Weil McLain F35 Error Code

What does Weil McLain F35 mean?

F35 is a warning indicating the controller is measuring a mains supply frequency that differs from the voltage supply frequency parameter (b17, set to 1 for 60 Hz in North American installations). This is a warning only — heat demand is not switched off. The fault clears automatically when the measured frequency matches the configured parameter.

Symptoms

• F35 warning displayed on the control module LCD.
• Boiler continues to operate in heating mode despite the warning.
• Fault may appear after a control board replacement if frequency parameter was not set.
• Fault may indicate electrical supply quality issues.

Common causes

• Parameter b17 (main supply frequency) set incorrectly — should be 1 for 60 Hz in the US.
• Electrical supply frequency instability from a generator or power quality issue.
• Faulty control board measuring frequency inaccurately.

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Diagnostic steps

1. Check parameter b17

   Access the configuration menu (hold DHW+ and DHW– for 10 seconds) and scroll to b17. Confirm it is set to 1 (60 Hz) for US installations. Change to 1 if it shows 0 (50 Hz).

2. Measure supply frequency

   Use a multimeter or power quality meter to measure the actual supply frequency at the boiler. Nominal North American frequency is 60 Hz. Significant deviation indicates a supply quality problem.

3. Check if running on generator power

   If the boiler is operating on a generator during an outage, the generator frequency may be unstable and trigger F35. Confirm the generator is operating at stable 60 Hz output.

4. Reset and monitor

   After verifying b17 is correctly set and supply frequency is stable, press Reset and monitor for recurrence of F35.

When to call a professional

Call a licensed electrician if the supply frequency is consistently outside the 60 Hz nominal range, as this indicates an upstream power quality problem requiring investigation at the service panel or utility connection. If F35 appears on a stable utility supply with a correctly configured boiler, the control board may need replacement.