How To pressurise your Boiler
How to repressurise your boiler
If your boiler pressure is too low, your heating or hot water may stop working. This guide explains how to check the pressure gauge and safely top up your system using a filling loop.
Ideal cold pressure
Most boilers should sit around 1 to 1.5 bar when the system is cold. Always check your boiler manual, as different models can vary.
Safety first
Only repressurise your boiler if you feel confident and can clearly identify the filling loop or filling mechanism. If you smell gas, suspect carbon monoxide, see water leaking heavily, or are unsure what to touch, stop and contact a Gas Safe registered engineer. For a gas emergency, call the National Gas Emergency Service on 0800 111 999.
Watch how to repressurise your boiler
This short Ideal Heating video gives a visual guide to topping up boiler pressure. Boiler models can vary, so always check your boiler manual before using the filling loop.
Check the gauge
Look for a dial or digital display on the boiler. If it is below 1 bar, the system may need topping up.
Top up slowly
Open the filling loop valves slowly and watch the pressure rise. Do not leave the boiler unattended.
Stop at 1–1.5 bar
Close the valves once the pressure reaches the correct range, then restart the boiler and check it holds pressure.
Check your boiler’s pressure gauge
First, find the pressure gauge on your boiler. It may be a round dial with a needle, or it may be shown on a digital screen.
As a general guide, most boilers should be around 1 to 1.5 bar when the heating is off and the system is cold. If the reading is below 1 bar, your boiler may need repressurising.
Instructions for an external filling loop
An external filling loop is usually a flexible silver braided hose underneath the boiler, connecting two pipes with small valves.
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1
Turn off the boiler
Switch off the heating and allow the boiler to cool before topping up the pressure.
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2
Find the filling loop
Check that the braided hose is securely connected to both valves underneath the boiler.
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3
Open the valves slowly
Turn the valves slowly so cold mains water enters the heating system. You may hear water flowing.
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4
Watch the pressure gauge
Keep your eyes on the gauge as the pressure rises. Do not walk away while filling the system.
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5
Close the valves
Once the pressure reaches around 1 to 1.5 bar, close both valves fully to stop water entering the system.
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6
Restart and recheck
Turn the boiler back on and check the pressure again after a few minutes.
Instructions for an internal filling loop
Some boilers have a built-in filling loop. This may be a key, lever or small filling mechanism built into the underside of the boiler.
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1
Turn off and cool down
Switch off the heating and let the boiler cool before starting.
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2
Find the filling mechanism
Check your boiler manual to identify the correct key, lever or filling control.
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3
Open the mechanism
Insert and turn the key, or move the lever, depending on your boiler model.
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4
Listen and monitor
You should hear water entering the system. Watch the pressure gauge carefully.
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5
Stop at the correct pressure
Close the mechanism once the gauge reaches around 1 to 1.5 bar.
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6
Restart the boiler
Turn the boiler back on and make sure the pressure remains stable.
What if the pressure is too high?
If you accidentally add too much pressure, do not keep using the filling loop. Make sure the filling valves are fully closed.
Boiler pressure may rise slightly when the heating is on, but if the gauge goes very high or reaches the red zone, turn the boiler off and contact an engineer. You can sometimes reduce pressure by bleeding a radiator, but stop if you are unsure or if water continues to discharge.
When should you call an engineer?
You should arrange help if the pressure keeps dropping, the boiler loses pressure again quickly, you cannot identify the filling loop, the boiler shows a fault code, or you can see signs of a leak.
Why does my boiler keep losing pressure?
Common reasons include a leak in the heating system, recently bled radiators, a faulty pressure relief valve, or an issue with the expansion vessel. If it keeps happening, book an engineer.
Can I repressurise my boiler myself?
Many homeowners can top up pressure using the filling loop, but only do it if you are confident and can follow the boiler manual. If you are unsure, get professional help.
Should the boiler be on or off when repressurising?
It is best to switch the heating off and let the system cool before checking or topping up the pressure.
What happens if I over-pressurise the boiler?
High pressure can cause the system to discharge water through the pressure relief valve. Close the filling valves, stop topping up, and contact an engineer if the pressure remains too high.
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