Need an AC technician? Call 904-621-4328
Jacksonville is the part of Florida where the heating side of an air conditioner earns its keep. The 1991–2020 normals for the airport put the January low at 42.9 °F, December at 45.8 and February at 46.1, and the clear nights behind a winter cold front drop below those numbers. Most First Coast houses heat with a heat pump, which runs the refrigerant cycle backwards to pull heat out of the outdoor air, with electric heat strips in the air handler as backup.
On those nights the outdoor coil grows frost, and the heat pump is built to clear it. The question worth answering is whether the ice you see is that routine frost or a coil that has stopped clearing.
Why the outdoor coil frosts
To pull heat out of 40-degree air, the outdoor coil has to run colder than the air, often below freezing. Jacksonville winter air is damp, so the moisture in it freezes onto the fins as a thin white frost. Left alone, the frost would block the air the coil needs, so the heat pump melts it off on a schedule.
What a normal defrost cycle looks like
The outdoor fan stops, the refrigerant flow reverses for a few minutes, and the coil warms until the frost melts. From the yard it looks alarming: steam rises off the unit, water runs out from under it, and the fan is silent. Inside, the supply air goes cool for the length of the cycle, and the thermostat may show “Aux” while the heat strips cover the gap.
Five to ten minutes, then the fan restarts and the air warms again. A defrost every hour or two on a wet 38-degree night is a heat pump working as designed. Afterward the coil should look wet or lightly frosted, with the fins visible.
Ice that does not clear
A block of ice that encases the coil, or ice still on the unit in the afternoon when the air is well above freezing, is a different thing. It points to one of a short list of causes:
- Water from above. A gutter or a roof edge that drips onto the unit freezes on top of it overnight. The heat pump cannot defrost ice that keeps arriving from above; moving the downspout or adding a gutter fixes it.
- Blocked airflow. Leaves packed against the coil, shrubs grown up around it, or a unit sitting in a low spot where water pools around its base.
- The defrost control. A failed defrost board or coil sensor means the cycle never starts, and the frost thickens into ice.
- The outdoor fan or the refrigerant charge. A fan motor that has stopped, or a system low on refrigerant, runs the coil colder than it should and ices it.
Do not chip the ice; the fins and the tubing under it bend and puncture easily. A garden hose melts it faster than waiting. If it builds back by the next morning, the cause is still there, and running the heat pump iced over strains the compressor.
Emergency Heat is for this
Emergency Heat, “EM Heat” on many thermostats, turns the heat pump off and heats the house with the strips alone. A heat pump that keeps icing is what the switch is for: it keeps the house warm and the compressor off until the repair. It costs roughly three times as much per hour as the heat pump, because the strips turn electricity directly into heat, so switch it back to Heat once the repair is done. A thermostat left in Emergency Heat after a cold snap is a common cause of a high February bill from JEA, Beaches Energy, Clay Electric or FPL.
Ice is one of several January heating problems. A breaker that trips when the strips come on, supply air that stays cool for an hour, or a smell that lasts past the first cold night are on the heat pump repair page.
Climate figures are the Florida Climate Center’s 1991–2020 normals for Jacksonville. The contractor who does the work is responsible for the diagnosis and the repair.
Need an AC technician? Call 904-621-4328