Defrost cycle

For homeowners

A heat pump runs in heating mode by absorbing heat from outdoor air, even when that air is cold. The outdoor coil acts as the evaporator — refrigerant boils inside it at temperatures below the outdoor air temperature. When outdoor air is humid and the coil runs below freezing, water vapor from the air condenses and freezes on the coil surface as frost.

Frost buildup blocks airflow and insulates the coil from outdoor air. Within an hour or two of heavy frost conditions, system capacity drops by 30–50% and the system can’t deliver enough heat to maintain indoor temperature.

The solution is the defrost cycle — periodically reversing the heat pump so the outdoor coil becomes hot enough to melt the accumulated frost.

The four phases:

1. Heating mode (normal operation). Refrigerant boils in the outdoor coil at 20–30°F. Cold outdoor air gives up heat to the colder refrigerant. Frost builds gradually on the coil. Typical interval between defrosts: 30–90 minutes of continuous heating runtime depending on conditions.

2. Defrost initiated. The control board energizes the reversing valve (reverses refrigerant flow), stops the outdoor fan, activates indoor backup electric heat strips, and keeps the compressor running. The outdoor coil now acts as the condenser — hot refrigerant flows through it at 90–110°F. Frost begins to melt.

3. Frost melting. Meltwater runs off the coil. Heat transfers from refrigerant through coil to ice. The cycle continues until the coil surface temperature rises significantly (typically above 55–65°F), indicating frost has cleared.

4. Termination. Control board switches the reversing valve back, restarts the outdoor fan, deactivates backup heat. System returns to heating mode. Total defrost duration: 5–15 minutes typical.

Two defrost timing strategies:

Time + temperature initiated — most common in residential. A timer in the control board allows defrost every X minutes of heating runtime. The coil sensor must also confirm the coil is cold enough (below ~26°F) for the defrost to actually run. If conditions don’t warrant defrost at the scheduled time, the cycle is skipped.

Demand defrost — modern higher-end systems. The board compares outdoor coil temperature to outdoor ambient temperature. Frost insulates the coil, causing the coil temperature to diverge from ambient. When the divergence reaches a threshold, defrost runs — regardless of timer. More efficient, fewer unnecessary defrosts.

What you see during defrost:

• Outdoor unit appears to be running but fan is stopped
• Steam rising from the outdoor coil (warm coil + cold air = visible water vapor)
• Indoor air temperature delivery drops briefly (backup heat usually compensates)
• A whoosh or click as the reversing valve operates
• The cycle completes in 5–15 minutes and normal heating resumes

This is all normal. The most common homeowner complaint — “my heat pump is making weird noises and steaming” — is just defrost operating as designed.

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For technicians

The physics of frost formation.

A heat pump in heating mode extracts heat from outdoor air through the outdoor coil. The refrigerant inside the coil boils at a saturation temperature determined by the suction pressure. For typical heat pump operating conditions:

• Outdoor air: 35°F
• Coil refrigerant saturation: 20°F (about 15°F below outdoor temperature)
• Coil surface temperature: 22–25°F (slightly above refrigerant due to thermal resistance)

Water vapor in the outdoor air condenses on any surface below the dewpoint. With outdoor air at 35°F and 80% RH, the dewpoint is about 30°F. The coil surface at 22–25°F is well below dewpoint, so water condenses. Because the coil is also below freezing, that water immediately freezes — forming frost.

Frost is different from ice: light, fluffy crystalline structure; low density; acts as a thermal insulator; builds up on the coil’s air-side fins, not the refrigerant tubes; and restricts airflow through the coil. The frost accumulation is self-reinforcing once it starts.

When frost forms vs doesn’t. Frost requires both conditions: coil surface below 32°F, and outdoor air dewpoint above the coil surface temperature. In dry conditions (low humidity), frost barely forms. In wet conditions (rain, fog, high humidity, 30–45°F outdoor), frost forms aggressively. Below about 25°F outdoor, frost formation actually slows down — cold air holds little moisture.

Florida and other humid climates rarely see severe frost because outdoor temperatures rarely drop low enough to keep the coil below freezing for extended periods. A typical Florida heat pump might defrost once a day during a January cold snap, vs every 30 minutes for a heat pump in coastal Oregon.

Defrost mechanism in detail.

Phase 1 — Defrost initiation:

Control board signals: energize reversing valve solenoid (or de-energize, depending on whether the valve defaults to heating or cooling — wiring varies by manufacturer), stop outdoor fan, energize indoor electric heat strips, compressor stays on.

