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How to fill a pressurised heating system

What we’ll cover…

  • Shutting all air vents and drain-offs prior to refilling
  • The addition of central heating inhibitor for corrosion prevention
  • Identifying the filling loop close to the boiler or internal filling loop
  • Filling process:
    1. How to open the filling loop to fill downstairs radiators first
    2. The filling of upstairs radiators
    3. Filling the towel radiator at the final stage as the highest point
  • Closing the filling loop when pressure reaches 1-1.5 bar level
  • Switching the boiler back on and checking all radiators are heating up correctly
  • Note on instances to seek professional help when unsure
  • FAQs regarding ideal boiler pressure levels for specific circumstances (0.7-2.5 bar)
  • Frequent causes of pressure loss such as leaks
  • Side effects of low pressure like downstairs radiators not heating up properly
  • Pressure drop after radiator bleeding

A quick guide to refilling a heating system

This guide will show you how to re-fill a pressurised heating system that has been drained down.

Follow the instructions or watch the video to discover the best way to fill a pressurised heating system.

How do I refill my heating system?

To begin with, you will have to close all of the air vents and drain offs by turning them clockwise.

Fill Pressurised Boiler_1_Close Vents

Before filling up it’s important to add central heating inhibitor.

Do this by simply removing a radiator blank and pouring the inhibitor into the system.

Fill Pressurised Boiler_1_Remove Air Vent
Fill Pressurised Boiler_3_Pour Inhibitor

Doing this will help to prevent corrosion throughout your heating system and reduce the chance of having heating problems in the future.

Follow the inhibitors manufacturing instructions; generally, one tub or bottle will be adequate for up to 10 radiators.

Where will I find my filling loop?

By your boiler, you should have a filling loop on the pipework.

Fill Pressurised Boiler_4_External Filling Loop

Or you may have an internal filling loop situated at the bottom of your boiler.

Fill Pressurised Boiler_5_Internal Filling Loop

To begin refilling your system, open the filling loop to the downstairs radiators first.

Fill Pressurised Boiler_6_Downstairs Bleed

Once you have done this, you can repeat the process for all of the upstairs radiators too.

Fill Pressurised Boiler_7_Upstairs Bleed

Turn your radiator key anti-clockwise until water begins to trickle out.

Fill Pressurised Boiler_8_Rad bleed to fill

Please note that if you have a towel radiator, be sure to do this last as it is normally the highest point of the heating system.

Fill Pressurised Boiler_9_Towel Rail bleed to fill

Once you have filled all of your radiators, close the filling loop ensuring that the system pressure is around 1-to-1.5 bar. It may help if someone monitors the pressure whilst you are filling up so you don’t over-pressurise the system.

Fill Pressurised Boiler_10_Boiler

After you have re-pressurised the system, you can then turn your boiler back on. Always make sure that all of your radiators are warming up.

And that’s it, you have (or should have) successfully filled your pressurised heating system!

Please note: Our ‘How-To’ manuals are intended only as a guide to assist you with common home heating tasks. Please do not attempt to undertake this task if any of the instructions are unclear, or if you are in any doubt about what to do. Instead, seek advice from a professional.

Frequently Asked Questions

An appropriate pressure level for boilers is somewhere between 0.7 and 2.5 bars on the central heating system gauge. This will allow for hot water radiators and heated towel rails to work on the network successfully and provide an appropriate level of heating for the household.

To find out more, take a look at our blog, Why does my boiler keep losing pressure?

To work safely and efficiently, boilers should be at somewhere between 0.7 and 2.5 pressure bars. This will enable any designer radiators or heated towel rails on the system to operate successfully for the adequate heating of the home.

For further information, browse our blog, Why does my boiler keep losing pressure?

The most common cause of a boiler losing pressure is from leaks forming on radiators, the boiler itself or attached pipework, or from internal faults within the central heating system. When left unattended, this can spiral towards fully fledged boiler and overall home heating troubles.

To find out how to combat the problem, and return your boiler to its ideal pressure levels, explore our blog, Why does my boiler keep losing pressure?

Low boiler pressure can stop downstairs radiators from heating up adequately, as the water isn’t pressurised enough to transport heat from the central heating system all the way down to your downstairs home radiators.

To find out more, explore our dedicated blog, Why are my downstairs radiators not working?

Yes. During the process of bleeding radiators, air is released from within them, which in turn reduces the pressure in your boiler system.

Find out everything you need to know about the radiator bleeding process in our full Advice Centre blog, How To Bleed A Radiator.

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