A pool heat pump that stops heating on a cold Houston morning is often doing what it was designed to do. Heat pumps pull their heat out of the air, and when the air gets cold enough they slow down, defrost, or shut themselves off until it warms up. Here's how to tell the weather from a real fault, what the Pentair UltraTemp and Hayward heat pump codes mean, and why water under the unit usually isn't a leak, from both manufacturers' own manuals.
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A pool heat pump heats the water by taking heat from the surrounding air and moving it into the pool, rather than burning gas. Pentair's manual puts it simply: the warmer and more humid the air, the more heat there is to work with.
Pentair rates its UltraTemp at 80°F air, 80% humidity and 80°F water, and says performance drops as conditions fall from there. A properly sized unit should raise the pool about 1°F an hour on average, depending on the air, humidity and water temperature, and Pentair says heat pumps are best used to hold a set temperature rather than for fast heating. That's the main trade-off against a gas heater, which we cover in pool heater repair or replace.
Houston winters swing. The National Weather Service's 1991–2020 normals for Houston Hobby put a typical January day between about 46°F and 64°F, so the same heat pump can struggle at sunrise and run well by mid-afternoon. Pentair's advice is to run it during the warmest time of day for maximum efficiency.
When the air gets cool, frost can form on the heat pump's evaporator coil, and the unit stops heating to clear it.
So a heat pump that's quiet, showing defrost or a low ambient code on a 45°F morning usually isn't broken. If it's still not heating once the afternoon is well above 50°F, look further.
Codes differ by brand and even by model, so check the label on your unit. These are the ones Pentair's UltraTemp guide and Hayward's heat pump manual list:
| Pentair UltraTemp | Hayward | What it means | What to check |
|---|---|---|---|
| LOW WATER FLOW | FLo | The water pressure switch is open: not enough flow through the heat pump | Pump running, filter clean, valves open and any bypass around the heat pump closed |
| HIGH REFRIG | HP | The refrigerant high-pressure switch is open. Pentair lists low water flow first; Hayward also lists high water temperature | Flow first. Pentair says a unit that sat unused can be cycled on and off up to three times; if that doesn't clear it, call for service |
| LOW REFRIG | LP | The low-pressure switch is open: low refrigerant, air or water too cold, restricted air flow or a fan problem | Clear leaves and shrubs from around the unit and wait for warmer air. Refrigerant work is for a technician |
| LOW AMBIENT TEMP | FS (defrost) | Too cold to run, or a defrost that didn't finish in time (Pentair); defrosting automatically (Hayward) | Wait for the coil to clear and the air to warm |
| FIVE ALARMS | — | Five faults within one hour | It resets after an hour or with the ON/OFF button; the alarm underneath it is the real problem |
| WATER THERM / SUCTION TH (short or open) | SO, SS, dSO, dSS | A water or coil temperature sensor circuit is shorted or open | A technician; usually a sensor or wiring repair |
Hayward's inverter HeatPro models, such as the HP31005T, use a different set of codes, such as E03 for the flow switch, E01 and E02 for the pressure switches and TP for low ambient temperature, but they point to the same checks; our guide to Hayward heat pump error codes lists them all, and our guide to Pentair UltraTemp error codes does the same for Pentair. A blank display that won't start is usually power: Pentair lists a tripped breaker, a blown fuse or the transformer.
Both manufacturers say the same thing: a heat pump's evaporator coil produces condensation while it runs, usually three to five gallons an hour, and more when the air is humid. In Houston, that can be a lot of water, and it's normal.
Hayward gives a simple test. Turn the heat pump off for several hours but leave the filter pump running. If the water slows down, it was condensation; if it keeps coming, there may be a leak, and that's a call for us.
Leave the inside to a professional. Pentair warns against any internal adjustments, and refrigerant, pressure switches, sensors, fans and control boards are technician work.
Call when the codes come back after the checks above, when it still won't heat on a mild afternoon, or when water keeps coming after Hayward's condensation test. Our heater repairs run from about $200 for a small component to $1,000 or more; a replacement heat pump is roughly $6,000 to $6,500 installed, so it's worth getting the fault diagnosed properly first.
One more Houston note: a heat pump isn't freeze protection. Our hard freeze playbook explains what to do when a real freeze is coming. Gas heater instead? See our MasterTemp, Hayward H-Series and Jandy JXi and LXi code guides. For everything else, see pool heater repair.
A heat pump takes its heat from the air, so it does less as the air cools. Around 50°F it may go into defrost, and both Pentair and Hayward heat pumps can shut down until the air warms. If it heats normally on a milder afternoon, it's the weather; if it still won't heat then, check flow and any code on the display.
Pentair says the air is too cold for heat pump operation, or a defrost cycle didn't finish within its 15 minutes. The fix is to wait until the ice melts off the coil and the air warms up. If it shows in mild weather, Pentair points to the temperature sensor, which a technician can test.
Usually condensation. Pentair and Hayward both say a running heat pump produces about three to five gallons of condensation an hour, more in humid weather. Hayward's test: turn the heat pump off for several hours with the filter pump still running; if the water slows down, it was condensation.
On the models covered by Hayward's standard heat pump manual, FLo means the water pressure switch is open, so check that the pump is on, the filter is clean and the valves are open. LP means the low-pressure switch is open, which can be an air flow obstruction, the low ambient temperature cutoff (wait for 60°F) or low refrigerant.
Pentair says a properly sized heat pump should raise the pool about 1°F an hour on average, depending on air temperature, humidity and water temperature, and that heat pumps are meant to hold a temperature rather than heat quickly like a gas heater.
Tell us the brand, the code on the display and what the weather's doing. If it's the cold, we'll tell you so; if it's flow, sensors, refrigerant or the fan, we'll come out and fix it. Licensed, insured, serving Cypress and Greater Houston.