Most combi boilers have a pressure gauge on the front. Check your boiler’s manual for where it should sit, but it’s commonly somewhere around 1 to 1.5 bar when the heating is cold.
A little is normal. A lot isn’t.
A small drop over a long time is nothing to worry about, and topping up once in a while is fine.
Topping up every few weeks is different. That water is going somewhere. And every top-up brings fresh water into the system, which can speed up corrosion inside the radiators and pipes.
Where the water usually goes
A radiator valve weeping. Look closely at the valves at both ends of each radiator. A green or white crust around a joint is a sign of a slow leak, often with no puddle at all.
A joint under a floor or inside a wall. Harder to spot. Sometimes a warm patch on a floor, or a mark on a ceiling below.
The pressure relief pipe. There’s usually a small copper pipe from the boiler out through an outside wall. If it’s dripping outside, that’s a big clue.
The expansion vessel. Inside the boiler there’s a vessel that takes up the expansion as the water heats. When it fails, pressure climbs too high when hot, the relief valve lets water out, and the pressure reads low once everything cools. High when hot and low when cold is the classic sign.
The boiler itself. Less common, and more serious.
Topping it up
Many boilers have a filling loop underneath: often a braided silver hose with a valve at each end, though some have a built-in lever or key instead. Your manual will show which yours has. Open the valves slowly, watch the gauge, and close them fully once it reaches the right level. A filling loop left open causes its own problems.
If you’re not sure which part is the filling loop, don’t guess.
When to get it looked at
- You’re topping up more than a couple of times a year
- The pressure drops noticeably within a week
- Water is dripping from the pipe outside
- The pressure goes very high when hot and low when cold
- There’s any water near the boiler
None of these are emergencies, but they all tend to get worse the longer they’re left.
Last updated September 2026
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