Most pool heat pumps that “won't heat” aren't broken.They're short on water flow, short on air, short on run time, or being asked to do a January job with a fraction of their rated output. I've been on Florida routes since the late 1990s — my father has been a licensed Florida pool contractor since 1989 — and in South Florida, where the heat pump is the default pool heater, the no-heat calls arrive in a wave with the first real cold front every winter. For gas heaters and the universal first checks, start with our pool heater troubleshooting guide. For brand-specific codes, see the AquaCal and Hayward HeatPro pages.
First question: is it broken, or just losing the race?
A heat pump doesn't make heat. It moves heat out of the air and into your water, the way an air conditioner moves heat out of your house. Manufacturers rate output at 80°F air, 80% humidity, and 80°F water, and output falls as conditions drop below that. Pentair's and Hayward's own spec tables show a unit at 50°F air delivering somewhere between about half and two-thirds of its rated BTUs — on the same nights the pool is losing more heat to cold, dry wind. A heat pump that held 84°F all October can lose ground in January without a single part failing.
Two quick tells that the unit itself is doing its job:
- The air coming off the fan is noticeably colder than the air around it.AquaCal's guidance puts a healthy split at roughly 9–12°F between the air going in and the air coming out.
- The water leaving is a few degrees warmer than the water going in.AquaCal's setup chart puts most of its residential models around 3–7°F at typical pool temperatures, and the exact target varies by model. Reading it accurately takes temperature ports or a tech's probe.
If both are there, stop hunting for a broken part. The problem is run time, heat loss, or sizing, and all three are covered below. If the air coming off the fan feels like the air going in, the unit isn't moving heat, and the table tells you where to look.
Symptom, likely cause, and fix
| Symptom | Likely cause | What to do |
|---|---|---|
| Runs all day, cold air off the fan, pool won't climb | The unit works; heat loss is beating output — short pump hours, no cover, wind, cold nights, or an undersized unit | Run the pump continuously while heating, cover the pool, check sizing |
| Fan runs, air off the fan isn't cold | Compressor not running: the start delay, a failed capacitor or contactor, or refrigerant loss tripping the low-pressure switch | Wait five minutes. Still no cold air — call a tech |
| Display on, set point above water temp, nothing happens, no code | Wrong pool/spa mode, remote mode handing control to an automation system that isn't calling for heat, or the pump timer is off | Check the mode, the set point, and the automation's heater setting |
| Won't power up, display blank | Tripped breaker, failed control transformer, or a control board fault | Reset the breaker once. Trips again — tech or electrician |
| Low-flow or flow-switch fault | Dirty filter, packed baskets, a closed or half-closed valve, an open bypass, or a variable-speed pump running too slow | Clean the filter and baskets, check valves, raise the pump speed |
| High-pressure fault in heat mode | Usually the water side: not enough flow carrying heat away. Less often a bad switch or a stuck expansion valve | Fix flow first. Repeats with good flow — call a tech |
| Low-pressure fault | The air side — a dead fan, a blocked coil, heavy ice, very cold air — or a refrigerant leak | Clear the coil and clearances. Repeats — refrigerant tech |
| Compressor starts and stops within minutes | Marginal water flow, a dirty coil, low refrigerant, or cold, humid air on the coil | Check flow and airflow. Keeps cycling — shut it off to protect the compressor and call |
| Frost on the coil on a cold morning that clears on its own | Normal defrost — frost can form below about 50°F air depending on humidity | Nothing. It's working as designed |
| Ice in mild weather, or ice that never clears | Restricted airflow, sprinklers hitting the coil, low refrigerant, or a failed defrost sensor | Turn it off, let it thaw, call for service |
| Water under the unit | Usually condensation, which can run three to five gallons an hour on humid days; occasionally a heat exchanger leak | Chlorine test strip: no chlorine means condensation |
Code names differ by brand, so match the fault on your display against the brand pages linked above. The frost row is covered in depth in our heat pump defrost guide.
The homeowner-safe check sequence
Work these in order. None of them needs more than a hose and a test strip, and none of them opens the cabinet.
- Confirm water is moving, and will keep moving.A heat pump only heats while the pool pump pushes water through it, and the full-size residential units we see most list a minimum around 30 gallons a minute. If the timer shuts the pump off at 4 p.m., the heat pump quits at 4 p.m. too. When you're bringing a cold pool up, run the pump around the clock until it gets there.
