost-trigger mode
• Refrigerant low (suction pressure low, coil running too cold even when warmer outdoor)

Defrost doesn't terminate (long-cycle defrost):

• Termination sensor failed (never reaches termination temp)
• Termination temp set wrong on board
• Refrigerant overcharged (high pressure side never lets coil get warm enough)

Aux heat strips not energizing during defrost:

• Strip relay failed
• W2 / aux call from board not reaching strips
• Customer experiences "cold blast" from registers during defrost (15-30 sec of cold supply air normal; 2+ minutes is the strips not firing)
• Real safety issue: without aux during defrost, the indoor coil pulls heat from indoor air = home temperature drops

Pool of ice under outdoor unit:

• Defrost is happening but drainage is poor → water pools, freezes
• Pad needs elevation (4-6" above grade with gravel below)
• Coil drain pan ice damming - check the drain holes
• Severe case: pad shifts, condenser tilts, drain doesn't function

Diagnostic procedure

1. Watch a heat call from start to finish. Time the heating run, watch for defrost initiation. Time the defrost.
2. Measure coil temperatures during operation:

• Top of outdoor coil
• Bottom of outdoor coil
• Refrigerant line at coil (suction)

1. Note when defrost initiates and terminates - start time, end time, conditions during.
2. If defrost never initiates: check defrost board with manual-defrost button (most boards have one - press it, defrost should start immediately). If yes, board works but the trigger conditions aren't being met (check sensor, settings).
3. If defrost is constant: disconnect the defrost sensor. Manual-defrost: board should still trigger on the timer alone. If it does, defrost sensor is the problem.

Cold-climate heat pumps (hyper-heat)

Newer cold-climate units (Mitsubishi H2i, Carrier Infinity Greenspeed, Daikin Aurora):

• Variable-speed compressor - can ramp up to maintain capacity below 5 °F
• Enhanced refrigerant management - vapor injection
• Smarter defrost - less frequent, faster termination
• Some don't switch to backup until -15 °F or colder

These units have proprietary defrost algorithms that the field tech doesn't tune; they're driven by the equipment's main controller. Diagnostic is mostly verify-the-sensor and verify-the-comm-link.

References

• Manufacturer service manuals (Carrier, Trane, Lennox, Goodman, Rheem, Mitsubishi) - for unit-specific defrost timing, termination temps, and board layout
• ACCA Manual S (Equipment Selection - heat pump sizing in cold climates)
• ASHRAE Handbook - HVAC Systems and Equipment (heat pump operation and defrost)
• AHRI 210/240 (rating standards including heating performance)