Boiler pressure low? How to repressurise it (step by step)

If your radiators have gone cold and the boiler is showing a low pressure warning or a fault code, this is often a five-minute fix you can do yourself. Boiler pressure should sit between 1.0 and 1.5 bar when the system is cold. Below 1 bar, many boilers shut the heating down to protect themselves. You bring it back up using the filling loop.

The important part is not the topping up. It is noticing whether the pressure holds afterwards. A boiler that needs repressurising once a year is normal. One that needs it every week has a leak.

Points to take away

• Correct pressure is 1.0 to 1.5 bar cold, rising a little when hot.
• Repressurise using the filling loop: boiler off, open both valves, fill to 1.5 bar, close both valves, restart.
• There are four common types of filling loop. Find yours before you start.
• Pressure that drops within hours or days means a leak. Do not keep topping it up.
• A dripping pipe outside the house is often the pressure relief valve letting go.

Reading time: about 6 minutes.

What the pressure gauge should read

The gauge is on the front of the boiler or on the pipework just below it. It may be a round dial or a digital readout.

• 1.0 to 1.5 bar, system cold: correct.
• Rises to 1.8 to 2.0 bar when the heating is on: normal expansion.
• Below 1.0 bar cold: too low, repressurise.
• Above 2.5 bar, or in the red: too high, which is a separate problem. Bleeding a radiator lets some water out and brings it down. If it climbs straight back up, the filling loop may be passing or the expansion vessel has failed.

How to repressurise your boiler: step by step

1. Turn the boiler off at its switch and let it cool for 15 minutes if it has been running.
2. Find the filling loop (see the next section). It connects the cold mains to your heating system.
3. Check both ends are firmly connected to the pipework.
4. Open both valves slowly, a quarter turn at a time, so the handle lines up with the pipe. You should hear water flowing.
5. Watch the gauge. When it reaches 1.5 bar, close the valve nearest the mains first, then the other. Do not overshoot much. If you go past 2 bar, bleed a radiator briefly to bring it back.
6. Turn the boiler back on and press the reset button if it was locked out.
7. If your loop is a detachable braided hose, close both valves, then disconnect it and keep it somewhere safe for next time. A permanently connected loop can be left in place with the valves shut.

Check the gauge again the next morning. If it has held, you are done.

Finding your filling loop

The filling loop is where most people get stuck. There are four common designs.

• Braided silver hose (most combis). A flexible metal hose, about 30cm long, with a small valve at each end, connecting two pipes under the boiler. Sometimes stored coiled up and clipped to a bracket.
• Internal filling key (many Worcester Bosch). A white plastic key that slots into a tray on the underside of the boiler. You push it in, turn it, and turn a separate lever. The key is often taped inside the boiler casing or in the manual pack.
• Lever filling valve. A single black or brass lever on the pipework below the boiler that you push up or across to fill. No separate hose.
• Keyed link valve. A slot that takes a plastic key or a flat-blade screwdriver, turned to open the fill.

If you cannot identify yours, search your boiler’s make and model plus “filling loop” and check the manufacturer’s manual. Never improvise a connection between the mains and the heating system.

What if there is no visible filling loop

Some older or awkward installations have the filling point hidden: in an airing cupboard, under floorboards, or in the loft near a header tank. A regular (heat-only) boiler with a loft feed-and-expansion tank has no pressure gauge and no filling loop at all, because it is not a sealed pressurised system. If your boiler has no gauge, low pressure is not your problem, and you should look elsewhere for the fault. Our guide on a boiler with no heating or hot water covers the other causes.

Why boiler pressure drops, and what a repeat drop means

Every drop in pressure means water has left the sealed system. Where it went tells you how serious it is.

• You recently bled radiators. Bleeding releases air and a little water, which lowers pressure. Top up once and it should hold. Normal.
• A pipe outside the house is dripping. That is usually the pressure relief valve discharging through its overflow pipe. It can be a one-off from over-pressurising, or a sign the valve or the expansion vessel has failed. If it keeps dripping, book an engineer.
• Slow drop over weeks. A small leak on a radiator valve, a towel rail, or a pipe joint under a floor. Look for damp patches, rust stains under valves, or a warm patch on a floor.
• Fast drop within hours. A more significant leak, possibly inside the boiler. Stop topping it up and get it looked at. Repeated pressurising to hide a leak lets the leak do damage.

When to stop and call an engineer

Top up once. If the pressure holds for a few days, carry on as normal. Call a Gas Safe registered engineer if: the pressure drops again within a week, the external overflow pipe keeps dripping, you cannot find or operate the filling loop, or the boiler still will not run after repressurising and resetting.

An engineer can pressure-test the system, find the leak, and check the expansion vessel and relief valve. Our guide to boiler service and repair costs gives a sense of what that visit should cost.

Get free quotes from Gas Safe registered engineers covering your postcode, or browse the directory to find heating companies registered in your area.