To repressurise a boiler, turn it off and let it cool, open the filling loop slowly until the gauge reads between 1 and 1.5 bar, then close it again and switch the boiler back on. That’s the whole job. It takes about five minutes and you don’t need any tools beyond, occasionally, a filling key.
We’re Clever Energy Boilers, a Gas Safe registered (no. 554138) and Worcester Bosch accredited installer based in Bingley. Our engineers fit boilers across Yorkshire, the North West and the Midlands, and topping up pressure is one of the questions we’re asked most often. Here’s how to do it safely on any modern combi or system boiler.
How do I know my boiler needs repressurising?
Your boiler needs topping up if the gauge reads below 1 bar when the heating is off. You might also notice the heating and hot water have stopped, or a low-pressure fault code on the display.
Not sure what’s normal? Our guide to what boiler pressure should be explains the readings.
How to repressurise a boiler, step by step
- Turn the boiler off and let it cool. Topping up a hot system gives a false reading, so give it 20 to 30 minutes.
- Find the filling loop. It’s usually underneath the boiler. More on what it looks like below.
- Check both ends are connected. If you have a detachable braided hose, attach it to both valves first.
- Open the valves slowly. Turn the first valve fully, then open the second a little at a time. You’ll hear water flowing in.
- Watch the gauge. Stop when it reaches about 1.2 to 1.5 bar.
- Close both valves firmly. Make sure they’re fully shut, or pressure will keep creeping up.
- Disconnect the hose if it’s removable. Water regulations say a temporary filling loop shouldn’t be left connected.
- Switch the boiler back on. If it shows a fault code, press the reset button once.
How do I increase or top up the pressure on a combi boiler?
Exactly the same way. Increasing, topping up and repressurising all mean adding water through the filling loop until the gauge reads 1 to 1.5 bar. Combi and system boilers both work like this. Regular boilers with a tank in the loft don’t need topping up at all.
Where is the filling loop on my boiler?
Most boilers have one of three types. Check which one you have before you start.
| Type | What it looks like | Where to find it |
|---|---|---|
| External filling loop | A silver braided hose between two small valves, often with lever or screwdriver-slot handles | On the pipes below the boiler |
| Built-in filling tap or lever | A small tap or lever, sometimes blue, on the underside of the boiler | Under the boiler casing, near the front |
| Filling key | A plastic key that slots into a socket underneath and twists | Key often clipped to the boiler or kept with the manual |
If you can’t find yours, the boiler manual will have a diagram. Most manufacturers also publish manuals online.
How do I repressurise a Worcester, Baxi, Ideal or Vaillant boiler?
The steps are the same for every brand. What changes is the filling device, so here’s a quick guide to each. Always double-check against your own manual, as layouts vary between models.
Worcester Bosch
Many Worcester Bosch combis have a built-in filling link underneath, and some older Greenstar models use a removable filling key. Newer models may have a lever instead. As accredited Worcester Bosch installers, we always show customers where theirs is on installation day.
Baxi
Baxi boilers commonly use an external braided filling loop below the boiler, though some older models have a filling tap. Our Baxi boiler installation page covers the current 600 and 800 ranges.
Ideal
Ideal Logic and Vogue boilers generally use an external filling loop with two valves. If pressure keeps dropping on an older Ideal, our Ideal boiler installation page shows the newer models we fit.
Vaillant
Depending on the model, Vaillant ecoTEC boilers use either a filling tap underneath or an external loop. See our Vaillant boiler installation page for the latest range.
What if I put too much pressure in?
Bleed a radiator to bring it back down. Hold a cloth under the bleed valve, open it a quarter turn and let water out until the gauge drops to around 1.5 bar. Our radiator bleeding guide shows you how, and our post on boiler pressure that’s too high covers what to do if it won’t come down.
Is it safe to repressurise a boiler myself?
Yes. Topping up pressure through the filling loop is a normal homeowner job, and manufacturers expect you to do it. You’re adding mains water to a sealed system, not touching any gas parts. Just don’t remove the boiler casing.
Why does my boiler need topping up again?
If you need to top up more than once or twice a year, water is escaping somewhere. Common causes are a small leak, a faulty pressure relief valve or an expansion vessel that’s lost its charge. Our guide on why a boiler keeps losing pressure goes through each one.
Should the heating be on or off when I repressurise?
Off, and ideally cold. Water expands as it heats up, so topping up a hot boiler to 1.5 bar can leave it too high once it’s running.
When is it time for a new boiler?
If you’re topping up every few weeks and your boiler is over 8 years old, it’s usually more cost-effective to replace it than to keep paying for parts. Older boilers also burn more gas than modern A-rated models.
Our boiler replacement service comes with a fixed price and Gas Safe engineers. On top of that, Clever Energy can set you up with long, market-leading warranty cover. Most swaps are finished in a day.
Repressurising a boiler FAQs
How long does it take to repressurise a boiler?
About five minutes, plus the time it takes the boiler to cool down first.
What pressure should I fill my boiler to?
Aim for 1 to 1.5 bar with the system cold. Check your manual, as some boilers recommend a slightly different figure.
Why won’t my boiler pressure go up when I open the filling loop?
Check both valves are open and the mains stopcock is on. If water still won’t flow, the loop may be blocked or faulty, and a heating engineer should look at it.
Do I need to repressurise a regular boiler?
No. Regular (heat-only) boilers are fed from a tank in the loft, so they don’t use a filling loop. This guide applies to combi and system boilers.
