A heat pump is an air conditioner that can run backwards. In winter the reversing valve flips the refrigerant circuit so the indoor coil gets hot and the outdoor coil gets cold. Most of the time that works quietly for fifteen years. When it does not, the symptom is almost always the same — the thermostat says Heat, the blower is running, and the air at the vents is cool.
Here is how we work through it. Two of these you can rule out yourself before calling anyone.
Normal: the defrost cycle
On a cold, damp morning the outdoor coil frosts over, and the unit periodically reverses into cooling mode for a few minutes to melt it. During defrost the indoor air goes cool — the electric strips are supposed to come on to cover it, but on some systems they are weak enough that you still notice. You will often see steam rising off the outdoor unit. A defrost lasts two to ten minutes and then heat returns. If that is your pattern, nothing is wrong.
Normal-ish: it is just cold outside
Heat pump supply air is only about 90–100°F, which feels lukewarm on your hand compared with the 120°F+ a furnace or strips produce. And a heat pump loses capacity as the outdoor temperature falls. On the handful of Orlando nights that drop into the 30s, the auxiliary strips should be staging in to make up the difference. If the outdoor unit is running and the air is lukewarm rather than cold, the system is likely working — it may just be at the edge of what it can do.
Broken: the reversing valve is stuck
If the outdoor unit runs, the indoor air is genuinely cold (not lukewarm), and the outdoor unit is blowing warm air out of its top in heat mode, the reversing valve has not shifted — the system is still air-conditioning. Sometimes it is only the small solenoid coil on the valve, which is a cheap repair. Sometimes it is the valve itself, which is a large one, and on a system over twelve years old we will talk about replacement before we quote it.
Broken: the defrost board or sensor
The defrost control decides when to run a defrost cycle. When it fails, the outdoor coil either never defrosts — you will see a solid block of ice building over hours (normal frost vs a failed defrost) — or the unit gets stuck in defrost and blows cold indefinitely. Ice that never clears is the giveaway, and it is worth switching the system to Emergency Heat until we arrive so the compressor is not trying to pump through a frozen coil.
Broken: the thermostat is wired or set wrong
Heat pump thermostats have an O/B setting that tells the reversing valve which way is heat. Most brands energise the valve in cooling ("O"); Rheem and Ruud energise it in heating ("B"). A thermostat replaced over summer with the wrong setting will cool perfectly and then run backwards the first time you ask for heat. This one is common after a DIY smart thermostat install and takes us minutes to correct.
Broken: the strips are out and the heat pump cannot keep up
If the heat pump itself is fine but the backup strips have a blown sequencer or a tripped breaker, everything works on a mild evening and fails on a cold one. Many Florida air handlers have a separate breaker just for the strips, and it is rarely labelled. That is exactly why the heat-mode test in our tune-up amp-clamps every stage of strip heat rather than just watching the thermostat.
If you have ruled out defrost and the Em Heat setting and the air is still cold, that is a service call. The diagnostic is $89, credited to the repair, and there is no surcharge for the night it happens to be cold.
Written by
The Eco Air Orlando team — licensed Florida HVAC contractors, #CAC1814992. Everything here comes from work we actually do in Central Florida homes.