How to top up boiler pressure
Quick answer: With the heating off and the system cold, check the pressure gauge. If it is below about 1 bar, open the two valves on the filling loop slowly, watch the gauge rise to about 1.2 to 1.5 bar, then close the valves tightly. This only applies to sealed heating systems. Never touch the gas side of the boiler.
If your radiators are cold or the boiler is showing a low-pressure fault, the first thing to check is the pressure gauge. Low pressure is common after bleeding radiators, and it often needs a simple top-up.
Topping up water pressure is a user job on most combi and system boilers. Anything involving gas, flues or the inside of the boiler is for a Gas Safe registered engineer.
What you will need
- Your boiler's manual, if you can find it
- The filling loop: a flexible braided hose with a valve at each end, usually under the boiler
- A cloth
- A flat screwdriver for some valves
Step by step
Check it is a sealed system
A combi or system boiler has a pressure gauge. If your home has a header tank in the loft, you have an open-vented system that does not need topping up this way.
Turn the heating off and let it cool
Switch the heating and hot water off and wait for the system to cool. Check the gauge. The normal range when cold is about 1 to 1.5 bar. If it is below 1 bar, you need to top up.
Find the filling loop
Look for a flexible silver hose joining two pipes under the boiler, with a valve (a lever or a screw) at each end. Some boilers have a built-in filling key or a button.
Open the valves slowly
Open the valve or valves on the loop. You will hear water flowing. Watch the gauge climb.
Stop at about 1.2 to 1.5 bar
Close the valves firmly as the needle reaches about 1.2 to 1.5 bar. Do not go over 2 bar. Check both valves are tightly shut.
Check the boiler
Turn the heating back on. Check the gauge does not rise past about 2 bar when hot. If the boiler has locked out, follow the manual to reset it.
Watch for pressure loss
If the pressure drops again within days, there is a leak or a fault. Do not keep topping up: call a heating engineer.
Tips
- Pressure rises a little when the system is hot. That is normal.
- A pressure that keeps dropping or going too high means a fault, such as a leak or a failing expansion vessel.
- Never remove the boiler casing and never attempt a gas repair.
Stop and get help if
- You smell gas: turn off the gas supply, open the windows, do not use switches and call the National Gas Emergency Service on 0800 111 999.
- The pressure keeps falling or the boiler shows a fault code you do not understand.
- Water is leaking from the boiler, a pipe or the pressure relief pipe outside.
- You have no filling loop or are not sure how it works. Ask a Gas Safe registered engineer.
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Common questions
What should my boiler pressure be?
When the system is cold, about 1 to 1.5 bar. It will rise a little when hot. Below 1 bar or above 2.5 bar suggests a problem.
Why does my boiler keep losing pressure?
Common causes are a small leak somewhere in the system, a failing pressure relief valve or expansion vessel, or bleeding radiators. If it keeps happening, get it checked.
Can I top up my boiler pressure myself?
Yes on most sealed systems, using the filling loop, as long as you follow the steps. Do not touch any gas parts or open the boiler.
More guides
Last reviewed 17 October 2026. This guide is general information for UK homes and is not a substitute for the manufacturer's instructions. If the job involves gas, mains wiring, asbestos, structural work or working at height, or you are unsure, ask a suitably qualified person.