Heaters & heat pumps
Pentair UltraTemp heat pump alarms: what LOW WATER FLOW, HIGH REFRIG and the rest mean
The Pentair UltraTemp heat pump shows its faults as named alarms and lights its red SERVICE light. Pentair's guide lists fourteen of them. Several come down to water flow or the weather and are worth checking yourself; the sensor, electrical and refrigerant alarms are inside the cabinet. What each alarm means, what to check first, and the delays that look like a fault but aren't.
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The Pentair UltraTemp is a heat pump, so it doesn't light a burner. It uses a fan, a compressor and refrigerant to move heat from the air into the water. When something stops it, the red SERVICE light comes on and the display names the alarm (UltraTemp guide, p. 16). Pentair's guide lists fourteen alarm messages. This article goes through each one, what Pentair says causes it, and which ones are worth checking yourself before calling anyone.
Safety first
- Don't open the cabinet. Pentair's troubleshooting section warns of a risk of electrical shock or electrocution, says not to attempt any internal adjustments inside the heater, and says to always turn power to the unit off before servicing (UltraTemp guide, p. 27).
- Keep hands and hair clear of the fan (p. 27).
- If any part of the heat pump has been under water, Pentair says not to use it and to call a qualified service technician to inspect it (p. 16).
- A breaker that keeps tripping is an electrical fault. Pentair's answer for a unit that runs but trips its breaker continuously is to contact an electrician (p. 30). Leave it off.
The alarms at a glance
From Pentair's control panel alarm list and troubleshooting chart (UltraTemp guide, pp. 20 and 27-29):
| Alarm | What Pentair says it means | Worth checking yourself? |
|---|---|---|
| LOW WATER FLOW | The pressure switch sensing water flow is open | Yes: pump, filter, valves, bypass |
| HIGH REFRIG | The refrigerant high pressure switch is open | The flow checks first; then a technician |
| LOW REFRIG | The refrigerant low pressure switch is open | Only the weather and the fan; then a technician |
| LOW AMBIENT TEMP | The air is too cold for the heat pump to run | Wait for warmer air or for ice to melt |
| HIGH WATER TEMP | Water above 107°F | Check the water temperature with a thermometer |
| LOW WATER TEMP | Water below 40°F | As above |
| WATER THERM SHORT / OPEN | The water temperature sensor circuit is shorted or open | Technician |
| SUCTION TH SHORT / OPEN | The defrost sensor circuit is shorted or open | Technician |
| BROWNOUT | The 24-volt supply to the control board is low | Technician or electrician |
| REMOTE POOL/SPA | The pool and spa remote inputs are both energized at once | Automation settings |
| SERIAL COMM LOST | No message from the automation controller for two minutes | Automation and its cable |
| FIVE ALARMS | Five faults of any kind within one hour | Find the alarm that caused them |
Pentair's alarm list gives the high water temperature as above 107°F, and its troubleshooting chart says above 108°F (pp. 20 and 27). Either way it's a few degrees above the spa maximum of 104°F that the chart mentions elsewhere (p. 30).
The control keeps a history of the last 30 alarms. On the alarm status screen, press the DOWN arrow: the first one shown is the most recent (UltraTemp guide, p. 17). If you're not sure which alarm started the trouble, that list is worth reading, or photographing for us.
LOW WATER FLOW
This is the alarm most worth checking yourself, because most of its causes are outside the heat pump. The UltraTemp has a water pressure switch that only lets it run when water is flowing through it. Pentair's chart lists, in order (UltraTemp guide, p. 27):
- The circulation pump isn't on. Check it's running, and on a variable-speed pump, at a speed that moves enough water (see why a variable-speed pump set too low stops your heater).
- Not enough flow. A dirty or worn filter or clogged lint traps. Pentair says to clean the filter, backwash if needed, inspect and clean the pump impeller, adjust the plumbing valves and fix suction air leaks.
