Boiler pressure keeps dropping: how often is too often?
Updated 1 October 2026 · 2 minute read · From our Gas Safe heating engineers
Every combi and system boiler has a pressure gauge on the front. Most people only look at it when the boiler stops. Knowing what it should say, and how fast it is falling, is the single most useful thing you can tell a heating engineer on the phone.
What normal looks like
- Cold, the gauge should sit roughly between 1 and 1.5 bar.
- With the heating on, it rises a little as the water expands. That is fine.
- Over months, a system loses a small amount, especially after you bleed radiators. A top up once or twice a year on the filling loop is nothing to worry about.
When it is a fault
The rate tells you. Write the date down every time you top up.
| How often you top up | What it means |
|---|---|
| Once or twice a year | Normal |
| Every few weeks | Something is wrong, worth looking at |
| Every few days | Find it now |
The two usual causes
1. A leak somewhere on the system. Look under each radiator and around its valves for staining or a damp patch. Then go outside near the boiler and find the small copper pipe coming through the wall. That is the pressure relief pipe. If it is dripping, or there is staining on the wall below it, the relief valve is passing water and that is where your pressure is going.
2. An expansion vessel that has lost its charge. The vessel has an air cushion that soaks up the water as it expands when hot. When the air charge goes, the pressure shoots up when the heating runs (often well over 2 bar) and sits low when cold. That can also make the relief valve open, which looks like a leak. The fix is a recharge or a new vessel. Topping up will never fix it.
Why weekly top ups are a false economy
Every top up puts fresh, oxygen-rich water into the system. Oxygen is what corrodes steel radiators from the inside, and that corrosion becomes the sludge that later blocks radiators and coats the boiler’s heat exchanger. Topping up every week is slowly damaging the system, and it ends up costing more than the repair would have.
What to tell us
How often you are topping up, and what the gauge does when the heating comes on. That usually tells us which of the two it is before we arrive. We find the cause, show it to you and price the repair before doing anything.
Keep reading
- Cold radiators: how a heating engineer tells bleed, balance and flush apart Where a radiator is cold tells you the fix. Cold at the top, cold at the bottom, or whole rooms never warm: three symptoms, three different jobs, and only one of them needs a powerflush.
- Radiators hot, taps cold: why a combi stops making hot water When a combi boiler heats the radiators but not the taps, the fault is usually one part: the diverter valve. What it does, what to check first, and why it is a repair, not a new boiler.
- When a powerflush is worth paying for, and when it is not A powerflush is the right answer for a genuinely dirty system and a waste of money for everything else. The signs that say flush, the ones that do not, and what has to happen afterwards.