Topping up the pressure on your Ideal Boiler
The following guidance is intended for combi boilers where the filling loop or filling controls are clearly identifiable underneath or built into the boiler.
System and heat-only boilers may use an external filling loop located elsewhere within the heating system, such as near the hot-water cylinder or inside an airing cupboard. The flexible hose may also have been disconnected or removed following installation. If you have a system or heat-only boiler with an external filling loop, contact your installer or a suitably qualified heating engineer for assistance.
*Is occasional pressure loss normal?
A central-heating system can need repressurising once or twice a year without this indicating a boiler fault.
Small pressure losses may occur when:
Trapped air is released through automatic air vents
Radiators are bled
The system repeatedly heats up and cools down
Tiny amounts of water escape through radiator valves, joints or other heating-system components on the system.
Maintenance or plumbing work removes a small amount of water from the system
The pressure gauge measures the pressure within the whole central-heating circuit, not just the boiler. A gradual pressure reduction over several months does not therefore necessarily indicate that the boiler itself has a fault.
It becomes more likely that there is a fault when the pressure drops repeatedly over a period of days or weeks, falls to zero, or requires frequent topping up.
Repressurising the system once or twice a year is generally not a cause for concern. If you are having to top up the pressure more frequently, please contact us so that the issue can be investigated.
Step 1. Switch the boiler off
Switch the boiler off and allow the heating system to cool. The system pressure should be checked and adjusted while the boiler is cold.
Step 2. Check the pressure gauge
Boilers manufactured in 2016 or earlier may have a separate pressure gauge fitted to the pipework beneath the boiler, close to the isolation valves.
On boilers manufactured from 2016 onwards, the pressure gauge or pressure display is generally located on the front of the boiler. It may be concealed behind a cosmetic drop-down flap.
As the location can vary by model, please refer to your boiler’s user guide for specific information.
The normal cold operating pressure is between 1.0 and 1.5 bar. On most pressure gauges, this range is indicated by green markings.
If the pressure falls below 1.0 bar, the heating system may need to be topped up. The boiler may also display a low water-pressure fault and stop operating until the system has been repressurised.
Step 3. Locate the filling controls
The filling arrangement may be:
A flexible silver filling-loop hose underneath the boiler, connecting two pipes in the shape of a horseshoe
A ridged connection pipe underneath the boiler, connecting two pipes in the shape of a horseshoe
Only continue if you can clearly identify the correct filling controls. Do not operate unidentified valves underneath the boiler.
Step 4. Open the filling controls slowly
Slowly open the filling valve or valves while watching the boiler’s pressure gauge.
Where two valves are fitted, both may need to be opened before water can enter the heating system. Open them gradually, as the pressure may rise quickly.
You may hear water entering the system while the filling controls are open.
Step 5. Stop between 1.0 and 1.5 bar
Once the pressure reaches approximately 1.0 to 1.5 bar while the system is cold, return the valves back to their original position.
Make sure the controls are completely closed. Leaving a filling valve partially open may cause the system pressure to continue rising.
Step 6. Vent (Bleed) all the radiators
Turn off the heating and allow the radiator to cool.
Place a tray or container beneath the bleed valve and put a cloth between the radiator and the wall.
Locate the bleed valve at the top of the radiator. Use a radiator key, screwdriver or spanner, depending on the valve type.
Turn the valve slowly anticlockwise by about a quarter turn.
Allow the air to escape. You should hear a hissing sound.
Close the valve clockwise as soon as water begins to trickle out.
Tighten the valve firmly, but do not overtighten it.
Check the boiler pressure afterwards and repressurise the system if necessary.
Step 7. Switch the boiler back on
Once the filling controls have been closed, switch the boiler back on and check that it operates normally.
The pressure may rise slightly while the heating is operating. This is normal, but it should remain within the operating range shown in the boiler’s user guide.
When to contact us or your installer
Contact us if:
The pressure drops repeatedly over days or weeks
The pressure falls to zero
You need to top up the system more than once or twice a year
The pressure rises too quickly or continues rising after the valves are closed
The boiler pressure will not increase
Water appears to be leaking from the boiler
Contact your installer or local Gas Safe engineer if:
You have a system or heat-only boiler with an external filling loop
The external filling-loop hose has been disconnected or removed
You cannot locate or confidently identify the filling controls
Water is leaking from radiators, valves, joints or heating pipework
Recent plumbing or heating work may have affected the system pressure
Repeated pressure loss can be caused by an issue elsewhere within the heating system and does not always indicate that the boiler itself is faulty. We recommend checking your system for visible leaks, weeping valves or wet patches along pipework regularly.
Is Your Pressure Dropping Frequently?
While it is normal to top up your pressure occasionally, doing so frequently (e.g., once a month or more) could indicate a small leak in your system or a faulty pressure relief valve.
Check Your Radiators: Bleeding your radiators can help stabilise system pressure.
Monitor the Gauge: If the needle continues to drop below 1.0 bar, consult a Gas Safe engineer.
Understand Your Model: Different boilers have different filling loop styles. Check your boiler model name for specific instructions.
