Boiler pressure keeps rising, then it leaks.
It is often the expansion vessel.

Inside most boilers there is an expansion vessel: a steel can with a rubber diaphragm across the middle, air on one side and your heating water on the other. Water expands as it heats, and the air behind the diaphragm is a cushion that absorbs the expansion, so the pressure on the gauge only rises a little when the heating comes on. When the vessel loses its air charge, or the diaphragm fails, there is nothing to absorb the expansion. The pressure climbs steeply as the boiler heats, and at 3 bar the pressure relief valve opens and lets water out, which is the water you find outside or on the floor. When the system cools the pressure drops low, so you top it up, and the cycle starts again.

We repaired exactly this in Ealing this week: a combi boiler where a faulty expansion vessel was pushing the pressure up every time the heating ran, and water had been leaking onto the floor beneath it. Found and put right in the same visit.

What you can
check yourself

Three things, all of them safe. First, watch the gauge. With the heating off and the boiler cold for a few minutes, the pressure should sit at about 1.5 bar, and anywhere from 1.5 to 2 is fine. If it reads a good deal higher after the heating has been running for an hour, and settles back down once everything has cooled, that is the pattern. Second, look at the pipe outside. Most boilers have a copper pipe that goes through the wall and only ever discharges when the relief valve opens, so a drip from it, a wet patch below it or a white limescale trail down the wall means the valve has been letting go. Third, look under the boiler. Water on the floor, or a damp casing, is a reason to stop and call us, because water and the electrics inside a boiler do not mix.

One thing not to do is chase the gauge. Every top-up adds more water for the vessel to cope with, and if the vessel has failed the relief valve will simply send the extra water outside again the next time the heating runs. Top up to 1.5 bar with the system cold, leave it, and watch what it does. If you are topping up from underneath the boiler, make sure the lever you close afterwards is the filling loop and not the gas, which is a mistake we have been called out to more than once and which shows up as the boiler refusing to light.

What it is likely
to be

In our experience there are three things that make a sealed system climb on its own, and the gauge tells you which. If the pressure rises when the boiler is hot and falls when it is cold, it is the expansion vessel. If the pressure rises even with the boiler cold, the water is coming in from the mains: either the filling loop has not closed properly, or there is a pinhole in the plate heat exchanger, which is the small heat exchanger that makes your hot water, and mains water is finding its way into the heating side.

Sometimes the expansion vessel itself is damaged and leaks inside the boiler, and you will see water coming from underneath it.

Whichever it is, the relief valve has usually been opening for a while by the time anyone notices, and a valve that has opened a few times often does not reseal cleanly. So a vessel repair often means a new relief valve as well, and we say so before we start.

What we check

We check the charge in the vessel, which on most boilers means isolating and depressurising the heating side and testing the air pressure behind the diaphragm against the figure the manufacturer gives for that model. A vessel that has simply lost its charge can be recharged; one with a failed diaphragm is replaced. We check the relief valve and the pipe it discharges into, and replace the valve where it has been letting by. We check that the filling loop is closed and, where it is the removable type, that it has been disconnected. And where the pressure climbs with the boiler cold, we isolate the cold mains to the boiler and watch the gauge, which tells us whether the water is coming through the loop or through the plate heat exchanger.

On a system boiler with a cylinder there is one more thing we do, and it is our own practice. The expansion vessel built into the boiler is small, and on a larger system it is working near its limit from day one. Where there is space we fit a separate external expansion vessel, sized for the system, in addition to the one inside the boiler. It is often not done by others, it adds to the cost, and we fit it for peace of mind.

The short version

Pressure that climbs when the heating runs, water from the pipe outside or under the boiler, and a gauge that is low again a week later is the pattern of a failed expansion vessel. Check the gauge cold and hot, look at the pipe outside, and do not keep topping up past 1.5 bar. The vessel is recharged or replaced and the relief valve usually goes with it. Pressure that climbs with the boiler cold is mains water getting in, through the filling loop or a pinhole in the plate heat exchanger, and that is a different repair. All of it is sealed-system work for a Gas Safe registered engineer, and you can check any engineer's registration at https://www.gassaferegister.co.uk.

Quick answers