Why Does My Boiler Keep Losing Pressure After a Repair?

The Most Common Reason: A New or Missed Leak

Often, a boiler losing pressure after a repair points to a leak that was either created during the work or was not the original fault. When an engineer opens the sealed heating system to replace a part, they must reconnect pipework, valves, and fittings. If a joint isn't perfectly sealed, it can weep water. This leak might be tiny and slow, causing pressure to drop over days rather than hours. Common spots are around the pump seals, radiator valves that were bled, or connections to a new component like an expansion vessel.

Issues with the Pressure Relief Valve (PRV)

The pressure relief valve is a critical safety component designed to release water if the system pressure gets too high. Sometimes, after a repair that involved repressurising the system or replacing related parts, the PRV can get a piece of debris stuck in its seat, preventing it from sealing fully. This allows a small, constant trickle of water to escape, usually via an external pipe outside your home (the discharge pipe). If you see a drip from this pipe, it's a strong indicator the PRV is passing and needs attention from an engineer.

Air in the System and Faulty Components

Introducing air into the system is common during repairs. While bleeding radiators removes air from them, air can also become trapped elsewhere, like inside the boiler's heat exchanger or pump, creating an 'airlock'. This can cause erratic pressure readings and heating problems. More seriously, if the repair involved the expansion vessel (a rubber diaphragm that absorbs expanding water), and it wasn't recharged or was faulty, the system has nowhere for the expanding hot water to go. This forces the PRV to open, releasing water and causing the pressure to drop when the system cools.

What an Engineer Will Check

A Gas Safe registered engineer will perform a systematic diagnosis to find the cause of the recurring pressure loss. Their checks typically include:

• Visual Inspection: Looking for signs of water leaks around the boiler, pipework, radiator valves, and under floorboards near the work area.
• Pressure Test: Isolating sections of the system and using a pressure testing pump to see if pressure holds, pinpointing the leak's location.
• Expansion Vessel Check: Testing the vessel's air charge with a tyre gauge and checking it hasn't failed.
• PRV Operation Test: Manually operating the valve to see if it reseats correctly and checking the discharge pipe for flow.
• Component Integrity: Ensuring any newly fitted parts, like an automatic air vent or fill valve, are working correctly and not leaking.

When to Call an Engineer Back

If your boiler starts losing pressure within a few days of a repair, you should contact the company that carried out the work. Reputable engineers will have a warranty on their labour and will want to rectify any issue arising from the repair. Do not keep repressurising the boiler yourself to compensate, as this can mask the problem and potentially lead to more damage or a safety issue if the PRV is involved. Persistent pressure loss needs a professional diagnosis to ensure your heating system is safe, efficient, and watertight.