- Check the mode and the set point.Pool versus spa on a combo, a set point that is actually above the water temperature, and whether the unit is in remote mode taking orders from an automation system. Then give it five minutes — the units we service hold the compressor off for three to five minutes after a call for heat. If the display and your pool thermometer disagree, see our thermostat calibration guide.
- Clean the filter, empty the baskets, check the valves.AquaCal notes that a filter running 7–10 psi above its clean pressure is typically already cutting flow. Make sure no valve after the filter is half-closed and any bypass around the heat pump is shut.
- Clear the air path.Manufacturers call for roughly one to two feet of open space on the sides, about two feet in front for service, and five to six feet of open air above if the unit sits under an overhang. Pull back shrubs, lawn furniture, and anything leaning on the cabinet. With the breaker off, rinse the coil gently with a garden hose — no pressure washer, no spray nozzle.
- Feel the discharge air. Hold a hand over the fan outlet, never in it. Cold air means the unit is moving heat.
- Photograph any fault code. Look it up for your brand before you reset anything more than once.
- Test any water under the unit. Dip a chlorine strip in the puddle, or shut the heat pump off for a few hours with the pool pump still running and see whether the water stops.
Where the homeowner stops
Everything past that list is licensed work. Refrigerant work — finding a leak, recovering the charge, brazing, recharging — requires a technician certified under EPA Section 608, and that's federal law, not a preference. Capacitors and contactors live inside a 240-volt cabinet, and capacitors hold a charge after the power is off. A unit that keeps tripping its breaker, or needs a new circuit, needs an electrician. Compressors and heat exchangers are their own repair-or-replace conversation.
The “it's not broken” causes
Cold air
Around 50°F air, two things happen at once: output drops toward half or two-thirds of the rating, and the coil starts to frost on humid mornings, so the unit spends part of its time defrosting instead of heating. Pentair and AquaCal both flag that 50°F range for frost and defrost. Colder still, many units have a low-ambient protection that shuts them off until the air warms up; the exact cutoff varies by model, so check your manual. The manufacturers' own advice is the right advice: run the heat pump during the warmest part of the day and hold the heat overnight with a cover.
Evaporation
The Department of Energy's Better Buildings program calls evaporation by far the largest source of pool energy loss and puts the savings from a pool cover at 50–70% of heating costs. A solar blanket on a heated pool on a windy January night is the cheapest “repair” on this page, and a windbreak helps too. More on how Florida pools lose water and heat in our Florida evaporation guide.
Run time and pump speed
AquaCal's own troubleshooting says that when the heat pump is operating normally but the pool isn't warming, inadequate run time is the most likely cause. Variable-speed pumps add a twist: a schedule that drops to a quiet overnight speed can fall below the heat pump's minimum flow, so the flow switch never closes and the unit sits idle all night while the display looks fine. During a heat-up, give the pump a speed and a schedule that keep the heat pump fed.
Undersizing
Heat pumps are built to hold a temperature, not to raise one fast, and Pentair's manual says plainly that bringing a cooled pool back can take days. A unit sized for April and November top-ups has no reserve when January takes a third or more of its output. If yours has always lost ground on cold weeks, run the numbers in our pool heater sizing guide before paying for any repair.
South Florida wear: salt air, sprinklers, and roof runoff
- Salt air. Coastal coils load up with salt film and corrode years ahead of inland units, and pinhole refrigerant leaks follow. A gentle fresh-water rinse is real maintenance on the coast. See our coastal salt-air guide.
- Sprinklers. Irrigation spraying the coil on a cool morning builds ice. Re-aim the heads away from the pad.
- Roof runoff. A unit under a roof edge with no gutter takes a waterfall of water and debris every afternoon storm.
Repair or replace: what the numbers look like
At Florida's Best Pool Service, heat pump repairs are managed by us under our Florida Certified Pool/Spa Contractor license. Our own techs handle most of the work, and an EPA 608-certified refrigerant tech or a licensed electrician comes in when the job calls for one. A diagnostic runs $0–$125 and is free for weekly-route clients. Most heater repairs land between $250 and $1,800, which covers the electrical parts, sensors, switches, and boards that fail most often. A refrigerant leak is quoted once the leak is found, because the location decides the job. A failed compressor or heat exchanger runs $4,000–$5,000, and a new heat pump is $5,000–$7,000 installed, which is why a compressor on an older unit is usually where replacement wins. Everything is quoted in writing before a part is ordered. If you've worked the checklist and the air off the fan still isn't cold, book a pool heater repair visit.
