Boiler frequently loses pressure over days or weeks, requiring regular topping up.

Safety: Do not repeatedly overfill the system above the recommended pressure.

• Inspect for visible leaks: Check all radiators, valves, and joints for dampness or staining. Repair any leaks found.
• Check safety valve: Look at the discharge pipe from the Defro safety valve outside. If it drips when the boiler heats, the valve may be passing and require replacement.
• Expansion vessel test: As in E11, check and recharge the Defro expansion vessel. A failed vessel can cause pressure swings and relief valve discharge.
• Plate exchanger leak: If pressure drops and the boiler gauge rises when cold water is turned on, the plate heat exchanger may be leaking internally and should be replaced.