People are surprised that a Florida heat pump frosts up at all. It is not even freezing out. But in heat mode the outdoor coil runs 10 to 20 degrees colder than the air around it, because it is pulling heat out of that air. On a 40-degree Orlando morning with fog on the windshield, the coil is below freezing and soaking in moisture. Humid Central Florida mornings frost a coil faster than a dry, much colder day further north.
So frost is expected. What matters is whether it clears.
How defrost is supposed to work
Every 30 to 90 minutes of running in frosting conditions, the defrost control briefly reverses the system into cooling mode. The outdoor fan stops, hot refrigerant runs through the outdoor coil, and the frost melts in two to ten minutes. You may hear a loud whoosh as the valve shifts. You may see a cloud of steam rise off the unit, which has convinced more than one neighbor that the thing is on fire. Water runs out of the base. Then the fan restarts and heating resumes.
Inside, the air may go cool for a few minutes while it happens. That is covered in heat pump blowing cold air in heat mode.
Normal or not
| What you see | Normal? | What it usually means |
|---|---|---|
| Thin, even white frost that clears every hour or so | Yes | Defrost is working |
| Steam cloud, a whoosh, water draining from the base | Yes | A defrost cycle just finished |
| Ice getting thicker over an hour or more and never clearing | No | Defrost board, sensor or reversing valve fault |
| Ice on the top, the fan grille or the fan blades | No | Water is falling on the unit from above |
| Ice only on the bottom few inches of the coil | No | Low refrigerant, or meltwater cannot drain away |
| Frost when it is 50°F or warmer outside | No | Low refrigerant or an airflow problem |
The five causes we find
- 1A failed defrost control or sensor. The board never calls for defrost, so the ice builds until the coil is a solid block. A stuck reversing valve looks similar.
- 2Low refrigerant. An undercharged system runs the coil colder than designed, so it frosts in milder weather and at the bottom first.
- 3An outdoor fan that is not turning. With no air moving across the coil, a bad fan motor or capacitor lets it ice over very fast. Look through the top grille: if the compressor is humming and the fan is still, that is your answer.
- 4Water from above. A gutter that overflows onto the unit, a roof valley that sheds onto it, or a sprinkler head aimed at it. Water on a cold coil turns to ice that defrost cannot keep up with. Sprinklers on a freeze night are a classic Central Florida version of this. See what to do on a freeze warning.
- 5A unit that cannot drain. Over the years pads settle and mulch builds up around them. When meltwater cannot get away from the base, it refreezes, and the ice climbs the coil from the bottom.
What to do right now
- 1Switch the thermostat to Em Heat. That shuts the outdoor unit off and heats the house on the strips. This is the job emergency heat is for.
- 2Do not chip, scrape or hit the ice. The fins and tubes are thin. One puncture turns a defrost repair into a refrigerant leak repair.
- 3Melt it with a garden hose once the outdoor unit is off. Ordinary tap water is fine. Never use boiling water.
- 4Fix the water source if it is coming from above, before you put the unit back in service.
- 5Watch it. If it ices up again within a few hours of running normally, it is a fault, not the weather.
What the repair involves
A defrost sensor or board is usually a modest repair, done in one visit. A failed outdoor fan motor or capacitor is the same: we carry the common sizes on the truck. A reversing valve is a refrigerant-circuit job and costs considerably more. On a system past twelve years old, we will talk about whether it is worth doing before we quote it.
The heating diagnostic is $89 and credited to the repair. If the ice is back the morning after a cold night, call before the unit spends another night trying to run through it. More on what we check is on our heating repair page.
Written by
The Eco Air Orlando team — licensed Florida HVAC contractors, #CAC1814992. Everything here comes from work we actually do in Central Florida homes.