Most sealed heating systems should hold pressure fairly well. You might need the odd top-up now and again, but if it keeps dropping regularly, something needs checking.

What pressure should a boiler be?

Most boilers sit around 1.0 to 1.5 bar when cold. When the heating is on and the system heats up, the pressure will usually rise a bit.

If it drops below the normal range regularly, the boiler may lock out or stop working properly.

Common reasons boilers lose pressure

1. A leak somewhere in the heating system

This is one of the most common causes. It could be a visible leak from a radiator valve, pipe joint or towel rail, but it can also be hidden under floors.

2. Expansion vessel problems

The expansion vessel allows the water in the system to expand as it heats up. If it loses charge or fails, the pressure can rise too high and then drop back down again.

3. Pressure relief valve passing

If the pressure relief valve has opened, it may not reseal properly. This can slowly discharge water outside and cause pressure loss.

4. Radiator valves or small weeps

Small leaks can be hard to spot because water evaporates when the heating is hot. Even tiny weeps can affect pressure over time.

Can you just keep topping it up?

You can top it up temporarily, but it isn’t a proper fix. If you keep adding fresh water, you’re also adding oxygen into the system, which can contribute to corrosion and sludge over time.

What you can check yourself

• look around radiators and valves for signs of leaks
• check under the boiler for drips
• look outside at the pressure relief pipe
• notice whether pressure rises too high when heating is on

When to get it checked properly

If the pressure keeps dropping, or you’re topping it up every few days or weeks, it’s time to get it looked at properly.

Final thoughts

A boiler losing pressure is common, but it shouldn’t be ignored. Sometimes it’s a simple repair, sometimes it takes a bit more investigation, but it’s always better to sort it before it gets worse.