Why does my boiler keep losing pressure?
If you are topping the boiler up every week, the water is going somewhere. Here is how to work out where, and which causes you can sort yourself.
By Dan, Gas Safe registered engineer5 min read
This is probably the question I get asked most. The boiler drops below one bar, the heating goes off, you top it up, and a fortnight later you are doing it again. The important thing to understand is that a sealed heating system should not lose water at all. If the pressure is falling, it is going somewhere, and the job is to find out where rather than keep refilling it.
What boiler pressure actually is
Your heating system is a closed loop of water. It is pressurised so the water can get round the radiators properly, and most boilers want somewhere between one and one and a half bar when the heating is cold. Below about one bar the boiler will usually shut itself down rather than run dry, which is the safety cut-out doing its job.
Check it cold
Pressure rises as the system heats up, so read the gauge with the heating off and the radiators cold. A gauge that reads high when hot and normal when cold is usually fine.
The three common causes
1. A leak on the system
This is the most common answer by a distance. It does not have to be dramatic. A weep from a radiator valve that evaporates before you notice it is enough to drop the pressure over a couple of weeks. Look under every radiator valve for staining or a green or white crust on the pipe, check around the boiler itself, and look at the ceiling below any pipework that runs upstairs.
Pipework buried under floors is the awkward one, because there is nothing to see. That is when it is worth tracing the leak properly instead of guessing and lifting a floor in the wrong place.
2. The pressure relief valve is passing
There is a safety valve on the boiler that dumps water outside if the pressure ever gets too high. It runs to a small copper pipe through an outside wall. If that pipe is dripping, or you have seen a wet patch below it, the valve is passing and the system is slowly emptying itself out of the wall. That is a boiler repair, not a top-up.
3. The expansion vessel has lost its charge
Inside the boiler is a vessel with a rubber diaphragm and a cushion of air behind it, which absorbs the expansion as the water heats up. When that cushion goes, the pressure swings hard: very high when hot, very low when cold, and often the relief valve starts dumping as a result. It is a common fault on boilers around eight to ten years old and it is repairable.
What you can safely do yourself
- 1Read the gauge cold and write the number down.
- 2Top up to the level in your boiler manual, usually around one to one and a half bar, using the filling loop under the boiler.
- 3Write the date on a bit of paper and stick it inside the boiler cupboard.
- 4Read it cold again in a week and note how far it has fallen.
That last step is the useful bit. How fast it drops tells an engineer a lot. Losing pressure over months is a very different problem from losing it overnight, and knowing which one it is saves time on the day.
When to stop and ring someone
If you are topping up more than about once a month, if the relief pipe outside is dripping, or if you can see water anywhere near the boiler, stop refilling it and get it looked at. Repeatedly topping up a leaking system pushes fresh oxygenated water through it, which corrodes it from the inside.
The honest version
Most pressure loss is a small leak, and most small leaks are cheap to fix once you know where they are. The expensive outcome is the one where somebody tops it up for two years and the system silently fills with sludge. If your gauge keeps falling, get it found. I cover Preston, Penwortham, Chorley, Leyland, Garstang and Kirkham, and I would rather tell you it is a five minute job than have you replace a boiler that did not need replacing.
Need this doing?
I am Dan, and I cover Preston and the towns around it.