Heat pumps move heat from outside air into pool water. When the outside air is cold, the outdoor coil gets colder than the air, and condensate freezes on the fins as frost. A frosted coil loses surface area; heat transfer drops; eventually the unit stops delivering heat at all. The defrost cycle is how a heat pump gets rid of that frost — and why a heat pump you're watching on a chilly Florida morning looks like it's doing something strange.
How defrost works
When the outdoor coil drops below a frost-forming threshold (the exact setpoint depends on the manufacturer but is typically in the 32–40°F coil-temperature range), the controller triggers a defrost cycle. It does one or more of the following, depending on the design:
- Reverses the refrigerant cycle(a reverse-cycle defrost) — the outdoor coil becomes the hot side briefly, melting the frost. Pool water is not heated during this step; some systems even pull a little heat out of the pool to drive the defrost.
- Runs the fan only with the compressor off, letting ambient air melt the frost. Slower but gentler on components.
- Uses electric defrost heaters on the coil (rare on residential pool heat pumps).
What you'll see on a Florida cold morning
- Compressor and fan running but the water returning feels cool, not warm.
- Steam or mist above the unit as frost melts.
- A short pause in operation; then a “whoosh” as the reversing valve clunks back to heating mode.
All three are normal. The cycle lasts 5–15 minutes typically. If the unit enters defrost and doesn't come out — or if it cycles between defrost and heat every few minutes — something is wrong.
Why Florida heat pumps defrost at all
Because the relevant number isn't air temperature, it's coiltemperature. At 45°F ambient with 85% humidity (a January morning in Tampa), the coil can still drop below freezing as the compressor does its work. Florida heat pumps see fewer defrost cycles than Pennsylvania ones, but they are not zero.
Scroll compressors vs. reciprocating — why it matters
Modern heat pumps mostly use scroll compressors. Two orbiting scrolls continuously compress refrigerant without valves. This gives scroll-equipped heat pumps several advantages over older reciprocating designs:
- Better performance at low ambient temperatures — heat output stays higher longer as air temperature drops.
- Quieter operation, which matters near neighbors or indoors.
- Fewer moving parts per revolution, which extends service life in humid Florida air.
If you're specifying a heat pump for a Florida pool that needs to keep working through the January cold-snaps, insist on scroll.
When the defrost cycle is broken
- Permanent ice on the coil— stuck reversing valve, low refrigerant, failed defrost sensor. Shut the unit down; this is refrigerant-tech territory.
- Short-cycling (defrost every few minutes)— typically a defrost sensor out of calibration or a controller fault.
- Fan runs, no compressor during defrost— failed contactor or capacitor.
If a homeowner calls in January saying “the heater is blowing cold air and making weird noises,” odds are it's a defrost cycle working exactly as designed. Confirm, reassure, and move on. If the call comes back a few weeks later with the same complaint in warmer weather, that's when you start diagnosing.