Weil McLain F37

What does Weil McLain F37 mean?

F37 indicates the central heating system water pressure has dropped below the low-pressure threshold — the boiler's low water pressure switch is set at 9 psi (±2 psi), and F37 triggers if pressure falls more than 8 psi below its proper operating level. When this fault is active, heat demand and the pumps are switched off. The fault is detected via both the low water pressure switch and the water pressure sensor.

Symptoms

• Boiler displays F37 and stops heating with pumps off.
• System pressure gauge reads below normal operating range.
• Fault may not auto-reset and requires manual power cycling in some installations.
• Repeated F37 events indicate a chronic pressure loss problem.

Common causes

• System water leak causing pressure to drop below the low-pressure threshold.
• Automatic fill valve (pressure reducing valve) not refilling the system.
• Expansion tank waterlogged, causing excessive pressure swings and low cold pressure.
• Water pressure sensor damaged or disconnected.

Diagnostic steps

1. 01

   Check system pressure gauge

   Read the boiler pressure/temperature gauge. System should be 12–15 psi cold. If below this range, the system needs refilling.

2. 02

   Inspect the system for leaks

   Walk the full piping system and inspect all joints, circulators, zone valves, and the boiler itself for water leaks. Repair any leaks before refilling.

3. 03

   Use the fill loop to restore pressure

   Slowly open the fill loop valves to add water until the pressure gauge reads 12–15 psi. Close the fill valves immediately. If the system holds pressure, the issue is a depleted fill cycle.

4. 04

   Check the automatic fill valve

   Verify the automatic fill valve is set correctly and operational. If the system pressure keeps dropping, the fill valve is not maintaining pressure and needs service or replacement.

5. 05

   Inspect the water pressure sensor and switch

   Confirm the pressure sensor wiring is connected and undamaged. The low water pressure switch is set at 9 psi (±2 psi) — verify the switch is not stuck open, which would produce a false F37.

When to call a professional

Call a licensed technician if the source of pressure loss cannot be identified, if pressure continues dropping after refilling, or if the pressure switch or water pressure sensor requires replacement. A recurring F37 pattern indicates an active leak in the system that requires pressure testing to locate and must be repaired before the boiler can operate reliably.