Boiler losing pressure? What it means and when to call an engineer

What the pressure gauge should say

Most combi boilers have a gauge on the front, either a dial or a number on the display. When the heating is off and the system is cold, it usually sits somewhere around 1 to 1.5 bar. It rises a little when the heating is running, which is normal.

Below about 0.5 to 1 bar, most boilers stop working to protect themselves, and you'll often see an error code. Your boiler's manual gives the exact figures for your model.

Why boilers lose pressure

Pressure doesn't just disappear. If it keeps dropping, water is getting out of the system somewhere, or a part isn't doing its job. The common causes:

• A leak somewhere in the system. A weeping radiator valve, a joint under the floor, or a leak inside the boiler itself.
• A faulty pressure relief valve. This lets water out when pressure gets too high. When it fails, it lets water out all the time. The tell-tale sign is that small copper pipe outside, near the boiler, dripping or leaving a wet patch.
• A flat expansion vessel. This tank inside the boiler absorbs pressure changes as water heats up. When it loses its charge, pressure swings high when the heating's on and drops low when it's off.
• Recently bled radiators. Bleeding radiators lets out air, and the pressure drops a little. A one-off drop after bleeding is normal.

Can I top it up myself?

Many boilers have a filling loop for topping up pressure, and the manual explains how. But topping up only treats the symptom. If you're having to do it more than once, the water is going somewhere, and that leak or faulty valve will only get worse. Leaks inside a boiler can also damage the electronics.

And anything inside the boiler casing is gas work. By law, only a Gas Safe registered engineer can do it.

When to call an engineer

• The pressure drops again after being topped up.
• You can see water under the boiler or round its pipes.
• The copper pipe outside is dripping.
• The pressure goes above 2.5 to 3 bar when the heating is on.
• The boiler shows a fault code and won't relight.

A Gas Safe engineer will pressure-test the system, find where the water's going, and replace the faulty part. Most pressure faults are fixed on the first visit.

Smell gas?

A pressure problem isn't a gas leak. But if you ever smell gas, call the National Gas Emergency Service free on 0800 111 999 first, open the windows, and don't use switches or flames.

Common questions

What pressure should my boiler be at?

Usually around 1 to 1.5 bar when cold, rising a little with the heating on. Check your boiler's manual for the exact figures.

Is low boiler pressure dangerous?

Not on its own. The boiler shuts down to protect itself. But the cause, like a leak, can damage your home or the boiler, so get it looked at.

How much does it cost to fix?

It depends on the cause. We give you a price on the phone before an engineer is sent, so you know what to expect.