On this page
- What pressure should a combi boiler be?
- How do you find the filling loop?
- How do you repressurise a combi boiler step by step?
- What about keyless fillers and built-in filling valves?
- What if you put too much pressure in?
- When does needing to top up mean something is wrong?
- Getting a pressure problem fixed in Stoke-on-Trent
- Frequently asked questions
Quick answer
To repressurise a combi boiler, turn the heating off and let it cool, find the filling loop under the boiler, open its valves slowly until the gauge reads the pressure in your manual, usually around 1 to 1.5 bar cold, then close both valves fully. Reset the boiler if the manual says to.
Key takeaways
- Check your manual for the correct cold pressure, usually around 1 to 1.5 bar.
- Always fill with the boiler cool and open the valves slowly.
- Close both valves fully and disconnect an external loop afterwards.
- Overfilled? Release a little water by bleeding a radiator.
- Needing to top up often means a leak or faulty part.
A combi boiler that has lost pressure usually shows it one of two ways: the gauge needle has dropped towards zero, or the display shows a low pressure fault code and the boiler will not fire. Topping it up is one of the few boiler jobs a householder can do safely, provided you follow your manufacturer's manual and take it slowly.
What pressure should a combi boiler be?
Most combi boilers should sit at around 1 to 1.5 bar when the system is cold, but always check the figure in your boiler's manual as it varies by model. When the heating is running the reading rises a little as the water expands, which is normal.
Use this as a general guide to what the gauge is telling you:
| Gauge reading (cold) | What it usually means | What to do |
|---|---|---|
| Below 0.5 bar | Too low, boiler may lock out | Repressurise following the manual |
| Around 1 to 1.5 bar | Normal for most combis | Nothing, note the reading |
| Around 2 to 2.5 bar | Higher than needed | Release a little water from a radiator |
| Near 3 bar | Too high, relief valve may discharge | Turn off and call an engineer if it keeps rising |
If your boiler has a digital pressure display rather than a dial, the same ranges apply. Many models show the reading on the main screen or in an information menu.
How do you find the filling loop?
The filling loop is usually underneath the boiler, among the pipes. On many combis it is a silver braided hose with a valve at each end. Some boilers have a built-in filling valve with a lever or tap on the underside, and others use a filling key that clips into the base.
If you cannot find it, look for a diagram in the manual or on the manufacturer's website under your model number. Do not remove the boiler casing to look for it. Everything a householder needs to reach is designed to be accessible from outside.
How do you repressurise a combi boiler step by step?
The process is the same in principle on most boilers with an external filling loop. Take your time, keep an eye on the gauge throughout and have an old towel ready for drips.
- Turn the heating off and let the boiler and radiators cool. A cold reading is the one that matters.
- If the loop is detached, connect both ends of the hose hand-tight to the two valves under the boiler.
- Open the first valve fully, then slowly open the second. You will hear water flowing into the system.
- Watch the gauge climb. Stop at the pressure stated in your manual, usually around 1 to 1.5 bar.
- Close both valves fully. Check they are tight so water does not keep creeping in.
- If your loop is removable, disconnect it and catch any water left in the hose.
- Turn the boiler back on. If it shows a fault, reset it once as the manual describes.
What about keyless fillers and built-in filling valves?
Keyless and integral fillers work the same way but without a hose. You turn a lever, knob or tap on the underside of the boiler, watch the gauge rise, then turn it back to closed. Some Worcester Bosch and Vaillant models use a filling key or a lever that must be returned to its original position.
The exact movement differs between brands, so follow the instructions for your model. Manufacturers such as Worcester Bosch publish user manuals and filling guides online by model name. If a built-in valve will not close fully, or water keeps dripping from the pressure relief pipe outside, the valve may need attention from an engineer.
What if you put too much pressure in?
If the gauge is above the recommended range, let a little water out by bleeding a radiator. With the heating off and cool, open the bleed valve slowly with a radiator key and hold a cloth underneath. Release water a little at a time and close the valve once the gauge is back in range.
Our guide on how to bleed a radiator covers the technique. If pressure reaches around 3 bar, the pressure relief valve is designed to open and discharge water through a pipe to the outside. That is a safety feature doing its job, but it should not happen regularly.
When does needing to top up mean something is wrong?
Topping up once or twice a year is not unusual, particularly after bleeding radiators. If you are repressurising every few weeks, the water is going somewhere. That usually means a leak on the pipework or a radiator valve, a failing expansion vessel or a pressure relief valve that is letting by.
We explain the common causes in why your boiler keeps losing pressure. Constant refilling also brings fresh oxygen into the system, which speeds up internal corrosion and sludge, so it is worth having the cause found sooner rather than later. A boiler that keeps locking out for other reasons is covered in boiler keeps turning off.
Getting a pressure problem fixed in Stoke-on-Trent
Early summer is a good time to deal with a slow pressure loss. The heating is barely used, so a repair can be booked without the urgency of a cold house. If your combi keeps dropping pressure, our boiler repair engineers can trace the leak or test the expansion vessel. A repair visit is £90 per hour.
We cover the whole of Stoke-on-Trent and nearby towns, including boiler service in Kidsgrove and Biddulph. Call 07896 677083 to book a visit.
Frequently asked questions
Is it safe to repressurise a boiler myself?
Yes, topping up through the filling loop is a householder task that most manufacturers describe in the user manual. It does not involve the gas side of the boiler. Keep to the external controls, never remove the casing, and call a Gas Safe registered engineer if anything does not behave as the manual describes.
Why does my boiler pressure rise when the heating is on?
Water expands as it heats, so a small rise is normal. The expansion vessel absorbs that extra volume. If the pressure climbs close to 3 bar when hot, the expansion vessel may have lost its charge, or the filling loop may be letting water in, and an engineer should check it.
Can low boiler pressure stop my hot water?
Yes. Most combis have a low pressure sensor that stops the boiler firing when pressure falls too far, which means no heating and no hot water. Repressurising and resetting often brings it back, but the pressure loss itself still needs to be explained.
Should I leave the filling loop valves slightly open?
No. Both valves should be fully closed after filling. Leaving them open can overpressurise the system and allow heating water to mix back towards the drinking water supply. If the loop is removable, take it off and store it somewhere easy to find.
Sources and further reading
Gasmaster Plumbing & Heating has served Stoke-on-Trent since 2005. Every article is written by our Gas Safe registered engineers (reg. 220932) from day to day work on boilers, heating and gas appliances across Staffordshire, Cheshire and Derbyshire.
