How to Repressurise a Boiler

Boiler pressure is the pressure of the water inside your sealed central heating system. It needs to be high enough for the boiler to circulate hot water to your radiators and (on a combi) to heat your taps, but not so high that it risks damage. Pressure can drop over time—for example after bleeding radiators, or because of a small leak—and when it gets too low the boiler may cut out or refuse to fire. Repressurising (filling the system via the filling loop) is a simple DIY task on most combi and system boilers. This guide explains the correct pressure, when to top up, and the steps to do it safely.

What Is the Correct Boiler Pressure?

When the system is cold (heating has been off for a while), the pressure gauge on your boiler should read between 1 and 1.5 bar. Many boilers have a green zone on the gauge indicating the safe range. When the heating is on and the water is hot, pressure will rise—typically to around 1.5 to 2 bar, sometimes a little higher. That is normal. If the pressure is below 1 bar when cold, you need to repressurise. If it regularly goes above 3 bar when hot, or the pressure relief valve discharges, the system may be overfilled or there may be a fault; see the section below on high pressure.

Signs Your Boiler Pressure Is Too Low

You may need to repressurise if you notice any of the following.

Boiler cuts out or will not fire: Many boilers have a low-pressure cut-out and will switch off or show an error code when pressure is too low.
No heating or hot water: The boiler may not run at all until the system is repressurised.
Pressure gauge below 1 bar: When the system is cold, the needle or digital readout is in the red or below the green zone (often marked below 1 bar).

If you have recently bled radiators, pressure often drops because you have released water from the system. Topping up via the filling loop is the normal next step.

What You Need

Repressurising is done using the filling loop—a flexible hose (usually silver braided) that connects the cold mains to the heating system. On many boilers it is permanently fitted under or next to the boiler; on others it is a separate kit that you attach when needed. You may also need a flat-head screwdriver to open the valves on some filling loops (others have lever-style valves you can turn by hand). Check your boiler manual if you are not sure where the filling loop is or how the valves work.

Step-by-Step Guide to Repressurising

Follow these steps in order. If your boiler or filling loop looks different from this description, refer to your boiler manual before proceeding.

Step 1 — Turn the boiler off and let it cool

Switch the boiler off at the control panel or programmer and wait until the system has cooled down. Repressurising when the system is cold gives you an accurate reading on the pressure gauge and avoids handling hot components.

Step 2 — Locate the filling loop under the boiler

The filling loop is usually under the boiler. It connects two pipes (typically one from the cold mains and one into the heating return) and has one or two valves—either small taps or screwdriver-operated slots. On integrated filling loops, the hose is already in place; you only need to open the valves.

Step 3 — Attach the filling loop if it is not permanently fitted

If your boiler uses a separate filling loop kit, connect it between the two filling loop connections (as shown in your manual) and ensure the connections are tight. Do not open the valves until the loop is properly attached.

Step 4 — Open both valves slowly

Open both valves on the filling loop slowly. You will hear water entering the system. Keep an eye on the pressure gauge on the boiler front—it will start to rise.

Step 5 — Stop when the pressure reaches 1.5 bar

When the gauge reads about 1.5 bar (or the middle of the green zone), stop. Do not overfill—pressure will increase further when the heating is on. Closing the valves promptly once you reach 1.5 bar is enough.

Step 6 — Close both valves fully

Turn both valves fully clockwise (or close the levers) so that no more water can enter the system. A small amount of water may drip from the valves when closing—this is normal. Ensure they are fully closed to avoid a continuous leak.

Step 7 — Remove the filling loop if it was a temporary one

If you used a separate filling loop kit, disconnect it from both connections and store it safely. Some systems have caps or blanks that should be refitted to the connection points. If the filling loop is permanently fitted, leave it in place with both valves closed.

Step 8 — Turn the boiler back on and reset if needed

Switch the boiler back on. If it was in lockout or showing an error due to low pressure, you may need to press the reset button (see your manual). The boiler should fire and the pressure should stay stable when the heating runs. If it drops again quickly, see the section below on recurring low pressure.

What If Pressure Keeps Dropping?

If you repressurise and the pressure falls again within days or weeks, there is likely a leak somewhere in the system. Check radiator valves (including bleed valves—ensure they are closed), visible pipework and any areas that have been recently worked on. Even a small leak can cause a gradual drop. If you cannot find the leak, or the pressure drops quickly, call a Gas Safe registered engineer. They can locate the fault and repair it safely. Do not keep topping up indefinitely without finding the cause—it can lead to corrosion and further damage.

What If Pressure Is Too High?

If the pressure gauge reads above 3 bar when the system is hot, or the pressure relief valve (usually a small pipe outside) has discharged water, the system may be overfilled. You can often reduce pressure by bleeding a radiator—letting a small amount of water (and air) out until the gauge drops to around 1.5 bar when the system has cooled. Do not bleed when the system is hot, and catch the water in a container. If pressure rises again quickly after bleeding, or you are not confident doing it, contact a Gas Safe engineer—there may be a fault with the expansion vessel or filling loop.

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Frequently Asked Questions

How often should I repressurise my boiler?

There is no fixed schedule. You repressurise when the pressure drops below the safe range—typically after bleeding radiators, or if a small leak has caused a gradual fall. A well-maintained system with no leaks may only need topping up once a year or less. If you need to repressurise every few weeks, have the system checked for leaks.

Is low boiler pressure dangerous?

Low pressure itself is not dangerous—the boiler will usually shut off to protect itself. The risk is having no heating or hot water until you repressurise. Very low pressure can sometimes allow air to enter the system, which may cause noise or cold spots in radiators. Repressurising to the correct range restores normal operation.

Can I repressurise my boiler myself?

Yes. Repressurising via the filling loop is a DIY task and does not require a Gas Safe engineer, as long as you follow the steps in your boiler manual and do not open any gas-carrying parts. If you are unsure where the filling loop is or how to use it, or if your boiler is in a difficult position, an engineer can do it for you and show you for next time.

When should I call an engineer?

Call a Gas Safe engineer if: pressure keeps dropping and you suspect a leak you cannot find; the pressure relief valve keeps discharging; the pressure is very high and bleeding does not bring it down; the boiler will not reset after repressurising; or you see water leaking from the boiler or pipework. For any work on the gas side of the boiler, or if you are not confident, always use a Gas Safe registered engineer.