Troubleshooting guide
Pool heat pump not heating
A heat pump pulls heat out of the outside air, so it heats slower than a gas heater and struggles when the air is cold. Many "not heating" calls are low water flow, a start-up delay, or simply cold weather.
Safety first
What's usually wrong
A heat pump only runs when water is moving through it. Pentair's UltraTemp guide says the pool pump must be on and water flowing for it to operate, and its LOW WATER FLOW alarm means the switch that senses flow is open. Hayward's HeatPro manual lists the same thing, plus no power, a thermostat set too low, a short compressor delay that hasn't finished, outside air that's too cold, and a fan that isn't working.
It's also slow by design. Pentair says a heat pump won't heat a pool as fast as a gas heater, its output drops at night, and a pool that has cooled off can take several days to come back up. Hayward shows separate error codes when the water or outside air is too cold to run.
Two safe first checks
- Flow. Make sure the pool pump is running and the filter and baskets are clean. Pentair also says to close any bypass valve around the heat pump, and Hayward suggests unhooking pool cleaners while you test. If your pump is variable-speed, try a higher speed.
- Settings and patience. Check the heat pump is on, set to the right body of water and set above the current water temperature. Then wait: Pentair's unit waits about a minute to sense water temperature and another 5 minutes before the compressor starts, and Hayward's has a 3-minute compressor delay.
Two things that are often normal: water dripping under the unit while it heats is usually condensation (Hayward says to turn the heat pump off for several hours with the pool pump still running; if the water stops, it was condensation), and slow heating on cold nights. Pentair recommends a pool cover to cut heat loss. If the unit runs but the water stays cold, the fan won't spin, it cycles on and off quickly, or a code keeps coming back, stop there. Hayward says to turn a heat pump that keeps short-cycling off to protect the compressor.
Get a tech on video
On a video call a tech can read the code or alarm on the display, look at the rating plate and the plumbing around the unit, and tell you whether it's flow, settings, weather, or a part such as a sensor, switch or fan. You'll know what you need before anyone orders anything. For gas heater codes, see pool heater won't fire.
Common questions
Why is my pool heat pump running but not heating?
Common reasons are low water flow (pump off or slow, dirty filter, an open bypass), a thermostat set at or below the water temperature, a dirty air coil, or air that's too cold for the heat pump to do much. Heat pumps are also slow: Pentair says a cooled-off pool can take several days to warm back up.
How long does a pool heat pump take to start?
There's a built-in delay. Pentair's UltraTemp waits about one minute to sense the water temperature, then up to five more minutes before the compressor starts. Hayward's HeatPro has a three-minute compressor delay.
Is water under my heat pump a leak?
Usually not. Heat pumps make condensation while they heat. Hayward says to turn the heat pump off for several hours with the pool pump still running. If the water stops or slows, it's condensation. If not, have it checked for a leak.
Does a heat pump work in cold weather?
Less well. Heat pumps take heat from the outside air, so output drops as it gets colder. Hayward's HeatPro shows protection codes when the air or water is too cold, and Pentair suggests running it during the warmest part of the day.
Related guides
Heater won't fire
No heat, a flow or pressure code, or an ignition lockout. Flow comes first.
Read the guidePentair error codes
MasterTemp ERR PS, HLS, IGN, AGS; IntelliFlo3 alarms; IntelliChlor lights.
Read the guideHayward error codes
Heater LO, IF, HS, AO; MaxFlo VS Check System messages; AquaRite lights.
Read the guideFilter pressure high or low
The gauge tells you when to clean the filter, and when something upstream is wrong.
Read the guideSources
- Pentair, UltraTemp heat pump installation and user's guide: operating the heat pump, timers and delays, control panel alarm messages, energy saving tips
- Hayward, HeatPro heat pump (HP31005T) installation and operation manual: Table 5 troubleshooting lookup chart and Table 6 error codes
- U.S. Consumer Product Safety Commission: Don't Swim with Shocks, electrical safety in and around pools, spas and hot tubs
Brand names belong to their owners. Pool Helpline isn't affiliated with or endorsed by any manufacturer. Steps and codes vary by model and year, so always check your own owner's manual.