Why your boiler keeps losing pressure

A sealed heating system is, as the name suggests, sealed. The same water goes round and round for years. So when the pressure gauge keeps falling, it is not being used up — it is leaving. Your job, or ours, is to work out where.

1. A leak on the system

The most common cause, and the one people least want to hear. It does not have to be dramatic — a weep from a radiator valve that evaporates before anyone notices will empty a system over weeks.

Worth checking yourself, carefully:

• Every radiator valve, top and bottom, and the blanking plugs at each end.
• Under the boiler, and the pipework immediately around it.
• Towel rails in bathrooms — a very common culprit.
• Any recent building work. A nail through a pipe under a floor is a classic.
• Ceilings and the underside of floors below pipework, for staining.

If the leak is buried under a floor or a screed there may be nothing visible at all. That is a job for pressure testing rather than guesswork.

2. The expansion vessel has lost its charge

Inside your boiler is a small tank divided by a rubber diaphragm, with air on one side. Water expands when it is heated, and that cushion of air absorbs the expansion. Over years the air leaks away, or the diaphragm splits.

When it fails, the pressure has nowhere to go when the heating fires. It climbs past three bar, the pressure relief valve opens to protect the boiler, and water is discharged outside. Once everything cools you are left with a low reading and no obvious puddle indoors.

The tell-tale sign: the gauge swings a long way between cold and hot — say 1 bar cold and close to 3 bar when hot. A vessel can often be recharged, which is a cheap job. If the diaphragm has gone, it needs replacing.

3. The pressure relief valve is passing

That relief valve discharges through a small copper pipe terminating outside, usually near the boiler on an external wall. If the valve seat has picked up a bit of debris it will never quite close again, and it will dribble away permanently.

Go and look at that pipe. If it is wet, dripping, or there is a limescale trail down the brickwork beneath it, you have found where your water is going. Note that a passing valve is often a symptom of cause 2 rather than a fault on its own — if the vessel is pushing pressure over three bar every time the heating runs, the valve is only doing its job, and eventually it stops sealing.

What you should not do

• Do not just keep topping it up and ignore it. Every top-up introduces fresh, oxygenated water, and oxygen is what corrodes the inside of your radiators into the sludge that blocks them.
• Do not leave the filling loop connected. It can allow mains water to back-feed into the heating system, which is exactly what it is designed to prevent.
• Do not over-pressurise it to buy more time. You will simply push more water out through the relief valve.

When to call someone

Ring an engineer if you are topping up more than about once a month, if the gauge swings wildly between hot and cold, if the outside discharge pipe is wet, or if you can see damp anywhere near pipework. All three causes are routine work — the expensive version is the one where a small leak has been quietly wetting a floor for a year.