Furnace frequently loses pressure over days or weeks without visible leaks

Safety first: Turn off power before inspecting inside the furnace. Be cautious of hot surfaces.

• Check discharge points: Inspect the pressure relief valve outlet and condensate drain for signs of water discharge.
• Inspect inside furnace: Look for dried streaks or corrosion indicating slow leaks on the heat exchanger or fittings.
• Isolate DHW (combi models): A leaking plate heat exchanger can pass system water into the DHW circuit. A technician can test by isolating and pressure testing the exchanger.
• Professional leak detection: If no obvious leaks are found, a heating engineer may need to use tracer gas or thermal imaging to locate hidden leaks.

Warning: Repeatedly topping up introduces oxygen and accelerates corrosion. Address the root cause promptly.