Heat Pump Not Heating: Defrost, Aux Heat, and What Is Actually Wrong

The Pause, the Vapor, and the Lukewarm Air Are All the System Working

On a cold Southwest Florida morning the outdoor fan can stop for several minutes while the compressor keeps running, sometimes with a plume of vapor lifting off the top of the cabinet. Nothing has quit. That is a defrost cycle: the system flips itself into cooling for a short stretch, pushes warm refrigerant out through the outdoor coil, and clears the thin layer of frost that built up while it was pulling warmth out of cold air. Then it flips back. The whoosh or the click some homeowners hear is that change of direction. It finishes on its own and heating comes back without anyone touching anything.

Once or twice through a cold snap is an ordinary rhythm for that, and each round is brief. There is no switch to flip and nothing to reset afterward. Some systems also bring the electric backup heat on for the length of a defrost so the air at the vents does not turn cold while the outdoor coil is being cleared, which is why a thermostat can show AUX during a stretch when the house feels perfectly comfortable. The vocabulary underneath all of it, heat strip and balance point and defrost cycle, lives on our heat pump terms page, which is where it belongs.

The second surprise is temperature at the vent. Anyone who grew up with a gas furnace expects a blast of hot air the second heat kicks on. A heat pump does not work that way. It moves warmth that already exists in the outside air into the house instead of burning fuel to create it, so what comes out of the register reads as warm to the hand and well short of furnace hot. What matters is whether the room climbs toward the thermostat setting over the next several minutes. Give it that time and judge the room, not the first thirty seconds at the vent. Air that stays barely warm the whole way through, with the house never catching up, is a different story and is covered below.

One of These Two Labels Fakes a Fault. The Other Hides One.

AUX and EM HEAT both turn up on a thermostat in cold weather, and each one misleads in its own direction. AUX is auxiliary heat, and the equipment brings it on by itself, without anyone touching the thermostat: the electric strips in the air handler add output while the outdoor unit keeps right on running. So AUX on a cold morning is the system working harder, and it is usually the least interesting thing on the display. EM HEAT is emergency heat. Nothing selects it on its own, so it is showing because a person put it there, and that selection stops the outdoor unit and leaves the strips carrying the house by themselves. The two are not interchangeable in either direction.

For anyone trying to tell working from broken, that difference decides which way to look. AUX generates calls about systems that are fine, since a thermostat announcing backup in the middle of a cold front reads like an alarm. EM HEAT generates no call at all, which is the worse of the two: the house stays warm on the strips, so a heat pump that quit weeks ago goes unnoticed. Read which of the two is actually showing before working through anything else here, because the rest of this page means something different depending on the answer. What that choice does to a power bill is a separate question, and heat pump versus electric strip heat takes that one up.

Heat That Runs All Night and Never Gets There

A defrost that comes and goes in a few minutes is routine. A system that drops into defrost over and over, sits in it for long stretches, or comes out of it without ever returning to real heating is something else. At that point the suspect is the defrost control: the board and the sensor that decide when a defrost should begin and when it should end. Which of those is misbehaving takes measurement to establish, and the answer changes the repair.

The other candidate is a reversing valve that never left cooling. The thermostat asks for heat, the system starts up, and it runs and runs, but what reaches the vents stays cool no matter how long it goes, because the valve never changed sides. Establishing that means testing the valve body, the coil that drives it, the signal arriving at that coil, and the suction and liquid line temperatures on either side. All of it sits on a sealed refrigerant circuit with electrical controls wrapped around it, which puts it squarely with a licensed technician.

The Backup Heat That Quietly Never Turns On

Electric heat strips fail on their own schedule, with nothing at all wrong on the heat pump side. Two symptoms point there. The thermostat sits on EM HEAT and the house does not gain a degree, or the coldest night of the year comes and goes without AUX ever appearing while the outdoor unit runs the whole time. The strips themselves, the relay that energizes them, and the breaker feeding them are all items on a technician's list for that, along with voltage, amperage draw, and a measured temperature rise across the heater. Those circuits carry serious current, and testing them is professional work.

A burning smell the first time the heat runs in a given year is a separate situation with its own safety guidance, and we answer that question here instead of restating it. That is a different failure with a different response.

The Short List a Homeowner Can Actually Work Through

This list stays short on purpose, and it is short in both directions: everything on it is safe, and everything left off it is not. Start at the filter, because restricted airflow across the indoor coil is a real reason a house never catches up, and it is the one piece of the system anybody can look at. The cadence a Florida home should hold to does not relax for heating weather. Next, look at the mode selection itself. HEAT and EM HEAT sit next to each other on plenty of thermostats, and a system parked on EM HEAT will never behave like a working heat pump, because the heat pump has been switched out of the job. Fresh batteries in the thermostat or the remote, if it uses them, are worth ruling out while you are there.

If a single breaker has tripped, resetting it one time and watching whether it holds is reasonable. If it goes again, that is information for a technician, not an invitation to keep resetting. Past those four items the homeowner list is finished. It does not reach refrigerant lines, the reversing valve, the strip heat circuit, or anything behind a panel, and it should not. What a heat pump asks for beyond the filter, and how that differs from a cooling-only system, is answered separately.

What a Heat-Side Diagnosis Covers, and What It Costs

A heat-side fault has a way of hiding in this climate. The equipment spends nearly the whole year cooling, so a reversing valve, a defrost board, or a bank of heat strips can sit broken for months with nobody the wiser, because nothing is asking them to work. It gets discovered on the one night it matters. Whether a Southwest Florida house needs heating at all is its own question, and the honest answer is that it needs it rarely and needs it badly when it does. That is the argument for switching the thermostat to heat on an ordinary afternoon and confirming warm air arrives, well before a front is on the way.

When it does need attention, a heat-mode diagnosis is measurement work. It means confirming which way the valve is actually sitting, reading suction and liquid line temperatures, and putting real numbers on the strip heat circuit rather than assuming the strips are healthy, which is the list heat pump repair works through. ECS covers Cape Coral, Fort Myers, Fort Myers Beach, Bonita Springs, Naples, Marco Island, and up to Port Charlotte. A repair visit runs on a flat $150 service call, and whatever gets found is quoted upfront before any work begins, with no hidden fees added after the fact.

The cheaper version of all this is catching it on a scheduled visit instead of on a cold night. The $99-per-visit Annual Comfort Plan puts a technician on the system twice a year, once before the season and once after, and each of those visits is a 34-point inspection covering the indoor air handler and the condenser outside. What that buys on a heat pump is timing more than content: a scheduled visit makes the heating side prove itself on an ordinary afternoon, months before a front turns the answer urgent. If you are still deciding between a heat pump and a cooling-only system for the next install, we put the two side by side as well.