A boiler that keeps losing pressure is losing water somewhere. In practice it is almost always one of three things: a leak in the pipework or a radiator, a failed expansion vessel, or a pressure relief valve that is passing water out through the overflow. Topping it up repeatedly does not fix any of them.
Most domestic boilers want somewhere between 1 and 1.5 bar when the heating is cold. It will rise when the system heats up, which is normal, and it should settle back down again as it cools.
If you are having to top it up more than once or twice a year, something is wrong. Once a week and you have a problem that needs finding, not managing.
This is the most common cause and the easiest to miss, because the leak is usually tiny. Water is escaping slowly, evaporating, and leaving no puddle.
Look under radiators and at the valves at each end of them. Look at the pipework where it comes out of the floor. Look at the boiler itself, and at the pressure relief pipe outside, which is often a small copper pipe poking out of an external wall. If that is dripping, water is being pushed out of the system.
A pinhole leak under a floor can lose pressure for months without ever showing itself.
Inside your boiler, or occasionally next to it, is a sealed vessel with a rubber diaphragm and a charge of air behind it. Its whole job is to give the water somewhere to expand into when it heats up.
When the diaphragm fails, the air escapes and the vessel fills with water. Now there is nowhere for the expansion to go, so pressure spikes hard when the heating comes on, the pressure relief valve opens to protect the system, and water gets dumped outside. The system cools, and the pressure is now too low.
The tell is pressure that swings dramatically between hot and cold, rather than drifting down slowly. An expansion vessel can often be recharged. Sometimes it needs replacing.
The valve is designed to open and release water if pressure gets dangerously high. If the seat gets damaged, or a bit of debris gets under it, it will start letting water past continuously even at normal pressure.
Go and look at the discharge pipe outside. If it is wet or dripping when nothing is wrong, that valve is passing and it needs replacing.
No, and this is the bit that matters. Every time you top up, you are putting fresh, oxygenated mains water into a sealed system that is supposed to keep the same water in it forever.
Fresh water brings dissolved oxygen. Oxygen plus steel radiators makes rust. Rust makes sludge. Sludge blocks radiators, wrecks pumps, and eventually kills boilers.
So a boiler you top up every week is not just losing pressure. It is being slowly poisoned. Find the leak.
Usually between 1 and 1.5 bar when cold. It will rise while the heating is running and drop back as the system cools. If you are topping it up more than once or twice a year, there is a fault to find.
Not dangerous, but the boiler will usually shut itself down rather than run, so you get no heating or hot water. The danger is the long-term damage from repeatedly refilling with fresh oxygenated water, which causes internal corrosion and sludge.
You can top it up, and you can look for obvious leaks under radiators. Anything involving the boiler itself, the expansion vessel or the pressure relief valve is Gas Safe work and needs a registered engineer.
It depends entirely on what is causing it. A recharged expansion vessel is a modest job. A leak under a solid floor is not. We find the actual cause before quoting, rather than guessing.
We diagnose the actual fault before quoting to fix it. No jargon, no scare tactics.
