Pentair UltraTemp Heat Pump Error Codes: LOW WATER FLOW, HIGH REFRIG, LOW REFRIG, and Every Alarm Message — A South Florida Pool Pro's Guide

The most likely fix on an UltraTemp alarm: check water flow before you touch anything inside the cabinet. The UltraTemp doesn't throw numeric codes — it lights a red SERVICE light and spells the problem out in words on the LCD. The two messages I see most on South Florida pads, LOW WATER FLOW and HIGH REFRIG, share the same first three causes in Pentair's own troubleshooting table: the pump is off, flow is too low, or an external bypass valve is open. Clean the filter, set the valves, reset once, and most of these calls end there. Every alarm below comes from Pentair's UltraTemp manual; a few thresholds shifted between revisions, so check yours.

This covers the heat-pump-only UltraTemp, 70 through 140, including the H/C heat-and-cool models. Pentair's 2010 guide lists the same alarm messages for the ThermalFlo HP. The UltraTemp ETi hybrid, a heat pump and gas burner in one cabinet, has its own manual and isn't covered here. For a gas Pentair heater, see our MasterTemp 400 error code guide.

How the UltraTemp tells you something is wrong

Three panel features save real time:

• The alarm history. The board stores the last 30 alarms. From the ALARM STATUS screen, press the DOWN arrow — the first entry is the most recent.
• The five-minute restart delay. When the unit is turned on or reset, the compressor waits five minutes to protect itself. A fan running with no cool air out the top during that window is normal.
• The input status screen. In the service menu (press UP and DOWN together), a tech can read a 1 (on) or 0 (off) for the water flow switch (WF) and the high and low refrigerant pressure switches (HP, LP).

UltraTemp alarm message quick reference

Fix before you call: the order I work every UltraTemp alarm

1. Read the message and the history. If it says FIVE ALARMS, scroll back — Pentair notes the alarm repeating underneath most likely caused the lockout.
2. Confirm the pump is running at a real speed. The UltraTemp needs at least 30 GPM. A variable-speed pump on a low overnight speed can fall short.
3. Clean the filter and empty the baskets. Pentair lists dirty filters, a clogged pump impeller, and suction air leaks under low flow.
4. Set the valves. Close any external bypass around the heat pump and set every other valve to send water through it. A pool-spa valve left on spa is a classic low-flow trap.
5. Clear the airflow. Pentair calls for 24 inches clear in front, 12 inches on the other sides, and 5 feet above under an overhang. Pull leaves off the coil, trim shrubs, and hose a dirty coil gently — never a pressure cleaner.
6. Reset once. Press ON/OFF to clear the alarm, wait out the five-minute delay, and watch a full start.
7. Stop if it comes back. The same alarm with verified flow and a clean coil means a switch, the TXV, the refrigerant charge, or the electrical supply. That's licensed work.

LOW WATER FLOW: the switch is usually telling the truth

The UltraTemp won't run until its water pressure switch closes, and Pentair presets that switch at 1.5 psi. The manual's four causes run in the order I'd check them anyway: pump off, not enough flow, an open bypass, and only then a bad switch, which a tech confirms with a resistance check while water is flowing. On Florida routes since the late 1990s, I've cleaned far more filters behind this alarm than I've replaced switches.

A very short return line to the pool may not build enough back pressure to close the switch; Pentair's fix is a directional fitting where the return enters the pool. And the switch is adjustable, but Pentair warns that setting it to close at too low a flow can damage the appliance. That adjustment is a tech job.

HIGH REFRIG: usually a flow problem wearing a refrigerant label

HIGH REFRIG means the refrigerant high-pressure switch opened. People assume a recharge, but Pentair's first three causes are the same flow problems behind LOW WATER FLOW: when water isn't carrying heat away fast enough, refrigerant pressure climbs until the switch trips. Pentair also lists a unit plumbed backwards — the manual puts the inlet on the right side and the outlet on the left — and a heat exchanger clogged with debris.

On a heat pump that hasn't run for an extended period, Pentair says to check the pool water level and cycle the unit on and off up to three times before calling for service. Past that, the remaining causes — a failed switch, a TXV stuck closed, or an overcharged system — are work for an EPA 608-certified tech.

