Why Does My Boiler Keep Losing Pressure?

The most common cause: a small leak

The majority of persistent pressure loss comes down to a leak somewhere in the sealed central heating system, which can be tiny and hard to spot. Check for damp patches around radiator valves, pipe joints under floors, and around the boiler itself.

Even a leak too small to see as dripping water can lose enough pressure over a few days to trigger a noticeable drop, since it only takes a small volume of water at system pressure to register on the gauge.

A faulty pressure relief valve

The pressure relief valve is designed to release water if the system pressure gets too high, but if it's worn or faulty it can leak even at normal pressure, usually visible as a dripping pipe outside the property. This is a common cause of pressure loss that isn't obvious from inside the house, so it's worth checking outside even if you can't find anything internally.

A faulty expansion vessel

The expansion vessel inside the boiler absorbs the natural expansion of water as it heats up. If the vessel's internal diaphragm fails or its air charge is lost, the system pressure will fluctuate and drop more than it should, sometimes accompanied by pressure spiking briefly when the heating fires.

Diverter valve or heat exchanger issues

Less commonly, an internal fault such as a failing diverter valve or a developing leak within the boiler's heat exchanger can also cause slow pressure loss. These are harder to diagnose without opening the boiler casing, which is gas appliance work and must be carried out by a Gas Safe registered engineer.

What to check yourself before calling out an engineer

• Look at the boiler pressure gauge — is it below the manufacturer's minimum, usually around 1 bar?
• Check for any visible water around the boiler, radiators or pipework
• Check the external pressure relief pipe outside your house for signs of dripping
• Top the pressure back up once via the filling loop and note how quickly it drops again
• Note whether the drop happens gradually over days or suddenly, as this helps narrow down the likely cause

Why you shouldn't just keep topping it up

Repeatedly topping up pressure without finding the cause introduces more water and dissolved oxygen into the system, which can accelerate corrosion and make the underlying problem worse over time. If pressure loss is frequent, it needs diagnosing rather than managing, and the sooner a leak is found, the less water damage it's likely to cause elsewhere in the property.

What a Gas Safe engineer checks and in what order

• Confirms the current pressure reading and asks how quickly and how often it has been dropping
• Visually inspects the boiler casing, valves and visible pipework for damp or staining
• Checks the external pressure relief pipe for signs of recent dripping
• Pressure-tests the sealed system to help narrow down whether the loss is from the boiler itself or the wider pipework
• Inspects the expansion vessel's air charge if no external leak is found

What this typically costs to diagnose and fix

Diagnostic work to trace a pressure loss is charged at our standard £99 per hour daytime rate, with a one-hour minimum then billed in 30-minute increments. Straightforward fixes, such as replacing a faulty pressure relief valve, are often completed within the first hour. If the cause turns out to be a hidden pipework leak or an issue inside the boiler itself, this may take longer and any replacement parts will be quoted before they're fitted.

Why this matters for Cheam and Sutton homes specifically

Many properties in Cheam and Sutton have combi boilers installed within sealed systems that were fitted a decade or more ago. As these systems age, joints and components that were fine when new become more prone to slowly weeping, which is why persistent pressure loss becomes more common as a boiler and its pipework get older. If your boiler is due a service anyway, mentioning a pattern of pressure drops beforehand helps the engineer focus their checks in the right place from the start.