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 the same equipment that cools them: 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. Older block houses in Arlington, Lakewood and the Beaches often have the strips and no heat pump at all.

A heat pump in cold, damp air does three things that look like something is wrong, and all three are normal. The fourth thing on this page, Emergency Heat, is a setting people leave on by mistake. This guide is the list a contractor would give you on the phone, so the call you make at 6 a.m. is the right one.

Normal: defrost

When the outdoor air is cold and humid, the outdoor coil runs below freezing and frost builds on it. The system notices and defrosts itself: 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: a cloud of steam rises off the unit, water runs from under it, and the fan is silent. From inside, the supply air goes cool for the length of the cycle, and the thermostat may show auxiliary heat while the strips cover the gap.

Five to ten minutes, then the fan restarts and the air warms again. A defrost cycle every hour or two on a wet 38 °F night is a heat pump working as designed.

Normal: air that feels cool

A heat pump delivers supply air at roughly 85 to 95 °F. That is warmer than the room, so the house warms, but it is cooler than your skin, so a hand in front of the register feels a lukewarm breeze rather than a furnace blast. People who grew up with gas heat call about this every January. Put a thermometer in the register if you want to know: if the air is 20 degrees or more above the room, the heat pump is heating.

Normal: auxiliary heat

On the coldest nights the heat pump cannot keep up alone, and the thermostat brings on the strips alongside it. Most thermostats show this as “Aux” or “Auxiliary Heat,” and it should come and go. That is the system using both stages, and it is the reason the electric bill is higher in January than in April.

Not normal: Emergency Heat

Emergency Heat, “EM Heat” on many thermostats, is a manual setting that turns the heat pump off and runs the strips alone. It exists for a heat pump that has failed, so the house stays warm until the repair. Selected by hand in a cold snap and forgotten, it heats the house with resistance electricity at roughly three times the cost of the heat pump, every hour, until someone notices the bill. On the First Coast that bill is JEA’s, Beaches Energy’s, Clay Electric’s or FPL’s, and a thermostat left in Emergency Heat can raise a February bill substantially.

If the thermostat says Emergency Heat and nobody remembers choosing it, switch it back to Heat and watch whether the outdoor unit runs. If it does, the problem was the setting. If it does not, keep reading.

A call

These are the things that mean the system has a fault, in the order a contractor would ask about them.

The supply air stays cool for an hour with the thermostat set to Heat and the setpoint above the room, or the house never reaches the setpoint on a night that is above freezing. A defrost cycle ends; a failed reversing valve, a low refrigerant charge, or a compressor that is not running does not.
A breaker trips when the strips come on. Heat strips draw a great deal of current, and January is when a weak breaker, a burned wire at the strip kit, or a failed sequencer shows itself. Leave the breaker off and say so on the call.
Ice that does not clear. Frost that comes and goes is defrost. A block of ice on the outdoor coil that is still there in the afternoon is a defrost board, a sensor, or a charge problem, and running the unit that way damages the compressor.
The outdoor unit does not run at all while the indoor blower does: a contactor, a capacitor, or the reversing valve stuck in cooling. In that case the strips may be heating the house alone, at Emergency Heat cost, without the thermostat saying so.
A smell. Dust burning off the strips the first cold night of the year is normal and lasts minutes. A smell that continues, a smell from the air handler closet, or anything from the electrical panel is a call, and the breaker goes off first.

Before the contractor arrives

Thermostat on Heat, setpoint a few degrees above the room, fresh batteries if it takes them. Both breakers on unless one has tripped twice. A look at the outdoor unit: running, silent, iced, steaming. The filter, if it has been more than a month. Then the call: say which of the above you are seeing, how old the system is, and whether the thermostat was in Emergency Heat when you found it.

Replacing it, and the strips

A heat pump replacement follows the same rules as any First Coast change-out: 14.3 SEER2 and 7.5 HSPF2 at minimum in the Southeast, R-454B or R-32 in new equipment, a mechanical permit through the City of Jacksonville or your beach city or county, pulled by the contractor. JEA’s application lists 16 SEER2 and 8.0 HSPF2 for a heat pump; Clay Electric’s Energy Smart credit is written for heat pumps at 15.2 SEER2 or higher; Beaches Energy and FPL have their own tiers. The local addition is the strip kit: ask that it be sized for the house from the load calculation rather than copied from the old air handler, because it decides the breaker and the wire that serve it, and it decides how much Emergency Heat costs the next time someone leaves the switch there.

An older house with strips and no heat pump can get a heat pump at replacement time. The same outdoor unit then cools in July and heats in January, and the strips become what they were meant to be: backup.

Climate figures are the Florida Climate Center’s 1991–2020 normals for Jacksonville; equipment rules are summarized from public sources as of September 2026, and the contractor who does the work is responsible for the diagnosis, the equipment and the permit.

Your heat pump on a January night in Jacksonville: what is normal, what is a call, and the thermostat setting that doubles the bill