F37
Viessmann
Water Pressure Too Low / Unstable
Boiler shows F37, pressure fluctuates or drops during operation
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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
Undersized expansion vessel, failed expansion vessel diaphragm, leak in system, faulty pressure sensor
How to Fix / Troubleshooting
Safety: Hot water can discharge from the safety valve if pressure is excessive.
- Observe pressure: Note cold and hot running pressures. Large swings suggest expansion issues.
- Top up system: Refill to correct pressure if low, then monitor over several days.
- Check PRV discharge: Look at the pressure relief valve outlet pipe outside for signs of dripping; constant discharge indicates overpressure.
- Expansion vessel check (technician): A professional should measure and recharge the expansion vessel pre-charge or replace the vessel if the diaphragm has failed.
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Repair Difficulty
Medium
3/5
Required Part
Expansion Vessel
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