F.20
Lochinvar Low Voltage / Power Supply Fault

Boiler shows F.20 and locks out, often immediately on a call for heat

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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

Incoming line voltage too low, loose power connections, failing transformer, shared circuit overload

How to Fix / Troubleshooting

Safety first: Turn off power at the breaker before tightening any electrical connections.

  • Check dedicated circuit: Ensure the boiler is on a dedicated circuit as recommended by Lochinvar. Avoid sharing with large loads.
  • Inspect line connections: At the boiler's junction box, verify hot, neutral, and ground connections are tight and not discolored or burnt.
  • Measure voltage (qualified persons only): With power on, a qualified person can measure line voltage at the boiler. It should be within the range specified on the boiler nameplate (typically 120V ±10%).
  • Reset boiler: After correcting any wiring issues, reset the boiler and observe operation.

Professional help: If voltage is unstable or the internal transformer/control board is suspected faulty, contact an electrician or heating technician.

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Repair Difficulty

Hard 4/5

Required Part

Power Supply / Control Transformer
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