The reversing valve switch happens in seconds. During the switch, pressures equalize briefly between high and low sides — there’s an audible whoosh or hiss from the outdoor unit as refrigerant rushes through the valve. Immediately after: high-pressure refrigerant now routes to the outdoor coil; outdoor coil pressure climbs from ~50 PSIG to ~250–300 PSIG; coil temperature climbs from 25°F to over 100°F within minutes.

Phase 2 — Frost melting:

The hot refrigerant transfers heat to the frost-covered surface. Meltwater drains off the coil through the base pan drain. Steam is visible because the hot coil heats nearby air, which then contacts cold outdoor air, condensing water vapor to visible droplets. This steam is often mistaken for smoke — it’s just water vapor.

Phase 3 — Termination:

When the coil temperature sensor reads above the termination threshold (typically 55–65°F): reversing valve switches back to heating, outdoor fan restarts (after a short 30–60 second delay), indoor heat strips deactivate. The fan delay is important — if the fan started while the coil was still warm, residual heat would dissipate into outdoor air instead of transferring to the next cooling cycle.

Why backup heat strips during defrost. When defrost is active, the indoor coil becomes the evaporator, dropping to perhaps 40–50°F. Without heat strips, supply air would blow 40–55°F — cooler than room temperature — on heating mode occupants. Typical residential systems include 5–10 kW of electric heat strips that run during defrost to compensate.

The two defrost initiation strategies.

Time-temperature initiated defrost. The defrost board has an internal timer that counts compressor runtime in heating mode. At programmable intervals (typically 30, 60, or 90 minutes selectable via jumpers), the board checks the outdoor coil temperature sensor. If coil temperature is below the defrost initiation threshold (~26°F), defrost begins; if above, timer resets. Factory default is typically 90 minutes; 60 minutes for humid climates; 30 minutes for severe conditions.

Pros: simple, reliable, easy to diagnose. Cons: can defrost unnecessarily if frost cleared between the last defrost and timer expiration.

Demand-initiated defrost. Uses two sensors — outdoor coil temperature and outdoor ambient air temperature. The board calculates the delta between coil and ambient. As frost insulates the coil, coil temperature drops further below ambient. When the delta exceeds a programmed threshold (typically 15–20°F), the board initiates defrost. More accurate, saves energy, better comfort. More complex board with twice as many potential failure points.

Common defrost problems.

Defrost too frequently — coil sensor failing (reads colder than actual), or wrong jumper setting. Customer hears defrost every 15–20 minutes, comfort suffers, electricity bill climbs.

Defrost not happening (coil ices over completely) — board failure, coil sensor failure (reads warm), reversing valve stuck, outdoor fan stuck on during defrost, or drain holes frozen closed preventing meltwater escape.

Defrost runs too long — coil termination sensor failing (reads cold longer than actual). Defrost cycle extends to 20–30 minutes; indoor side suffers.

Defrost terminates too early — termination sensor reads warm too quickly, frost isn’t actually melted, coil re-frosts immediately, pattern of frequent short defrosts that don’t clear the coil.

No reversal during defrost — board signals defrost, fan stops, strips activate, but reversing valve doesn’t switch. Outdoor unit runs with fan off but coil stays cold (or gets colder). Diagnosis: bad reversing valve solenoid, stuck slide, or bad wiring to solenoid.

Heavy ice buildup despite normal defrost. In extreme cold-wet conditions a heat pump can’t keep up — frost forms faster than the standard defrost cycle removes. Solutions: switch to emergency heat mode (strips only, no compressor) until weather improves; adjust defrost interval to shorter time (jumper change); diagnose for charge or airflow problems that worsen frost rate.

Florida considerations.

Florida heat pumps see defrost rarely — outdoor temperatures drop below 32°F only during occasional cold snaps, often only at night. The defrost system exists on most Florida heat pumps but rarely activates.

Common Florida-specific issues: defrost boards often fail or are misdiagnosed because techs rarely see them operate; coil sensors corrode in humid coastal environments and read incorrectly when actually needed; reversing valves can stick after years of inactivity (no cycling to exercise the slide); during the few cold-weather events, the system may struggle if components have degraded undetected.

Best practice in Florida: test heat pump heating mode and defrost function annually during fall maintenance, even though it’s rarely used. Better to find a failed component in October than during a January freeze when heat is critical.