Weil McLain F27 Error Code

What does Weil McLain F27 mean?

F27 indicates an exhaust gas over-temperature event specific to the vent material type selected (PVC, CPVC, Polypropylene, or AL29-4C). During flue gas protection mode, if the power reduction reaches 0% and the exhaust temperature still exceeds the maximum for the selected vent material, the controller stops and holds the fault for 5 minutes before automatically attempting recovery.

Symptoms

• Boiler displays F27 and shuts down for at least 5 minutes.
• Fault clears automatically after the 5-minute hold if exhaust temperature normalizes.
• Boiler repeatedly trips F27 under high heating demand conditions.
• Preceding F07 events are common before F27 escalates.

Common causes

• Vent material parameter b22 not set to match the actual installed vent pipe type.
• Vent piping that exceeds allowable length or uses incorrect elbow types.
• Air trapped in the heat exchanger reducing water-side heat absorption.
• Near-boiler piping undersized, restricting flow and raising exhaust temperatures.
• CH setpoint too high combined with an aggressive ramp rate (P02 at factory 11).

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Diagnostic steps

1. Verify vent material parameter b22

   Access the configuration menu (hold DHW+ and DHW– for 10 seconds) and scroll to parameter b22. Confirm the value matches the installed vent material: 1=PVC, 2=CPVC, 0=Polypropylene/AL29-4C.

2. Inspect vent piping lengths and fittings

   Verify the vent and air intake piping lengths and fittings comply with the AquaBalance Series 2 boiler manual tables. Replace standard elbows with long-sweep elbows as required.

3. Purge air from the heat exchanger

   Listen for gurgling or sloshing when the circulator starts. If air is present, bleed the heat exchanger thoroughly until water flows quietly.

4. Lower CH setpoint and reduce ramp slope

   Reduce the CH setpoint to 175–180°F. Access Transparent Parameters (hold Reset 20 seconds, navigate to P02) and reduce P02 from 11 to 6 to slow the temperature ramp.

5. Verify near-boiler pipe sizing

   Confirm the primary loop uses full 1-inch pipe for AB-155C models and that the pump speed is set correctly. Undersized piping or low pump speed increases exhaust temperature.

When to call a professional

Call a licensed technician if F27 persists after verifying vent material parameter, correcting piping, and reducing setpoints. Persistent F27 requires combustion analysis at high and low fire with a calibrated combustion analyzer to confirm the air-gas ratio is correctly set and that flue temperatures remain within safe limits for the installed vent material.