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HVAC Diagnostic Guide HIGH URGENCY

Heat Pump Not Defrosting

Heat pump outdoor unit not running defrost cycles — ice accumulates instead of being cleared.

Quick Check Before You Call a Pro

  1. Watch the unit for an hour — should defrost every 30-90 minutes in freezing weather
  2. Defrost cycle: outdoor fan stops, you hear refrigerant flow reverse (whoosh), steam rises as ice melts
  3. Check for ice buildup that doesn't clear after 90+ minutes of operation
  4. Inspect defrost sensor (small probe clipped to outdoor coil)

01 · Most Likely Causes (Ranked)

35%

Failed Defrost Control Board

What's happening

The board that times defrost cycles and monitors coil temperature has failed. No defrost commands ever issued, so ice accumulates indefinitely. Common after 7-10 years.

How to check

Visual inspection of defrost board (in outdoor unit). Burn marks, swollen capacitors, loose connections. Manual defrost button (if present) doesn't initiate cycle.

How to fix

Replace defrost control board. Match manufacturer part number. $80-200 part. Most are accessible from outdoor unit's electrical panel.

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25%

Failed Defrost Temperature Sensor

What's happening

Sensor on the outdoor coil tells the defrost board when ice is forming. Failed sensor = board never knows defrost is needed.

How to check

Locate sensor — small probe clipped to outdoor coil tubing with thermal paste. Damaged wires, loose mounting, or wrong resistance per manufacturer specs.

How to fix

Replace sensor. $20-50 part. Ensure good thermal contact with coil tubing using thermal paste or proper spring clip.

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18%

Stuck Reversing Valve

What's happening

Even if defrost board commands defrost, the reversing valve must physically switch to cooling mode briefly to send hot refrigerant outside. Stuck valve = no defrost.

How to check

Listen for the valve switching during defrost (clear 'whoosh' as refrigerant reverses direction). Silence = valve not switching. Tap with rubber mallet to attempt freeing.

How to fix

Tapping sometimes frees mechanically stuck valves. If solenoid failed or valve internals are damaged, replacement needed ($300-500 by tech).

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10%

Failed Defrost Termination Switch

What's happening

Some systems use a termination switch (separate from the defrost initiation sensor) to end defrost cycles when coil is clear. Failed switch can cause defrost to never start (if it's in the start-defrost circuit) or never end.

How to check

Tech diagnosis required. Multimeter testing of the switch at various temperatures per manufacturer specs.

How to fix

Replace termination switch. $30-80 part. Match by manufacturer part number.

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7%

Defrost Timer Set Too Long

What's happening

Some older units have manually-settable defrost timers (30, 60, 90 minutes between cycles). If set too long, ice accumulates before next defrost.

How to check

Older heat pump (15+ years). Defrost timer typically near the defrost control board. Should be set to 30 or 60 minutes in cold climates.

How to fix

Set timer to a shorter interval (30 minutes recommended for cold climates). Newer demand-defrost systems eliminate this issue.

5%

Outdoor Fan Running During Defrost

What's happening

During defrost, the outdoor fan should STOP to allow heat to melt the coil ice. If fan keeps running (failed relay or wiring issue), the heat is blown away and ice never melts.

How to check

During what should be defrost cycle (compressor running, reversing valve switched), check if outdoor fan continues spinning. Should stop.

How to fix

Defrost relay or wiring repair. Tech diagnosis required.

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Defrost BoardsReversing Valves

02 · Frequently Asked Questions

How often should a heat pump defrost?
Depends on outdoor temperature and humidity. Modern demand-defrost systems: only when sensors detect needed defrost. Older time-defrost: every 30, 60, or 90 minutes during operation in freezing weather. Cycle duration: 3-10 minutes typical.
Can I manually defrost my heat pump?
Temporary fix: switch to EMERGENCY HEAT mode for 4-6 hours, letting outdoor coil thaw naturally. Or carefully pour lukewarm (not hot) water over ice. After thawing, diagnose the defrost system failure.
What's the difference between time defrost and demand defrost?
Time defrost: runs defrost at fixed intervals (30/60/90 min) regardless of need. Inefficient but simple. Demand defrost: uses sensors to detect actual ice buildup, only defrosts when needed. More efficient, modern, but more complex (more failure points).
Why is my heat pump iced over but the inside is warm?
You may be running on aux heat (resistance strips) exclusively because the heat pump can't function with ice-blocked coils. Indoor stays warm because aux heat is doing all the work — but aux is expensive (2-3x cost). Diagnose defrost system promptly.

03 ·Related Symptoms

Heat Pump Frozen OutsideHeat Pump Not HeatingHeat Pump Auxiliary Heat Stuck On
⚠ Safety notice: This guide is for informational purposes only. HVAC systems involve high-voltage electricity, refrigerants under pressure, and natural gas — all of which can cause serious injury, death, or property damage. Refrigerant handling requires EPA Section 608 certification by federal law. When in doubt, contact a licensed HVAC professional.
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