Why does my boiler keep losing pressure?

If you find yourself topping your boiler up every few weeks, the water is going somewhere. A sealed heating system is exactly that: sealed. The same water should circulate round the radiators for years without needing anything added. So a pressure gauge that keeps sliding back towards zero is not a quirk to live with, it is a symptom, and September is a good month to get to the bottom of it before the heating goes on properly.

First, the normal picture. Most combi and system boilers want to sit somewhere around one to one and a half bar when the heating is cold, and they will creep up a little as the system warms through. That small rise and fall through the day is fine. What is not fine is a gauge that drops below the green zone within a few weeks of being filled, or one that falls far enough for the boiler to shut itself down and show a low pressure fault. Boilers do that deliberately, to protect the pump and heat exchanger from running dry.

The most common cause is a small leak somewhere in the pipework or on a radiator. These are rarely dramatic. A weeping radiator valve, a nut that has loosened slightly, or a pinhole under a floorboard can lose enough water to drop the pressure noticeably while leaving barely a mark you would spot. Worth checking the obvious places yourself: run your fingers underneath every radiator valve and along the tails, look for rust streaks or staining on the radiator body, and check for a damp patch or a musty smell in an airing cupboard or under the boiler. In a lot of West London properties the pipework runs under solid floors or behind boxing, so a leak can be losing water quietly for months without ever showing on a ceiling.

The second common cause has nothing to do with leaks at all. Every sealed system has an expansion vessel inside or near the boiler, with a rubber diaphragm and a cushion of air behind it. That air cushion absorbs the expansion as the water heats up. Over the years the charge in the vessel drops, the cushion disappears, and the system has nowhere to expand into, so the pressure relief valve lifts and dumps a little water outside every time the heating runs. The classic sign is a pressure gauge that swings wildly, very high when hot and near zero when cold, plus a small drip or a lime stain on the outside wall near the boiler where the discharge pipe comes out. Recharging the vessel is a straightforward job for an engineer with a pressure gauge and a pump, and it fixes a fault that is very often misdiagnosed as a mystery leak.

Bleeding radiators will also drop the pressure, which catches people out. If you have gone round the house bleeding air out, you have removed water as well, and the gauge will need topping back up afterwards. That is a one-off, though. If you are refilling repeatedly without having bled anything, something else is going on. A worn pressure relief valve is another culprit, letting a trickle past even when the system is behaving, and in hard water areas like ours limescale on the valve seat makes that more likely over time. Brentford, Chiswick, Ealing and the surrounding streets are all on genuinely hard water, and it takes its toll on valves, seals and heat exchangers in ways that softer parts of the country never see.

There is a cost to ignoring it. Every time you top up from the mains you are introducing fresh oxygenated water into the system, and oxygen is what drives corrosion inside your radiators. That is how magnetite sludge forms, and sludge is what eventually blocks radiators, wears out pumps and chokes heat exchangers. A pressure problem left running all winter is a good way to turn a small repair into a much bigger one. If you are topping up regularly, it is also worth having a magnetic filter fitted and the inhibitor level checked once the leak itself is sorted.

Topping up to get through a cold snap is fair enough, and most boilers have a filling loop under them for exactly that. But if you are doing it more than once or twice a year, it is worth someone taking a proper look. We are Gas Safe registered and Which? Trusted Traders, and we cover Brentford, Chiswick, Hammersmith, Fulham, Ealing, Acton, Richmond, Twickenham, Kew, Chelsea and nearby. If your gauge keeps dropping and you are not sure why, get in touch and we will come and find out where the water is going.