What pressure should my boiler be?

The quick reference

How to check your boiler pressure

The pressure gauge is on the front of the boiler or on the pipework underneath it. Older boilers have a dial with a needle, often with a green or shaded band marking the normal range. Newer boilers show pressure on the digital display as a number in bar, sometimes only after you press a button. Check it with the heating off and the system cold, because the reading climbs as the water warms up.

How to top up (repressurise) a boiler

On a combi or system boiler you top the pressure up yourself using the filling loop, which lets water from the mains into the heating system. It is a normal homeowner job and it does not involve opening the boiler. What the filling loop looks like depends on the make:

• An external filling loop: a silver braided hose under the boiler with a small valve at each end. The most common type.
• A built-in filling tap or lever on the underside of the boiler, found on many Vaillant models.
• A filling key that fits into the bottom of the boiler, used on many Worcester Bosch models. It is often stored in a clip underneath.

1. 1

   Turn the boiler off and let it cool

   Switch the boiler off and give the system time to cool. You want a cold reading: a hot system reads higher than it really is, and you will overfill it.

2. 2

   Find the filling loop

   Look under the boiler for the braided hose, tap or key. If the hose is only connected at one end, fit the loose end onto its valve first. Put a towel underneath.

3. 3

   Open the valves slowly

   Open one valve fully, then open the other a little at a time. You will hear water running into the system. Keep your eyes on the gauge the whole time.

4. 4

   Stop at 1 to 1.5 bar

   As soon as the gauge reaches 1 to 1.5 bar, or the figure in your manual, close both valves. It rises quicker than most people expect, so do not look away.

5. 5

   Close up and switch back on

   Check both valves are fully closed. If it is a detachable hose, disconnect it. Switch the boiler back on and, if it is showing a fault, press reset once.

6. 6

   Check it again later

   Look at the gauge once the heating has been running, and again the next day. A small rise when hot is normal. A drop back down means water is escaping somewhere.

Why does my boiler keep losing pressure?

Topping up once after bleeding the radiators is normal, because bleeding lets water out with the air. Having to top up again and again is not. The water is going somewhere, and these are the usual places, roughly in the order an engineer would check them.

As a rule of thumb, if you are topping up more than once every few weeks, get it looked at. A slow leak does damage out of sight long before it shows, and a boiler running on low pressure keeps locking out.

Why is my boiler pressure too high?

• It has just been overfilled. With the system cold, bleed a radiator a little at a time until the gauge is back to 1 to 1.5 bar. Bleeding releases water as well as air.
• The filling loop is letting water through even when it is closed, so the pressure creeps up over a few days. The valve needs replacing.
• The expansion vessel has lost its charge, so the pressure spikes every time the heating runs. That needs an engineer.

Low pressure fault codes

When pressure drops too far, most boilers stop and show a fault code rather than a message. The code depends on the make and model, so your manual is the reliable source. On Vaillant boilers, for example, low water pressure shows as F22. If you top up and the code clears, that was the cause; if it comes back within days, you are losing water.

When to call an engineer

• You are topping up more than once every few weeks
• Water is dripping from the copper pipe on the outside wall
• The gauge reads above 2.5 bar with the system cold, or reaches 3 bar at any time
• The boiler will not fire after topping up and one reset
• There is water under the boiler
• You cannot find the filling loop, or its valves will not open or close