Pentair MasterTemp ERR PS: Water Pressure Switch Open

What ERR PS Actually Means

The MasterTemp uses an internal water pressure switch to verify that enough water is flowing through the heat exchanger before it allows the burner to fire. When the switch is open, the control board sees zero flow confirmation and displays ERR PS on the LCD. The heater will not attempt ignition in this state.

This is a critical safety feature. Firing the burner without adequate water flow would overheat the heat exchanger, potentially cracking the copper headers or triggering the high limit switch. The pressure switch must close before the ignition sequence begins.

Unlike some error codes that require a manual reset, ERR PS will auto-clear as soon as the pressure switch detects sufficient flow. If you restore flow and the heater fires normally, the problem was purely hydraulic.

Step-by-Step Troubleshooting

Owner-Level Checks

Verify pump operation and flow

1. Confirm the filter pump is running and fully primed. Look at return jets for strong flow.
2. Check that all suction and return valves are fully open. A partially closed valve after the heater is a common oversight.
3. Look at the filter pressure gauge. If pressure is significantly above your clean baseline, clean or backwash the filter.

Check for flow restrictions

1. Empty the skimmer basket and pump strainer basket.
2. Make sure no suction or return lines are blocked by debris, closed valves, or winterizing plugs left in place.
3. If the pool has a solar heating system or water features, verify those valves are not diverting flow away from the heater.

Confirm minimum flow for your model

1. MasterTemp 175 and 200: minimum 20 GPM.
2. MasterTemp 250: minimum 25 GPM.
3. MasterTemp 300: minimum 30 GPM.
4. MasterTemp 400: minimum 40 GPM.
5. If you run a variable speed pump, make sure the heater call triggers a speed that meets the minimum GPM for your model.

Tech-Level Checks

Test the pressure switch directly

1. With the pump running at full speed and adequate flow confirmed, locate the pressure switch on the heater header.
2. Disconnect the two wires from the switch and check continuity with a multimeter. With flow present, the switch should be closed (near zero ohms). Without flow, it should be open.
3. If the switch stays open even with strong flow, replace it.

Inspect the bypass valve assembly

1. The MasterTemp has an internal bypass that regulates flow through the heat exchanger. If this is stuck or incorrectly adjusted, flow may not reach the pressure switch threshold.
2. Remove and inspect the bypass assembly for debris, corrosion, or a stuck piston.
3. Clean or replace as needed and retest.

Check for air locks or header blockage

1. Air trapped in the heat exchanger can prevent the pressure switch from closing. Open the drain plug briefly to bleed air if you suspect this.
2. On older units, inspect the heat exchanger headers for scale or debris buildup that restricts flow internally.

Common Parts That Fix This Problem

• Water pressure switch (Pentair part number varies by model revision)
• Bypass valve assembly or bypass spring
• Heat exchanger drain plug and O-ring for bleeding air
• Filter cartridge or DE grids if flow restriction is at the filter

Model-Specific Notes

• The MasterTemp 400 has the highest minimum flow requirement at 40 GPM. Many variable speed pumps at low RPM will not meet this threshold. Program a dedicated heater speed into the automation.
• The MasterTemp 175 and 200 share the same 20 GPM minimum, making them the most forgiving models for smaller plumbing systems.
• All MasterTemp models use HSI (hot surface ignition) with no standing pilot. The pressure switch must close before the HSI sequence initiates.
• On systems with IntelliCenter automation and RS-485 communication, ensure the heater address (1-16) is configured correctly so automation can ramp pump speed before calling for heat.

How to Prevent ERR PS From Coming Back

• Maintain clean filters so flow stays above the model's minimum GPM requirement.
• Program variable speed pumps to run at an adequate speed during heater calls. Do not rely on the lowest energy-saving speed.
• Keep suction and return valves fully open when the heater is expected to run.
• Inspect the bypass valve assembly annually during pre-season startup.
• Do not operate the heater with water temperature below 68 degrees F, as condensation can accelerate internal corrosion and affect flow paths.