- An external bypass is open. If there's a bypass valve around the heat pump, it should be closed.
- A bad water pressure switch. That's a resistance test with water flowing, for a technician.
Pentair also says the pressure switch is adjusted at installation so the heat pump turns off when the pump turns off, and warns that setting it to close at too low a flow can damage the unit (p. 27). If the alarm started after someone adjusted it, say so when you call.
HIGH REFRIG
The high pressure switch opens when refrigerant pressure gets too high, and the most common reason is the same as LOW WATER FLOW: not enough water carrying the heat away. Pentair's first checks are that the pump is on, the filter and skimmers are clean, and the external bypass is closed with all other valves set to let water through the heat pump. It gives a minimum flow of 30 gallons per minute (UltraTemp guide, p. 28).
Pentair adds a simple one: this alarm can appear if the heat pump hasn't been run for a long time. Its advice is to check the pool's water level, then cycle the heat pump on and off up to three times. If that doesn't clear it, call for service (p. 28).
Beyond that, Pentair's list moves inside the unit: a bad high pressure switch, a stuck expansion valve, an overcharged system, a clogged heat exchanger, a damaged internal bypass valve, a unit plumbed backwards, or the compressor's thermal protector having tripped (p. 28). Those are for a technician to diagnose.
LOW REFRIG
The low pressure switch opens when refrigerant pressure is too low. Pentair lists cold weather first among the causes you can do something about: if the air and water are too cold for the heat pump, wait until it warms up and see whether the unit runs. It also lists a faulty fan or fan capacitor, and insufficient airflow through the air coil, where the fix is to clear debris or shrubs from around the unit (UltraTemp guide, p. 28).
The rest of the list is a refrigerant leak, a bad low pressure switch or a stuck expansion valve (p. 28), which need a technician to find.
LOW AMBIENT TEMP and defrost
Heat pumps take heat from the air, so cold air limits them. When Defrost is enabled in the service menu, the UltraTemp may run a defrost cycle when air temperatures are around 50°F: the compressor stops and the fan keeps drawing air over the coil for about 15 minutes. If the coil hasn't warmed enough by then, the unit shuts down and shows Low Ambient Temp (UltraTemp guide, p. 22). Pentair's fix is to wait until the ice melts from the coil or the air warms up; a bad sensor is the other possibility (p. 29).
Pentair also notes that a heat pump's efficiency falls as the air gets colder, and suggests running it during the warmest part of the day (p. 22). On a cold East Valley morning, that's worth trying before anything else.
Ice or frost on the coil. Pentair lists restricted airflow, plants too close, a clogged coil, sprinklers spraying the unit in cool weather, defrost not enabled, or a refrigerant leak. If ice forms, it says to turn the heat pump off and let it thaw, and that you may use water at low pressure on the coil with the unit off (UltraTemp guide, p. 29). Keep sprinklers pointed away from it: Pentair says they can spray water into the heater, and that well water can leave mineral build-up on the coil and electronics (p. 2).
Water temperature and sensor alarms
HIGH WATER TEMP and LOW WATER TEMP. Check the water with a pool thermometer. If the water really is that temperature, the alarm is doing its job. If it isn't, Pentair's chart points to the thermistor or the control board, tested with a 10k ohm resistance test (UltraTemp guide, p. 27).
WATER THERM and SUCTION TH, SHORT or OPEN. One of the two temperature sensors, for the water or for the refrigerant line used in defrost, has a shorted or open circuit. Pentair's remedy is the same resistance test: replace the thermistor if it fails, or the control board if the thermistor is good (p. 27). That's a technician job inside the cabinet.
BROWNOUT
The 24-volt supply to the control board is low. Pentair's chart opens with a high-voltage caution and lists two causes: a bad transformer, or not enough supply voltage to the heat pump, which should be between 208 and 260 volts with no phase missing (UltraTemp guide, p. 27). After a storm or an outage it can be the supply; if it keeps coming back, the voltage needs measuring. See pool pump not working after a power outage for the rest of the pad.
