How To Repressurise A Boiler
Topping up a combi boiler through the filling loop is a quick job you can do yourself without paying for a callout.
The short answer
To repressurise a combi boiler, switch it off and let it cool, then slowly open the filling loop valves underneath until the pressure gauge reads between 1 and 1.5 bar. Close both valves, check the needle holds steady and switch the boiler back on, pressing reset if it shows a fault. A boiler that keeps losing pressure has a leak or faulty part that needs finding.
A combi boiler that's lost pressure locks itself out, leaving cold radiators and no hot water at the taps. The fix is a top-up through the filling loop, and it's a job any homeowner can do without waiting in for an engineer.
We've put this guide together from looking after the boilers in our own homes and the rentals and Airbnbs we manage. It covers reading the gauge, topping the system up and spotting when low pressure is a sign of something bigger. If yours drops again soon after, our guide on why a boiler keeps losing pressure goes through the likely causes.
How To Check Your Boiler Pressure
Every combi boiler has a pressure gauge, from Baxi, Glow-worm, Potterton and Vaillant to Worcester and the rest. On most models it sits on the front panel or just below the casing, and newer boilers show the reading on a digital display in place of a dial. The main photo above shows a typical analogue gauge marked in both bar and psi.
Take the reading with the heating off and the system cool. Water expands as it warms up, so a reading taken while the boiler is running will look higher than the true cold pressure and can hide a problem.
What Pressure Should A Combi Boiler Be
UK boiler manufacturers recommend a cold pressure of between 1 and 1.5 bar for most combis. Plenty of gauges have a green band that marks this range, and keeping the needle inside it is the aim. The manual for your boiler gives the exact figure if the gauge isn't marked.
A reading below 1 bar means the system has lost water somewhere, through a leak, a weeping valve or after radiators have been bled. Anything higher than 2 to 2.5 bar when cold means there's too much water in the system. You can bring that down by bleeding a radiator a little at a time until the gauge drops back into the green.
How To Repressurise A Boiler Step By Step
The filling loop is a short pipe or braided hose with one or two valves that links the cold mains to the heating system, and on most combis it's tucked under the boiler. Some makes use a built-in lever or key instead, so check the manual if you can't see a loop. Once you've found it, this is the order to work in:
- Switch the boiler off and give it time to cool down.
- Find the filling loop among the valves and pipework under the boiler, like the set-up in the photo below.
- Look over the loop and the pipes around the boiler for drips or damp patches before adding any water.
- If the loop is a removable hose, connect both ends firmly by hand.
- Open the valves slowly. You'll hear cold water running into the system and see the needle start to climb.
- Close both valves as soon as the gauge reaches 1.5 bar.
- Switch the boiler back on and press reset if it's showing a fault code.
- Check the gauge again once the heating has run, and disconnect a removable loop so it can't let water through by accident.
How Long Repressurising A Boiler Takes
The top-up itself is quick. With the valves open, the gauge should climb back to 1.5 bar within 10 to 20 seconds, depending on how much water the system has lost.
If the needle creeps up very slowly, or sinks again straight after you close the valves, water is escaping somewhere. That's worth tracking down before you top up a second time.
When Regular Top-Ups Point To A Fault
A sealed heating system holds its pressure, so a top-up should be an occasional job after bleeding radiators or draining down a central heating system, not a weekly routine. Needing to repressurise every few days points to a leak, a faulty pressure relief valve or an expansion vessel that's lost its charge.
Start with the radiator valves, the joints you can see and the pressure relief pipe on the outside wall, which drips when the relief valve is letting water go. Leaks under floors are harder to find, and a thermal imaging camera can trace the warm water in a hidden pipe. If nothing obvious turns up, book a Gas Safe registered engineer, as the expansion vessel and relief valve sit inside the boiler casing.
Boilers That Don't Need Repressurising
Everything above applies to sealed systems, which covers combi boilers and system boilers with a hot water cylinder. A conventional boiler on an open-vented system is fed from a small header tank in the loft that tops the water up on its own, so you shouldn't ever need to repressurise one. If the heating on that kind of system stops working, the fault lies somewhere else.
Keeping Your Boiler Pressure Steady
Once you've done it the first time, you'll know the routine, so have a go yourself before paying for a callout. Keep the manual somewhere easy to find, because the filling loop layout varies between makes.
A clean system is kinder to the boiler as well. A magnetic filter on the return pipe catches sludge before it reaches the heat exchanger, and our pick of the best central heating filters compares the main options. Adding thermostatic radiator valves keeps each room at a steady temperature without the boiler working harder than it needs to.
Your Questions Answered
What happens if boiler pressure is too low?
A boiler with pressure below 1 bar will refuse to fire or shut down mid-cycle and show a fault code, which leaves you with no heating or hot water. The lockout protects the pump and heat exchanger from running without enough water. Topping the system up through the filling loop to between 1 and 1.5 bar and pressing reset brings most combis back on.
Can boiler pressure be too high?
Yes, boiler pressure above 2.5 bar when the system is cold is too high. The pressure relief valve opens to protect the system and lets water out through the pipe on the outside wall. A filling loop left open is the common cause. Bleed a radiator slowly, with a cloth underneath, until the gauge settles back between 1 and 1.5 bar.
Why does my boiler pressure drop when the heating goes off?
Boiler pressure rises as the water heats up and drops back as it cools, so a small swing on the gauge between hot and cold is normal. A reading that keeps sinking below 1 bar over a day or two is different. That points to a leak or a faulty expansion vessel, so check visible pipework and radiator valves and call an engineer if it carries on.
Is it safe to repressurise a boiler yourself?
Repressurising a combi boiler yourself is safe, and manufacturers explain how to do it in the user manual. You're only letting mains water into the heating circuit through the filling loop, so there's no need to open the boiler casing or touch any gas parts. Replacing an expansion vessel or relief valve is different and needs a Gas Safe registered engineer.
Why should I check the boiler pressure with the heating off?
A hot system reads higher than a cold one because the water expands as it warms, and that extra can mask a real loss of pressure. Check the gauge with the heating off and the system cool, then compare the reading with the 1 to 1.5 bar range most combis recommend.
Why disconnect the filling loop after topping up?
A removable filling loop is best disconnected so it can't let water through by accident. Once the gauge has reached 1.5 bar, close both valves, switch the boiler back on and check the gauge again after the heating has run. Then take the loop off, so the sealed system stays closed to the mains.
How long should it take to repressurise a combi boiler?
Repressurising a combi boiler takes seconds once the valves are open, with the gauge climbing back to 1.5 bar in 10 to 20 seconds depending on how much water has been lost. A needle that creeps up very slowly, or sinks again as soon as you close the valves, means water is escaping and needs tracking down.
Do I need to repressurise an older boiler with a tank in the loft?
No, a conventional boiler on an open-vented system never needs repressurising, because a small header tank in the loft tops the water up on its own. Repressurising only applies to sealed systems, such as combi boilers and system boilers with a hot water cylinder. If that heating stops working, the fault lies somewhere else.
Richard has spent years renovating properties and turning them into rentals and Airbnbs. Like a lot of people he started out as a beginner, and these days there isn't much DIY he won't take on.