Why Does My Boiler Pressure Keep Dropping Even When There's No Visible Leak?

You top the boiler up to 1.2 bar, everything works, and a week later the needle is back on the red. You've looked under the boiler, felt round the radiators, checked the airing cupboard — nothing is wet. It's one of the most common calls we take as a plumber in Kingston, and the good news is that a sealed heating system that loses pressure with no puddle usually has one of a small number of causes.

A sealed system holds a fixed volume of water. Pressure only falls if water leaves the system, or if the component that absorbs expansion has stopped doing its job. That's the whole diagnosis in one sentence — the skill is working out which.

Before anything else, note the numbers. Fill to 1.2 bar cold and write down what the gauge reads each morning.

• Falls within hours: an active leak, or the pressure relief valve is passing water outside.
• Falls over a week or two: a pinhole leak, a weeping joint under a floor, or a failing expansion vessel.
• Only drops after the heating has been running hard: points strongly at the expansion vessel or the pressure relief valve.

That single week of readings often saves an hour of diagnosis when we arrive.

Most heating pipework in Kingston homes runs under floorboards, through screed, or behind dry-lined walls. A pinhole in a buried pipe can lose a litre a week and never show on the surface — under a suspended timber floor it soaks into the joists, and in a screeded floor it evaporates through the tiles without ever making a mark.

Things worth checking yourself: the underside of every radiator valve (run a dry tissue round the spindle and nut), the pipes where they emerge through the floor, and the towel rail in the bathroom, which is the single most common source we find. Also look for a patch of floor that always feels slightly warmer than the rest.

If nothing shows, the leak needs tracing properly rather than guessing. Our leak detection service uses thermal imaging and acoustic tracing so the floor comes up in one place, not five.

Inside your boiler is a small sealed vessel with a rubber diaphragm and a cushion of air behind it. When the system heats up and the water expands, that air cushion takes the extra volume. Over years the air slowly escapes or the diaphragm splits, and the cushion disappears.

What you then see is the classic pattern: pressure climbs to 2.5–3 bar when the heating is running, the pressure relief valve lifts and dumps water outside to protect the boiler, and once everything cools you're back below 1 bar. No leak inside the house — the water genuinely left the building.

The giveaway is a drip or a limescale stain on the outside wall under the small copper pipe near the boiler. Re-charging the vessel with a pump is a routine job; if the diaphragm has failed, the vessel itself is replaced. Either way it's a boiler repair, not a new boiler.

Even with a healthy expansion vessel, a PRV that has lifted once and caught a fragment of system debris on its seat will never fully close again. It then weeps constantly — slowly enough that the outside pipe looks dry between visits, fast enough to empty a bar of pressure over a fortnight.

Go outside and find the small pipe discharging from the wall near the boiler. Feel inside the end. Damp, or crusted with white scale, means the valve is passing. It's an inexpensive part and a short visit.

Older TRV bodies weep at the spindle, compression joints relax after a power flush, and a radiator that has corroded from the inside can develop a pinhole at the bottom seam that dries as fast as it leaks, leaving only a faint rust mark. Auto air vents on the pipework can also pass water when their seal perishes.

If your system has just been drained and refilled for other work, a small drop over the first few days is normal as trapped air works its way out. Persistent loss after that is not.

• Don't keep topping up indefinitely. Every top-up brings fresh oxygenated water into the system, which accelerates internal corrosion and sludge — turning a small repair into a power flush later.
• Don't leave the filling loop open. It can over-pressurise the system and it lets mains water back-feed the heating.
• Don't add leak-sealer as a first resort. It can block the heat exchanger and some manufacturers void the warranty.

Call an engineer if you're topping up more than once a month, if the pressure swings above 2.5 bar when hot, if there's water discharging from the outside pipe, or if the boiler is locking out on a low-pressure fault code. Those are all diagnosable faults with straightforward fixes — but they don't get better on their own, and the corrosion caused by constant refilling does real damage over a winter.

We cover Kingston upon Thames, Surbiton, New Malden, Richmond and the surrounding KT and TW postcodes for central heating repairs and boiler diagnostics.

If your boiler keeps losing pressure, we'll find out why rather than topping it up and leaving. Call Maxfix on 020 3488 0912 or request a free quote — we're a Gas Safe registered team offering plumbing services in Kingston and across South West London.