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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: October 9, 2019. Updated: February 15, 2024

What is a Boiler Filling Loop?

boiler filling loop example

Filling loops can come in different designs, and can be detached from the boiler in many cases

A Boiler Filling Loop provides a temporary connection to the mains so that you’re able to repressurise and fill your heating system with water.

In this article, we’ll break down what a boiler filling loop is actually used for, and why they’re so important. Once you know how to repressurise your boiler using this small but useful component, you’ll save yourself a lot of headaches!

For information on related issues and other components in your heating system, you can also read some of the following blog posts:

Why do I need to repressurise my boiler?

There could be a few reasons that you need to repressurise your boiler, such  as:

It’s helpful to know as much as you can about your Boiler Filling Loop. They play an important role in making sure your boiler pressure is at a reasonable level (around 1.4 bar), but if you don’t have the right knowledge, your boiler could be permanently damaged.

This is even more frustrating if it’s solved simply by repressurising the system. For detailed instructions on how you can repressurise your boiler, please watch the short video at the top of this guide >

Filling loops are used to provide a connection to your mains water supply. You’ll need this if you want to top up your system, which can happen if you’ve had to bleed your radiator.

For detailed instructions on how to bleed a radiator, please watch the short video on this page >

How do you use a boiler filling loop?

Thankfully, even if you think you don’t know enough about how boilers and central heating systems work, using a boiler filling loop is a surprisingly easy task.

You’ll need one if your boiler pressure has dropped too much. Anything below half a bar will have a major effect on how your boiler operates. If it’s only recently been installed, your filling loop may be attached using blanking caps.

Boiler Filling Loops parts

You won’t need anything else to repressurise your boiler, as you can just turn the filling loop valves using your hands. However, you’ll need to make sure that the valves are in the ‘off’ position before you start.

boiler filling loop off

  1. Using your hands, make sure that the filling loop is fixed firmly into place
  2. Turn one valve so that it matches the direction of the pipe
  3. Slowly turn the other filling loop valve. Water will start coming through the mains and into your systemboiler filling loop on
  4. Water should start to come through the system. As this happens, your boiler’s pressure displayed on the gauge should rise
  5. Once it hits 1.4 bar, close both of the valves back up

Some people find it more convenient to leave the filling loop attached or next to the boiler. This is actually against regulations, as they state your filling loop needs to be disconnected from your boiler when it’s not in use. If it’s left on, you can cause irreparable damage to your filling loop boiler. As well as this, it can be dangerous if you have children in the property. They can accidentally knock the filling loop valves, which would allow water to flow back into the system, raising the boiler pressure and cause more problems.

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Why aren’t some filling loops connected to the boiler all the time?

Easy one, this. The connection between a filling loop and the boiler is temporary because water regulations strictly prohibit any water from flowing back into the mains. If you end up using the same water in your radiators as the water that comes out of the shower and taps, then you’ll be drinking from a highly contaminated source…not a nice thought!

In short, if you’re having to use the boiler filling loop often for repressurising, there might be a problem with your current boiler or your current heating system that needs fixing.

When a boiler is installed, the engineer should leave the filling loop easily accessible for you in case you need it to repressurise your boiler. If this isn’t the case however, boiler filling loops are relatively cheap to replace.

After a quick Google search, you’ll find that most services and online stores charge between £7-£20 for a new filling loop. Considering how this component allows you to repressurise your boiler, you’d be wise to make sure you have one at hand to use whenever you need it!

Your boiler’s filling loop will have at least one stop valve that is used to control the flow of water, and a double check valve that helps prevent the flow of water coming back through from the mains.

Why do I need to use my filling loop?

A filling loop’s function is to help you repressurise the boiler if your pressure is too low. However, this in itself is something you shouldn’t have to do too regularly.

Pressure gauge location

You can check if your pressure is too low by looking at the pressure gauge located at the front of the boiler

If you think you’re having to top up your boiler too often – usually more than twice a year –  then you might have problems like leaks in your system. There are a variety of areas that could be affected like pipes, radiators, or other parts of your heating system like expansion vessels or pressure release valves.

Regardless of the cause, your boiler shouldn’t be low on pressure all the time. You’ll have to find the leak as soon as possible before your boiler suffers more damage.

If you have to top up your boiler quite often, the oxygen that comes from the mains water supply will encourage further corrosion. This corrosion ultimately results in a build of sludge inside your radiators and pipes. When this occurs, your boiler will struggle to reach the right temperature and with corrosion and component degradation thrown into the mix, will give you far too many problems to deal with.

Magnetic sludge can build up in your system causing more issues

In short, if you’re having to use the boiler filling loop to repressurise quite often, it’s probably worth speaking to heating experts to get some friendly, reliable advice. You can do so by calling us directly on 029 2009 9898.

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Quote in 30 seconds. No obligation. Next Day installation.
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4 replies
  1. Laura
    Laura says:

    I’m seeking some advice, yesterday I opened the filling loop valves to put the pressure up, I then closed both valves today I had no water running threw any off my hot water taps! So I opened one off the valves water is now running, but I’m now wondering why that happened , as the valves have both always been closed, my boiler is only a year old, I’m now scared to use it as I’m concerned , it doesn’t sound the same as it did before doing what I did!!

    • Bryn Lloyd
      Bryn Lloyd says:

      Hi Laura,

      Thanks for getting in touch.

      In order for us to get a better idea of your situation, we’ll need to know a few things, including which boiler manufacturer and model is installed, and which valves have been turned on and off. If you’d prefer, we’ll be happy to answer any questions you may have via email on [email protected], or calling us directly on 029 2009 9898.

      We hope this helps!

      Kind Regards,
      The Boilerhut Team

  2. Brenda
    Brenda says:

    I cannot close one of the blue taps on my ideal vogue boiler after topping up pressure drop due to bleeding a radiator. Can I use the boiler temporarily until I get this fixed?

  3. Tracy Hopkinson
    Tracy Hopkinson says:

    I have a oil filled combi boiler and the braided filler hose is leaking, maybe split.. My boiler is only 2 years old, is this a regular problem or should I be getting my boiler replaced by those I bought it from? I had the plumber out yesterday but today I can smell the oil quite strongly. I’ve just had my oil tank filled up.


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