How to repressurise a combi boiler
If your combi boiler's pressure gauge reads below 1 bar, or the boiler has locked out with a low-pressure fault code, you can usually top it up yourself in a few minutes using the filling loop.
- Time
- 10 minutes
- Difficulty
- Easy
- Steps
- 7
- Checked
What you’ll need
- Your boiler's user manual (or the manufacturer's website)
- Filling key, if your boiler has a keyed filling link
- Old towel
Safety first
- Don't remove the boiler's casing or try to fix anything inside it. By law, gas work must be done by a Gas Safe registered engineer.
- Don't overfill. If the needle goes into the red zone, stop and get advice: your manual or the manufacturer's helpline will tell you how to bring it down, or call an engineer.
- Leave both filling loop valves fully closed when you've finished. A loop left slightly open can over-pressurise the system.
Step by step
-
Step 1: Check the pressure gauge
Find the pressure gauge on the front or underneath the boiler. It may be a dial or a digital reading. With the heating off and the system cold, most boilers should read between 1 and 1.5 bar. Below 1 bar, it needs topping up. Check your manual for your model's exact figure.
Next: step 2 -
Step 2: Turn the boiler off and let it cool
Switch the boiler off and let the system cool down before you start. Your manual will say whether to turn off the electricity supply as well.
Next: step 3 -
Step 3: Find the filling loop
Most combis have one of two types:
- External filling loop: a flexible silver hose under the boiler, with a valve (or tap) at each end.
- Internal filling link: built into the boiler. Some have a lever or tap under the boiler, and some need a small filling key pushed into a port.
If you can't see it, your manual will show you where it is.
Next: step 4 -
Step 4: Open the valves slowly
For an external loop, make sure both ends are attached, then open both valves (usually a quarter turn, so the handle lines up with the pipe). You'll hear water flowing into the system. For an internal link, follow the manual: often it's a matter of pulling down a lever or turning the key and opening the tap.
Next: step 5 -
Step 5: Watch the gauge and close at the right pressure
Keep your eye on the gauge. When it reaches the pressure your manual gives (often just over 1 bar, or 1 to 1.5 bar), close the valves or release the lever. Close both valves fully on an external loop.
Next: step 6 -
Step 6: Restart the boiler
Turn the boiler back on. If it shows a fault code, you may need to press the reset button. Check that the heating and hot water work.
Next: step 7 -
Step 7: Tidy up
If your filling loop is designed to be removed, take it off and store it (a little water may drip out, so have the towel ready). Check that the pressure stays steady over the next few days.
Tips
- Pressure rises a little when the heating is on, so take readings with the system cold.
- You'll usually need to top up the pressure after bleeding radiators.
- Take a photo of your boiler's model plate and filling loop so you can find the right instructions online quickly next time.
When to call a professional
If you have to top up more than once every few months, there is probably a leak somewhere in the system (check under radiators and around pipes and valves for damp) or a fault with the boiler's pressure relief valve or expansion vessel. Book a Gas Safe registered engineer. You can check an engineer's registration on the Gas Safe Register website.
Questions people ask
Why does my boiler pressure keep dropping?
The most common causes are bleeding radiators and small leaks in the pipework or radiator valves. If you haven't bled anything and it keeps falling, ask a Gas Safe registered engineer to find the cause.
What should a combi boiler pressure be?
Most manufacturers say between about 1 and 1.5 bar when the system is cold. Check your manual, as some models differ.
Is it safe to repressurise a boiler myself?
Yes. Topping up the pressure with the filling loop is a normal user task described in boiler manuals. Never open the boiler casing.
Sources
Checked 9 October 2026. We wrote this guide in our own words, using:
- British Gas: how to re-pressurise your boiler (britishgas.co.uk)
- British Gas: how to re-pressurise a Worcester Bosch boiler (britishgas.co.uk)
- Viessmann: how to repressurise a combi boiler (viessmann.co.uk)
- Gas Safe Register (gassaferegister.co.uk)
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Printed from howtoguides.uk/guides/how-to-repressurise-a-combi-boiler/ · Checked 9 October 2026 · General information only. For gas, electrics, structural work, medical, legal or money decisions, use a qualified professional.