Plumbing Radiator Boiler

How to Repressurise a Boiler – DIY Guide

ByDIYnut

Plumbing › Boilers

How to Repressurise a Boiler

DIY Guide

Restore your heating fast — repressurise your boiler in under 15 minutes.

Quick Answer

To repressurise a boiler, locate the filling loop beneath the boiler, open both valves slowly until the pressure gauge reads 1.0–1.5 bar, then close the valves and reset the boiler. The whole job takes under 15 minutes and requires no specialist tools.

Before: Repressurise a Boiler
Before
VS
After: Repressurise a Boiler
After
Difficulty
Beginner
Time
10–15 Minutes
Cost
£0–£5
Tools Needed
  • Flat-head screwdriver
  • Adjustable spanner
  • Torch
  • Cloth or small towel
Materials
  • Filling loop (usually already fitted to boiler)
  • PTFE tape (if resealing a removable filling loop)
How To

Step-by-Step Guide

1

Check the pressure gauge reading

Look at the pressure gauge on the front of your boiler — it is usually a dial or a digital display. A reading below 1.0 bar means the system is low on pressure and needs repressurising. Most boilers operate correctly between 1.0 and 1.5 bar; anything below 0.5 bar may have triggered a lockout fault. If the gauge reads above 2.5 bar already, do not add more pressure — call a Gas Safe registered engineer. If you have also noticed cold spots at the bottom of radiators, it is worth bleeding your radiators before repressurising, as trapped air causes pressure to drop.

2

Locate the filling loop

The filling loop is a short braided flexible hose — or a built-in keyway — that connects the mains cold water supply to the central heating circuit. It is almost always found beneath the boiler, either behind a small access panel or clipped underneath the boiler casing. Some boilers have an internal filling loop with a single key or lever; others have an external loop with two separate quarter-turn valves. Identify which type you have before proceeding. Check your boiler manual if you are unsure — most are available as a PDF on the manufacturer’s website.

3

Open the filling loop valves

If you have an external filling loop, turn both valves slowly — one at each end of the braided hose — to the open position (inline with the hose). You will hear water entering the system. Watch the pressure gauge continuously as you do this. If you have a single internal keyway or lever, turn it slowly in the direction marked on the boiler. Do not rush this step — pressure rises quickly and overshooting is a common mistake. Keep your cloth to hand in case of any minor drips at the connections. This is also the point to check whether your mains water pressure is adequate to fill the system — if nothing happens when you open the valves, the mains supply may be the issue.

4

Close the valves at 1.0–1.5 bar

As soon as the gauge needle or digital reading reaches 1.0 to 1.5 bar, close both valves fully. Close them in the reverse order to which you opened them — this prevents any surge in pressure. Double-check both valves are fully shut; a valve left slightly open will allow pressure to continue rising or will cause a slow leak into the heating circuit. The target pressure when the system is cold is 1.0–1.2 bar — it will rise slightly to around 1.5–2.0 bar once the heating warms up, which is normal.

5

Reset and test the boiler

Once the valves are closed, press the reset button on your boiler — the location varies by model but is usually a clearly labelled button on the front panel. The boiler should fire up and the fault code or warning light should clear. Run the heating for 20 minutes and recheck the gauge. If pressure holds steady, the job is done. If it drops again within a few days, you likely have a system leak — check radiator valves and visible pipe joints for drips, and consult a Gas Safe engineer if you cannot find the source. For related heating faults, see our guide to fixing no hot water.

Watch Out

Common Mistakes to Avoid

Overfilling past 1.5 bar Filling the system above 1.5 bar (cold) will cause the pressure relief valve to discharge water — usually through a pipe outside the building. This wastes water, can introduce air back into the system, and may trigger another lockout. You will then need to bleed the excess pressure off before the boiler will run correctly. Leaving a filling loop valve open If either filling loop valve is not closed fully after repressurising, mains water continues to enter the heating circuit. Pressure climbs silently until the relief valve opens, potentially causing water damage and voiding your boiler warranty. Always double-check both valves are fully shut before leaving the job. Repressurising without diagnosing the cause If pressure keeps dropping every few days or weeks, simply topping it up repeatedly masks a system leak. Undetected leaks cause damp, structural damage, and boiler corrosion over time. Repeated repressurising without finding the cause is a warning sign — a Gas Safe registered engineer should inspect the system. FAQ Frequently Asked Questions Why does my boiler keep losing pressure? Pressure loss is usually caused by a slow leak somewhere in the system — at a radiator valve, a pipe joint, or the boiler itself — or by a faulty pressure relief valve. Bleeding radiators also releases a small amount of water each time, which lowers pressure. If you are repressurising more than once a month, have a Gas Safe registered engineer inspect the system. You may also want to check for leaking pipe joints across your heating circuit.

What pressure should my boiler be at?

For most sealed central heating systems, the correct cold pressure is 1.0 to 1.5 bar. When the heating is running at full temperature, pressure will rise to around 1.5 to 2.0 bar, which is normal. Consult your boiler manual for the exact recommended range as this varies slightly between models.

Can I repressurise a boiler myself, or do I need a plumber?

Yes — repressurising a boiler is a task any competent homeowner can do safely. You are not working on any gas components; you are simply adding water to the central heating circuit via the filling loop. No Gas Safe registration is required for this task. However, if the boiler continues to lose pressure, a Gas Safe engineer should investigate the underlying cause.

What if I cannot find the filling loop on my boiler?

Some boilers have an internal filling loop accessed by a key or lever hidden behind the front panel — check your boiler manual or search for the model number online to find a PDF guide. If your boiler does not have an integral filling loop and the external hose is missing, you will need a plumber to fit one before you can repressurise the system.

Is it safe to repressurise a boiler while it is still on?

It is safer to repressurise with the boiler switched off and the system cold. Hot systems are under higher pressure, which makes it harder to judge the correct fill level accurately. Switch the boiler off, allow the system to cool for at least 30 minutes, then carry out the repressurising procedure.

Pro Tip

On older systems, scale and debris can cause the filling loop’s check valve to stick open slightly — meaning pressure creeps up slowly even after you have closed the valves. After filling, disconnect or isolate the external filling loop entirely rather than leaving it permanently connected; this removes one route for undetected pressure rise and is best practice on any sealed system.

Sources

  • Heating and Hot Water Industry Council — Sealed Systems Guidance — centralheating.co.uk
  • HSE — Gas Safety: Advice for Homeowners — hse.gov.uk
  • Which? — How to repressurise a boiler — which.co.uk
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Safety Notice: Electrical and plumbing work can be dangerous if done incorrectly. In the UK, certain electrical work must comply with Part P of the Building Regulations and certain plumbing work with Part G. If in doubt, consult a qualified electrician (NICEIC/NAPIT registered) or plumber (CIPHE/WaterSafe registered). This guide is for general information only — it is not a substitute for professional advice.

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