LOW REFRIG: a cold morning or a leak

LOW REFRIG means the low-pressure switch opened. A leak is on Pentair's list, but so are air and water temperatures too cold to operate the heat pump — and the manual's fix for that one is to wait for warmer temperatures and see if it runs. On a January cold-snap morning in Palm Beach County, that's the first thing I consider. Next come restricted airflow through the coil, a faulty fan (capacitor, motor, or a slipping blade), a bad switch, and a TXV stuck closed.

A real leak means find it, repair it, pull a vacuum, and recharge — sealed-system work that federal law reserves for EPA Section 608-certified techs. Topping off without finding the leak just schedules the next call.

Defrost is not a fault

The UltraTemp may enter defrost when the air is around 50°F, if defrost is enabled in the service menu. The documented cycle is passive: the compressor stops, the fan runs about 15 minutes, and the compressor restarts only if the refrigerant sensor reads 20°F above the defrost start point — otherwise the unit shuts down. A heat pump that goes quiet on a cold morning may be doing exactly what it was designed to do. Pentair also lists an Active Defrost 140 H/C, so confirm which you have. If ice stays on the coil, shut the unit off, let it thaw, and fix the cause. More in our heat pump defrost guide.

The sensor and power messages

WATER THERM SHORT or OPEN, SUCTION TH SHORT or OPEN, HIGH WATER TEMP, and LOW WATER TEMP all point at a thermistor or the control board. Check the real water temperature with a thermometer first. South Florida pool water doesn't get anywhere near 40°F, so LOW WATER TEMP here is a sensor problem until proven otherwise. A tech measures the thermistor against Pentair's 10k-ohm chart — about 10,000 ohms at 77°F — and replaces it if it reads shorted or open, or replaces the board if it tests good.

BROWNOUT means the 24-volt supply to the board is low. Pentair's numbers are 208–260 volts into the transformer and 24–28 volts out. Low output means a new transformer; low supply means a lost phase or a voltage sag that may need a boost transformer near the unit. Pentair warns that voltage below 208 can damage the heat pump, and a breaker that keeps tripping goes to a certified electrician.

REMOTE POOL/SPA and SERIAL COMM LOST are automation issues: the first is fixed in the automation output, and for the second, the heat pump's RS-485 address must match the controller.

No alarm, but no heat

A fan running with no cool air out the top is the five-minute delay or a compressor, capacitor, or contactor problem. A fan that won't turn points to the fan relay, capacitor, motor, or board. A pool that heats slowly with no alarm usually isn't broken: Pentair lists low flow, cold air, too short a pump run, no cover, wind, and shade, and says a heat pump is built to hold a temperature, not heat fast.

South Florida failure modes

• Salt air on coastal pads. Pentair's manual flags salt water near the coast. Oceanside coils and connectors corrode years ahead of inland units, and a corroded sensor connector can look like a dead sensor — we check connections before ordering parts. Rinse the coil with fresh water.
• Cold snaps. Output drops as the air cools, so a few January mornings a year bring LOW REFRIG or a defrost shutdown on a healthy unit. Run it during the warmest part of the day.
• Condensate. The UltraTemp normally drains 3–5 gallons an hour while running, more when it's humid. A puddle under a running unit is usually condensate — test it for chlorine. Water that keeps draining more than an hour after shutdown points to an internal leak.

What it costs, and when to replace

A diagnostic runs $0–$125, and it's free for our weekly-route clients. Most heater repairs — the sensors, pressure switches, relays, and control boards behind most UltraTemp alarms — land between $250 and $1,800. A failed compressor or heat exchanger runs $4,000–$5,000, and a new heater installed is $5,000–$7,000. When a compressor quote gets that close to a new unit on an older heat pump, I'll tell you to put the money toward the replacement.

When to call a pro

The filter, baskets, valves, clearances, and a gentle coil rinse are yours. Everything past that is licensed work. At Florida's Best Pool Service, repairs are managed by us under our Florida Certified Pool/Spa Contractor license. We do most of the work ourselves and bring in licensed specialists when a job needs one: an EPA 608-certified tech on the refrigerant side, or an electrician for a new circuit. For the same first checks on any brand, see our pool heater problems guide; AquaCal owners can use the AquaCal heat pump troubleshooting guide. When you're ready, book a pool heater repair visit and we'll diagnose it with a written quote before any part is ordered.