Automation alarms: REMOTE POOL/SPA and SERIAL COMM LOST
These are about what's controlling the heat pump, not the heat pump itself.
- REMOTE POOL/SPA: the pool, spa and common terminals are all energized at once. Pentair's fix is to change the automation output so only pool and common, or spa and common, are energized (UltraTemp guide, p. 27).
- SERIAL COMM LOST: the control board hasn't heard from the automation controller for two minutes (p. 20). When it's set up correctly, a "heartbeat" indicator appears on the main screen each time a message arrives (p. 19), so a missing heartbeat confirms the link is down.
For blank, frozen or out-of-touch automation panels, see pool automation not working.
FIVE ALARMS
Five faults of any kind within an hour. It resets itself after an hour, or you can clear it with the ON/OFF button. Pentair says to watch the heat pump over the next few minutes or hours to see which alarm comes back, because that alarm most likely caused the five (UltraTemp guide, p. 29).
Not heating, but no alarm
Before calling, it's worth knowing the UltraTemp's built-in delays. After the water pressure switch closes there's a one-minute delay while it senses the water temperature, then, if it's calling for heat, the fan starts and there's another five-minute delay before the compressor (UltraTemp guide, p. 16). The five-minute restart delay protects the compressor from cycling on and off too quickly (p. 20). A heat pump that's been on for a minute or two isn't broken.
If it's past the delays, Pentair's chart for no heat and no alarm says to check that the heat pump is in the right pool or spa mode, and that the Remote feature isn't enabled with nothing connected to control it (UltraTemp guide, p. 29).
Water under the heat pump is usually normal. Pentair says the coil makes condensation while the unit runs, usually three to five gallons an hour depending on the heat and humidity, and that the drain holes in the base should be kept clear (UltraTemp guide, p. 2).
Slow to heat? Pentair says a heat pump won't heat a pool as fast as a large conventional pool heater, and a pool that has cooled a lot can take several days to come back up (UltraTemp guide, p. 16). Its chart for slow heating lists flow first, then cold weather, a pump timer that doesn't run long enough, no pool cover, wind and shade (p. 30). More on that in why your pool heater isn't heating.
Where to stop
LOW WATER FLOW, and often HIGH REFRIG, start with the pump, the filter, the valves and the bypass, and those are worth checking first. LOW AMBIENT TEMP and LOW REFRIG in cold weather may need nothing but warmer air. Everything else, the sensor alarms, BROWNOUT, and a refrigerant alarm that returns once flow and weather are ruled out, is inside the cabinet and belongs to a technician. Don't keep clearing an alarm that comes back.
Getting it diagnosed
Call when a flow alarm comes back with the filter clean and the pump running, when HIGH REFRIG doesn't clear after Pentair's three restarts, or when the heat pump shows a sensor, BROWNOUT or refrigerant alarm in mild weather. Our heat pump diagnostic checks the water flow and pressure switch, the supply voltage, the fan and the sensors, reads the alarm history against Pentair's guide, and tells you what has failed and what putting it right involves before anything is quoted.
If you don't know what's wrong, a diagnostic is $120, and it's credited toward your repair if you go ahead. Start from heat pump repair, or answer a few questions in the Pool Problem Finder. A photo of the display and the unit's rating plate helps. If you have a Hayward or Jandy heat pump instead, see the Hayward heater and heat pump codes or the Jandy heater and VersaTemp codes.
Sources
Every manufacturer statement above links to this source. Page numbers are the printed page numbers in the guide.
- Pentair, UltraTemp Heat Pump Installation and User's Guide, P/N 474099 Rev. AA (5/4/23).
Pentair and UltraTemp are trademarks of Pentair. Hayward and Jandy are trademarks of their owners. They are named here only to identify equipment.