Why does my boiler keep losing pressure?

Published 13 October 2026 by Reaction Property Service

A sealed central heating system is a closed loop. Once it has been filled and set to the right pressure, it should stay there, give or take a little as the water warms up and cools down. If you are topping the boiler up every few weeks, water is getting out somewhere. Here is how to read the gauge, where the water usually goes, and what to do about it.

What the pressure gauge should say

Most combi and system boilers have a pressure gauge on the front or a reading on the display. As a general guide, it should sit at around 1 to 1.5 bar when the heating is off and the system is cold, rising a little when the heating is running. Many gauges have a green zone marked on them. Your boiler manual gives the exact figure for your model, so it is worth checking.

Below about 1 bar many boilers will lock out and show a low pressure fault. Too high is a problem as well, which comes up below.

The common reasons boiler pressure drops

You have recently bled the radiators

Bleeding radiators lets air out, and the pressure drops as it goes. That is normal. Top it back up once and it should hold. If it keeps falling after that, something else is going on.

A small leak on the system

This is the most common cause. It does not have to be a dramatic drip. A slow weep from a radiator valve or a pipe joint can lose enough water to drop the pressure over a week or two, and it often evaporates before it leaves a puddle. Look for:

• Staining, rust marks or crusty white deposits on radiator valves and pipe joints.
• Damp patches on ceilings, or skirting boards and flooring that have swollen or lifted.
• Drips under the boiler itself.

If you can see a suspect joint, a dry piece of kitchen roll wrapped around it overnight will tell you quickly whether it is weeping. Pipes buried in walls and under floors are harder, and that is where a proper look is worth it.

A failed expansion vessel

Water expands as it heats. The expansion vessel, usually inside the boiler on a combi, gives that extra volume somewhere to go. If the vessel loses its air charge or its internal diaphragm fails, the pressure shoots up when the heating comes on. Then the next problem kicks in.

The pressure relief valve is letting water out

The pressure relief valve is a safety device that opens when the pressure gets too high. It discharges through a small pipe, usually copper, that runs through the outside wall. If the expansion vessel has failed, or the system has been overfilled, the valve can open every time the heating runs and lose water each time. Once these valves have opened a few times they sometimes do not seal properly again.

A useful check: look at the end of that pipe outside. If it is dripping, or there is staining on the wall beneath it, the relief valve is a likely suspect.

The filling loop has been left open or is letting by

The opposite fault is pressure that creeps up on its own. That is often a filling loop valve that is not fully closed or is passing water, and it can end up pushing water out through the relief valve.

How to top up the pressure safely

Most boilers are topped up through a filling loop, either a braided hose with a valve at each end or a built-in key or lever. Your manual shows which yours has. As a rule:

1. Turn the heating off and let the system cool, so the reading is accurate.
2. Open the filling valve slowly and watch the gauge.
3. Close it once the gauge reaches the pressure in your manual, usually around 1 to 1.5 bar.
4. Make sure the valves are fully closed, then reset the boiler if it has locked out.

Do not open the boiler casing or try to adjust anything inside it. Work on a gas appliance has to be done by a Gas Safe registered engineer.

When topping up stops being the answer

Topping up once after bleeding radiators is fine. Topping up every week is not a fix, for a couple of reasons:

• Every top up brings in fresh water with fresh oxygen, which speeds up corrosion inside steel radiators and adds to the black sludge that clogs systems.
• Fresh water also dilutes the inhibitor that protects the system.
• A slow leak inside a wall or under a floor can do real damage before it shows.

If the pressure is dropping and you cannot find an obvious cause, or the relief pipe outside keeps dripping, that is the point to have it checked. A boiler service or repair visit includes checking the expansion vessel, the relief valve and the pressure behaviour, and a leak on a radiator valve or pipework is usually a straightforward plumbing repair.

If the boiler is older and losing pressure alongside other faults, it may be one of the signs covered in how long a boiler replacement takes. You will be told honestly whether a repair is the sensible option.

Boiler pressure problems in Ashford and Hythe

I look at boiler pressure faults, leaks and expansion vessel problems across Ashford, Hythe and the villages around them. There is no call-out charge and quotes are free. If the pressure keeps dropping, call or send a message with a photo of the gauge and the pipe outside, and it is usually possible to say what is likely before anyone arrives.