If the air coming off the fan is cold, your heat pump works — feed it water, give it hours, and cover the pool. If the air coming off the fan feels like the air going in, that's when you call.
Pool heater sizing calculator
Surface area × temperature rise × 12 = required heater output. Surface area drives it — evaporation is where pools lose heat.
84–88°F is the family sweet spot.
South Florida unheated pools sit in the low-to-mid 70s most winters and can dip into the 60s after a hard cold front; size for the cold weeks.
Gas heater: 200k BTU nameplate
A 19°F rise on 392 sq ft needs ≈ 89,000 BTU/hr of output (≈ 109k input at ~82% efficiency). Oversizing a gas heater is fine — it just reaches temperature faster. Undersizing means it runs forever on cold weekends. Heat pumps are rated differently (typically 110k–140k BTU output) and heat slower but far cheaper per degree — the right call for set-and-hold temperature in South Florida; gas wins for fast weekend heat-ups.
Reference sizing for an uncovered residential pool with typical wind exposure, per manufacturer surface-area charts. A solar cover cuts the real load dramatically. Final sizing and gas-line capacity are the installer's responsibility.
Book heater service·Get a heater sized and installed·My heater needs repair
Frequently asked questions
Work the simple causes first. Check the breaker and reset it once. Confirm the set point is above the water temperature and the unit is in the right pool or spa mode. Make sure the pool pump is running with enough flow, because the flow or pressure switch will not let the heat pump start without water moving through it. Then wait out the compressor start delay, which runs three to five minutes on the units we service. If the heat pump is in remote mode, your automation system has to be calling for heat or nothing happens. Air that is too cold can also trigger a low-ambient lockout on many units. A blank display after a breaker reset, or a breaker that trips again, is a call for a tech or an electrician.
Frost on a cold, humid morning is normal. Below about 50 degrees air the coil can frost over, and the unit periodically runs a defrost cycle that pauses heating until the coil clears. Ice in mild weather, or sheets of ice that never clear, is not normal. The usual causes are restricted airflow, sprinklers spraying the coil, low refrigerant, or a failed defrost sensor. Turn the unit off, let the ice thaw on its own or with low-pressure water, clear anything blocking the airflow, and call for service if the ice comes back.
There is no single cutoff, because it varies by model. Output falls steadily as the air cools: Pentair and Hayward spec tables show a unit at 50 degrees air delivering roughly half to two-thirds of its rating at 80 degrees. Around 50 degrees, defrost cycles start appearing on humid mornings, and colder still many units shut themselves off with a low-ambient protection until the air warms. In South Florida that happens during January cold fronts. Run the heat pump during the warmest hours of the day and cover the pool at night to hold the heat.
Usually not. A heat pump pulls moisture out of the air as it works, and Pentair and Hayward both describe three to five gallons an hour of condensation as normal on humid days. Dip a chlorine test strip in the water: no chlorine means condensation. You can also turn the heat pump off for a few hours with the pool pump still running. If the water stops, it was condensation. Chlorine in the puddle points to a heat exchanger leak, and that is a tech call.
Short cycling usually starts with marginal water flow, such as a dirty filter, a partly closed valve, an open bypass, or a variable-speed pump running too slow, or with a dirty coil starving the unit of air. Low refrigerant and cold, humid air on the coil can cause it too. Fix flow and airflow first. If it keeps cycling, turn the heat pump off to protect the compressor and have it diagnosed. Refrigerant work requires an EPA Section 608-certified technician.
A diagnostic runs $0 to $125 and is free for our weekly-route clients. Most heater repairs land between $250 and $1,800. A failed compressor or heat exchanger runs $4,000 to $5,000, and a new heat pump is $5,000 to $7,000 installed, so a compressor failure on an older unit is usually where replacement makes more sense. A refrigerant leak is quoted once the leak is found. Everything is quoted in writing before any part is ordered.
Air off the fan still not cold?
If the filter is clean, the coil is clear, and the heat pump still isn't moving heat, the problem is inside the cabinet. We diagnose heat pumps of every major brand across Palm Beach and Broward — free for weekly-route clients, quoted in writing before any part is ordered, with refrigerant work handled by an EPA 608-certified tech.