Boiler pressure low: how to top it up, and why it keeps dropping

If your boiler has stopped working and the needle on its pressure gauge has dropped into the red, low pressure is almost certainly why. It is one of the most common boiler problems there is, and in many cases you can put it right yourself in a few minutes. The more useful question is why the pressure dropped in the first place, because a boiler that needs topping up every week is losing water from somewhere.

What boiler pressure actually is

Combi and system boilers run on a sealed heating circuit: the same water goes round and round between the boiler and the radiators, kept under a little pressure so it circulates properly. The gauge on the front of the boiler, or a digital reading on its display, shows that pressure in bar.

• Most boilers want to sit at around 1 to 1.5 bar when the heating is cold. Your manual will give the exact figure for your model, and many gauges have a green zone marked on them.
• The pressure rises a little when the heating is running, because warm water expands. A rise to around 2 bar when hot is normal on most systems.
• When the pressure falls too low, often below about 0.5 to 1 bar, the boiler locks itself out to protect itself. On a combi that means no heating and no hot water.

Older homes with a regular boiler and a small feed tank in the loft are usually open-vented, and have no pressure gauge to worry about. This guide is about sealed systems.

How to top up the pressure

You top up a sealed system through a filling loop, which lets a little mains water into the heating circuit. There are two common types.

• An external filling loop: a short silver braided hose under the boiler, joining two pipes, with a small valve at one or both ends.
• A built-in filling loop: a lever, tap or blue knob underneath the boiler, or a slot for a plastic filling key that came with it. The exact method varies by make, so check the manual.

For the common external hose, the steps are:

1. Turn the boiler off and let the system cool, so you get a true reading.
2. Make sure both ends of the hose are screwed on firmly.
3. Open the valves slowly, usually a quarter turn with a flat screwdriver or by hand. You will hear water running in.
4. Watch the gauge, and close both valves as soon as it reaches the level your manual gives, usually around 1 to 1.5 bar.
5. Disconnect the hose at one end and leave it off. The filling loop is meant to be a temporary connection, and leaving it attached risks water from the heating system getting back into your drinking water.
6. Turn the boiler back on and press reset if it shows a fault.

Take your time. It is easy to overfill, and if the pressure goes too high the boiler's pressure relief valve will open and dump water through a copper pipe to the outside. If you overshoot, bleeding a radiator for a few seconds will let a little water out and bring the reading down.

Bled a radiator recently? That explains it

Bleeding radiators lets air out, and water moves in to take its place, so the pressure drops. That is normal and expected. Check the gauge whenever you bleed a radiator and top up if it has fallen.

Why the pressure keeps dropping

A small top-up now and then is nothing to worry about. If you are topping up every week or two, the system is losing water, and topping up is only hiding the fault. The common causes are:

• A leak on the heating pipework. Check around radiator valves, the joints under the boiler and anywhere pipes run along skirting boards. Leaks under floorboards or in ceilings show up as stains, damp patches or bubbling paint. If you cannot find it, leak detection can trace it without pulling up floors at random.
• The pressure relief valve letting by. Find the copper pipe that runs from the boiler to the outside wall. If it is dripping, or the wall below it is wet or stained, the valve is releasing water. That is often a symptom of the next problem rather than a fault in itself.
• A flat expansion vessel. This is a small tank, usually inside the boiler, that absorbs the extra volume when the water heats up. If it loses its air charge, the pressure shoots up when the heating runs, the relief valve opens, and the pressure drops right down again once the system cools. A pressure that swings a long way between hot and cold is the classic sign.
• A weeping automatic air vent on the boiler or pipework, which can let out small amounts of water as well as air.

What not to do

• Do not keep topping up indefinitely. Every top-up brings in fresh water, which carries oxygen that corrodes radiators from the inside, and dilutes the inhibitor that protects the system.
• Do not take the boiler casing off to reach the expansion vessel or relief valve. Anything inside a gas boiler must be worked on by a Gas Safe registered engineer.
• Do not try to adjust or cap the pressure relief valve or its discharge pipe. It is there to stop the system over-pressurising.

What if the pressure is too high?

A reading near 3 bar is too high, and is usually what makes the relief valve open. If it happens right after you topped up, you have overfilled: bleed a radiator to bring it down. If it climbs every time the heating comes on, the expansion vessel is the likely cause. It is also worth checking that the filling loop valves are fully closed, as one that is letting by will slowly push the pressure up.

When to call us

If the pressure will not hold, the relief pipe outside is dripping, or the boiler still will not fire after you have topped it up, book a boiler repair. Our Gas Safe registered engineers can find where the water is going, recharge or replace the expansion vessel, and deal with a leak on the pipework. If you can already see where it is leaking, our leak repair service covers that. An annual boiler service is a good chance for the engineer to look at the system pressure and expansion vessel, which helps catch a fading vessel before it starts dumping water.

If low pressure has also left you without hot water, our guide to no hot water covers the other causes to rule out. You can call us on 020 3576 5825.