Pentair UltraTemp Thermistor Alarms

Thermistor alarms on the Pentair UltraTemp indicate a problem with one of the temperature sensors. These sensors are critical for proper operation as they monitor water temperature and refrigerant temperature for defrost cycles.

The four thermistor-related alarms are: WATER THERM SHORT, WATER THERM OPEN, SUCTION TH SHORT, and SUCTION TH OPEN.

Understanding Thermistors

The UltraTemp uses two main thermistors:

• Water Temperature Thermistor: Monitors pool/spa water temperature
• Suction (Defrost) Thermistor: Monitors refrigerant temperature for defrost control

These are 10K ohm NTC (Negative Temperature Coefficient) thermistors - resistance decreases as temperature increases.

Alarm Meanings

WATER THERM SHORT

Water temperature sensor circuit shows a short (very low resistance).

Causes:

• Thermistor failed short
• Wiring shorted together
• Debris or moisture shorting connections
• Control board fault

WATER THERM OPEN

Water temperature sensor circuit shows open (infinite resistance).

Causes:

• Thermistor failed open
• Broken wire
• Loose or disconnected connector
• Corroded terminals
• Control board fault

SUCTION TH SHORT

Defrost refrigerant temperature sensor circuit is shorted.

Causes: Same as WATER THERM SHORT but for defrost sensor

SUCTION TH OPEN

Defrost refrigerant temperature sensor circuit is open.

Causes: Same as WATER THERM OPEN but for defrost sensor

10K Ohm Resistance Test

To test a thermistor, measure resistance and compare to this chart:

Temperature F	Temperature C	Resistance (Ohms)
32	0	32,650
50	10	19,900
68	20	12,500
77	25	10,000
86	30	8,060
104	40	5,330

Troubleshooting Steps

Step 1: Access the Thermistor

1. Turn off power to heat pump at breaker
2. Remove access panel
3. Locate thermistor (water sensor is in water connection area, suction sensor is on refrigerant line)

Step 2: Inspect Wiring

• Check for damaged insulation
• Look for loose connections
• Inspect for corrosion on terminals
• Check for debris or moisture

Step 3: Test Thermistor

1. Disconnect thermistor wires from control board
2. Set multimeter to resistance (ohms)
3. Measure thermistor resistance
4. Compare to chart based on ambient temperature

Step 4: Interpret Results

• Reading matches chart: Thermistor is good, check wiring and control board
• Very low reading (near 0): Thermistor is shorted, replace it
• Very high or infinite reading: Thermistor is open, replace it
• Out of range for temperature: Thermistor is failing, replace it

Replacing Thermistors

Water Temperature Thermistor

1. Turn off power
2. Drain water from heat pump if needed
3. Disconnect old thermistor
4. Install new thermistor in sensor well
5. Use thermal compound if specified
6. Reconnect wiring
7. Restore power and test

Suction (Defrost) Thermistor

1. Turn off power
2. Locate sensor on refrigerant suction line
3. Remove insulation and old sensor
4. Install new sensor, ensure good contact with pipe
5. Reinsulate the sensor
6. Reconnect wiring
7. Restore power and test

When to Replace Control Board

If thermistor tests good but alarm persists:

• Double-check all wiring connections
• Try a known-good thermistor
• If still alarming, control board may need replacement
• Contact Pentair technical support for guidance