Boiler Pressure Too Low? How to Repressurise a Combi Boiler

What your boiler pressure should be

A combi boiler heats water that travels round a sealed circuit of pipes and radiators. That circuit has to hold a certain amount of pressure for the pump and safety devices to work properly. The gauge on the front of the boiler shows this pressure in bar. On older models it is a dial with a needle, and on newer ones it is a number on the digital display.

With the heating off and the system cold, most combis should read between 1 and 1.5 bar. When the heating runs, the water expands and the reading climbs, often to around 2 bar. That rise is normal. Once the pressure falls too far (the exact trigger varies by model, but it is usually somewhere under 1 bar), the boiler locks out to protect itself and shows a low pressure code. Common examples include Ideal F1, Vaillant F22, Worcester Bosch CE 207 and Baxi E119.

Find your filling loop

Topping up the pressure is one of the few boiler jobs a homeowner can safely do. Everything you need sits underneath the boiler or on the pipes below it, so you never need to take the front casing off. There are two main types of filling loop.

External filling loop with two valves

This is a short flexible hose, usually silver braided, that connects the cold mains pipe to the heating pipework under the boiler. There is a valve at each end. Some valves have a small lever or handle, and others have a slot you turn with a flat-head screwdriver. As a rule, a handle or slot in line with the pipe means open and across the pipe means closed. On many installations the hose is meant to be disconnected after use, so if you cannot see one, check for a spare hose in a drawer or bag near the boiler.

Built-in filling loop or filling key

Many newer combis, including a number of Worcester Bosch, Baxi and Ideal models, have a filling system built into the underside of the boiler. Depending on the model, you operate it with a lever, a small tap or a plastic filling key that you push in and turn. The exact method is shown in your user manual. If you have lost the manual, the make and model are printed on the front of the boiler or on the data badge, and most manufacturers publish manuals online.

How to repressurise a combi boiler, step by step

1. Turn the boiler off at the controls and let the system cool for a while. A cold reading is the accurate one to work from.
2. If you have a removable hose, connect both ends by hand. Check the rubber washers are in place so the joints do not drip.
3. Open the first valve fully.
4. Slowly open the second valve. You should hear water running and see the pressure start to rise.
5. Watch the gauge closely and close the second valve as soon as it reaches 1 to 1.5 bar.
6. Close the first valve, then check both are fully shut.
7. Wait a minute and make sure the reading stays steady. If it keeps creeping up, one of the valves is not fully closed.
8. If the hose is removable, disconnect it and put it somewhere safe for next time.
9. Turn the boiler back on. If the low pressure code is still showing, press reset as described in your manual.

With a built-in filling system the idea is the same: open the lever or key slowly, watch the gauge, and close it at 1 to 1.5 bar. If you overshoot, you can lower the pressure by bleeding a radiator, which is covered in our guide to boiler pressure that is too high.

Why does my boiler keep losing pressure?

A small drop after bleeding radiators is expected, because you have let some water out of the system. Beyond that, a healthy sealed system should hold its pressure for months. If you find yourself topping up every week or two, water is escaping somewhere.

A leak on the system

Check radiator valves, towel rails, visible pipe joints and the pipes under the boiler. Look for drips, green or white crusty deposits, rust marks, damp patches on ceilings and stained floorboards. Pipes that run under floors or inside walls can leak without any obvious sign, which is why a slow, steady loss sometimes needs an engineer to track down.

The pressure relief valve is letting water out

Every combi has a pressure relief valve that opens at around 3 bar to stop the pressure getting dangerously high. It discharges through a copper pipe that usually runs through the wall to the outside, often near the condensate pipe. If you see drips from the end of that pipe or a wet patch on the ground below it, the valve has been opening. It may be worn, or something is pushing the pressure too high when the heating runs.

The expansion vessel has lost its charge

The expansion vessel is a tank inside the boiler with an air cushion that absorbs the extra volume when water heats up. Over time it can lose its air charge. The tell-tale pattern is a pressure that rises sharply when the heating is on, then falls well below 1 bar once the system cools. The high pressure makes the relief valve open, water is lost, and the cycle repeats every time you top up. Recharging or replacing the vessel is an engineer job, because it sits inside the casing.

When to call a Gas Safe registered engineer

Topping up is a safe first step, but it treats the symptom rather than the cause. Book a Gas Safe registered engineer if:

• you are topping up more than about once a month
• the pressure drops quickly after you top up and you cannot find a leak
• the relief valve pipe outside is dripping or has left a wet patch
• the pressure swings from very high when hot to very low when cold
• you can see water dripping from inside the boiler or from the casing
• the pressure creeps up on its own even with the filling loop closed

Typical costs if the pressure keeps dropping

These are typical UK ranges for parts and labour. Prices vary with your area, the boiler model and how easy the fault is to reach.

If the boiler is due a service anyway, ask the engineer to check the expansion vessel charge and the relief valve at the same time. It often costs less than a separate call-out.