Pentair MasterTemp ERR HLS: High Temperature Limit Switch Open

What ERR HLS Actually Means

The MasterTemp monitors water temperature at two points in the heat exchanger. The high limit switch (HLS) sits after the first pass and is calibrated to open at 135 degrees F. When water flowing through the exchanger gets that hot, it means the heat exchanger is not dissipating BTUs into the water fast enough. The control board shuts down the burner immediately to prevent damage.

This is distinct from ERR AGS, which monitors temperature after the second pass and trips at 140 degrees F with a harder lockout. ERR HLS is the first line of defense and has a softer recovery: it auto-clears once temperature drops below the threshold.

A single HLS trip during unusual conditions (pump speed change, filter backwash mid-cycle) may not be alarming. Repeated trips point to a persistent flow problem, an oversized heater for the plumbing, or a failing bypass valve.

Step-by-Step Troubleshooting

Owner-Level Checks

Verify adequate water flow

1. Confirm the pump is running at adequate speed. If on a variable speed pump, increase RPM and see if the HLS stops tripping.
2. Check the filter pressure. A dirty filter reduces flow through the heater, causing the heat exchanger to overheat.
3. Ensure all valves between the pump, filter, heater, and pool returns are fully open.

Check for recent changes

1. Did someone recently lower the pump speed, close a valve, or add a water feature that diverts flow?
2. Was a new heater installed on existing plumbing that may be undersized for the BTU output?
3. Has the thermostat been set unusually high, such as above 104 degrees F for a spa?

Tech-Level Checks

Test the high limit switch

1. Let the heater cool completely. With power off, locate the high limit switch on the heat exchanger header.
2. Disconnect the switch leads and test continuity. At room temperature, the switch should be closed.
3. If the switch is open at room temperature, it has failed and needs replacement.

Inspect the bypass valve

1. The internal bypass valve regulates how much water circulates through the heat exchanger versus bypassing it. A stuck-open bypass diverts too much water around the exchanger, reducing cooling.
2. Remove and inspect the bypass assembly. Look for corrosion, debris, or a stuck piston.
3. Replace or clean and reassemble, then retest.

Verify flow rate matches model requirements

1. Use a flow meter or calculate GPM from pump curves. Compare against minimums: 20 GPM (175/200), 25 GPM (250), 30 GPM (300), 40 GPM (400).
2. A MasterTemp 400 on undersized 1.5 inch plumbing may never achieve 40 GPM regardless of pump size.
3. Consider whether the heater model is appropriate for the plumbing infrastructure.

Check for scale in the heat exchanger

1. Hard water and low-pH conditions can build scale inside the copper heat exchanger tubes, reducing heat transfer efficiency.
2. If you suspect internal scaling, a professional acid flush of the heat exchanger may be necessary.

Common Parts That Fix This Problem

• High limit switch (opens at 135 degrees F)
• Bypass valve assembly or bypass spring
• Water temperature sensor (thermistor) if readings are inaccurate
• Heat exchanger assembly in severe scaling or damage cases

Model-Specific Notes

• Higher BTU models (300K, 400K) are more prone to HLS trips because they put more heat into the exchanger per unit time. Adequate flow is critical on these larger models.
• The MasterTemp operates in both Natural Gas and Propane configurations. BTU output is the same, but LP models should be verified for correct orifice sizing to avoid overfire.
• Do not operate below 68 degrees F water temperature. Cold water combined with hot exhaust creates condensation that accelerates heat exchanger corrosion.
• On spa applications, maximum water temperature should not exceed 104 degrees F. Setting the thermostat too high for a small spa body can cause rapid HLS trips.

How to Prevent ERR HLS From Coming Back

• Maintain clean filters and open valves so flow always exceeds the model's minimum GPM.
• Size the heater appropriately for the plumbing diameter and pump capacity.
• Service the bypass valve annually to prevent sticking.
• Keep water chemistry balanced to minimize scale buildup inside the heat exchanger.
• On variable speed pump systems, program a heater call speed that exceeds minimum GPM by at least 10 to 20 percent.