A dropping pressure gauge and a boiler that keeps locking out is stressful, especially in cold weather. The good news is that repressurising a boiler is often a simple job you can do yourself, as long as you follow a careful, safety-first process.
Understanding boiler pressure and the gauge
Your boiler and central heating work as a sealed system. Water expands as it heats, so the pressure changes between a cold system and one that is up to temperature.
As a general guide, most domestic boilers sit around 1.0 to 1.5 bar when cold, and often between 1.5 and 2.0 bar when warm. Some manufacturers allow a little outside these ranges, so always check your boiler manual for the correct figures for your model.
The pressure gauge is usually on the front of the boiler or just underneath. It may be a round dial with a needle, or a digital display. Look for a green or shaded "normal" area if your boiler has one.
Before you start: safety checks
Repressurising involves adding mains water into a pressurised, hot water system. Taking a few moments to prepare properly helps avoid damage or injury.
Switch the boiler off at the controls and, if possible, at the fused spur
Allow the system to cool so radiators and pipework are not too hot to touch
Check the area is dry and you can access the underside of the boiler safely
Have a cloth or small towel ready in case of minor drips
Stop and call an engineer if anything looks damaged, loose or unclear
If you are ever unsure about a valve, pipe, or what you are looking at, do not guess. Incorrectly opening the wrong valve can cause leaks or over-pressurising.
Filling loop types and how they look
The filling loop is the arrangement that lets you top up water from the mains into your heating system. You will usually have one of two types.
Integral (built-in) filling loop
On some modern boilers, the filling loop is built in. You may see one or two small plastic or metal levers or knobs directly under the boiler, sometimes marked with symbols or numbers.
These valves are often inline with small copper pipes. When the lever is in line with the pipe, the valve is usually open. When it is across the pipe, it is usually closed. Labels or arrows may also show the on/off positions.
External braided hose filling loop
Older or some system boilers use a flexible braided metal hose between two valves. This hose may be permanently attached, or detachable using screw fittings at each end.
Each end of the hose will usually have a valve with a small lever or a flat slot designed for a screwdriver. Again, levers inline with the pipe normally mean open, across the pipe usually means closed.
If any part of the filling loop looks corroded, damaged, or you are not confident which valves to use, stop and contact a heating engineer.
Step-by-step: how to repressurise a boiler safely
1. Turn the boiler off and let it cool
Switch the boiler off at the controls and wait until the system is cool enough that radiators are comfortable to touch. This protects you from hot water and gives a more accurate cold pressure reading.
2. Locate the filling loop and check valves are closed
Find your filling loop as described above. Make sure both valves are in the closed position. If you have a detachable braided hose and it is not connected, attach it firmly to both valve fittings before you start, ensuring the rubber washers are in place.
3. Open the valves slowly
Turn the first valve a quarter turn to the open position, then gently open the second valve. You should hear water flowing. Never snap valves fully open; slow, small movements give you better control of the pressure rise.
4. Watch the pressure gauge closely
Keep your eyes on the pressure gauge while the valves are open. The needle or digital reading should start to rise steadily as water enters the system.
Close both valves when the pressure reaches the recommended cold pressure in your manual. If you do not have the manual, aim for around 1.2 to 1.5 bar when cold, but do not exceed any marked green "normal" zone on your gauge.
5. Close valves fully and remove the hose if detachable
Turn both valves back to the closed position. If you have a braided hose that is meant to be removed, unscrew it carefully and keep it somewhere safe. A small drip of water is normal, so have your cloth ready.
Leaving a detachable filling loop connected and the valves accidentally open can cause over-pressurising, so it is good practice to remove it if your system is designed that way.
6. Reset the boiler and check operation
Turn the boiler back on and reset it if needed, following the manufacturer instructions. Allow the heating to run and double-check that radiators are warming evenly.
Watch the pressure gauge as the system heats up. A rise of around 0.3 to 0.5 bar between cold and hot is common. If the pressure climbs close to the red zone or safety limit on the gauge, switch the boiler off and seek professional help.
What to do if something goes wrong
Pressure has gone too high
If you accidentally overfill and the pressure sits too high when the system is cold, you can usually reduce it slightly by bleeding a radiator.
Turn the boiler off and let it cool, then use a radiator key on the bleed valve at the top of a radiator, ideally one upstairs. Open the valve slowly until you hear a hiss of air, and allow a small amount of water to escape into a cloth or container, keeping an eye on the gauge as you do so.
Once the pressure drops back into the normal range, close the bleed valve firmly. Never open multiple bleed valves at once or allow large amounts of water to drain, as this can introduce air and may lead to more problems.
Pressure keeps dropping after repressurising
If your boiler repeatedly loses pressure after you top it up, there is likely an underlying fault. Common causes include:
A leak on a radiator, valve or pipework, sometimes in hidden areas
A problem with the boiler’s expansion vessel losing charge
A faulty pressure relief valve (PRV) letting water out through the discharge pipe outside
Look around radiators, valves and visible pipe runs for damp patches, staining or green/white crusty marks. Outside, check the metal discharge pipe from the boiler for signs of regular dripping when the heating is on.
Internal boiler components, including the expansion vessel and PRV, must only be worked on by a Gas Safe registered engineer. If you suspect an internal problem, do not remove the boiler case yourself.
When to call an engineer instead
Repressurising is usually a safe user task, but you should call a professional if:
You are unsure which valves form the filling loop, the pressure will not rise, the pressure drops repeatedly after topping up, the pressure shoots up into the red zone, or you see visible leaks anywhere on the system.
Regular pressure loss is often a sign of a developing fault that is best dealt with early, before it causes damage to your boiler or home.
Need help with recurring boiler pressure loss?
If your boiler will not hold pressure or you are uncomfortable repressurising it yourself, it is time to get an expert involved. A Gas Safe registered engineer can safely diagnose leaks, expansion vessel issues and faulty safety valves, and get your heating running reliably again.
For friendly, professional boiler repairs and help with ongoing pressure problems, contact Fixbot on 01234815101 or visit our boiler repairs page to arrange a call-out.
