Lochinvar Interlock / External Control Wiring Issue

Heater shuts off when the pool pump turns off, but sometimes also shuts off randomly while pump is running.

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

Loose external control wires, Faulty fireman’s switch or automation relay, Incorrect wiring between pool controller and heater

How to Fix / Troubleshooting

Safety: Turn off power to both the heater and any external controllers before working on wiring.

Steps:

  • Review wiring diagram: Use the Lochinvar wiring diagram to verify that external control wires (fireman’s switch, automation contacts) are connected to the correct terminals.
  • Check connections: Tighten any loose screws on low-voltage terminals. Look for corroded or broken wires.
  • Bypass test: Temporarily disconnect external control wires and configure the heater to operate in standalone mode (per manual). If random shutdowns stop, the issue is in the external control system.

Permanent wiring changes should follow Lochinvar and automation manufacturer instructions; consult a pool professional if unsure.

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Repair Difficulty

Medium 3/5

Required Part

External Control Wiring / Fireman’s Switch
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