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Boiler Filling Loop

What is a Boiler Filling Loop?

It is important to recognise that there are two different types of boiler filling loops. An external filling loop has two valves, one to control the water flow and the other to stop the boiler water from going back to the mains. A part of the external filling loop connects to your main water line, while the other connects to your boiler.

There are also internal filling loops which are typically found in combi boilers. They practically work in the same way but are fully integrated into the system.

Now that you are aware of the two types, this guide will go on to explain how and why you might need to use a boiler filling loop.

Why do I need to use my filling loop?

If your boiler pressure is too low, then you can use a filling loop to help fix it. However, repressuring your boiler is not something you have to do very regularly.

If you are having to refill your boiler more than twice a year, it can be detrimental to your heating system’s condition. This is because an excessive amount of oxygen coming from the mains water supply may lead to corrosion, resulting in a build-up of sludge inside your radiators and pipes. Not only will this impact the productivity of your radiators and pipes, but it will also cause other parts of your heating system to suffer, including expansion vessels and pressure release valves. This will then hinder your boiler’s ability to reach the right temperature, reducing your system’s efficiency, meaning energy and money are wasted.

No matter the cause, your boiler should not have low pressure all the time. If this is the case, then you will have to find the source of any leaks before your boiler suffers further damage.

How do you use a boiler filling loop?

Although you might consider yourself to have limited knowledge when it comes to understanding how boilers and central heating systems work, there is really no need to worry because using a boiler filling loop is a surprisingly easy task.

To repressurise your boiler you will need to make sure that the filling loop is fixed firmly into place. Turn one valve so that it matches the direction of the pipe, then slowly turn the other filling loop valve. Water will start coming through the mains and into your system. When this happens, your boiler’s pressure should rise. You should wait for the pressure gauge to reach at least 1.5, before closing the valves back up



Why is the filling loop not connected to the boiler all the time?

A filling loop is only attached to a boiler temporarily because water regulations strictly prohibit any water from flowing back into the mains. Although the convenience of leaving the two connected might be tempting, if the water coming out of the taps has also been used in the radiators, then you will be drinking from a highly contaminated source. Not only this but allowing too much water to flow back into the system could well raise your boiler’s pressure, causing further issues.

Worcester Bosch Boiler

Homeowners are still able to get a FREE boiler grant or insulation grant via the ECO scheme, which is still available to households that qualify. Warma UK are currently working throughout the UK helping privately owned and rented homes to improve home energy efficiency and save money on rising energy bills.

For more information email –

Or call 03304600065

Apply For Your Government Backed Heating Grant

Following your successful application, we will arrange a free, no obligation survey to ensure the maximum grant is obtained and advise you on the best products to reduce your homes energy usage.

Michael Cornish