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Boiler Filling Loop

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.

boiler filling loop example

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

But why might you need to do this? Although it seems like ‘common knowledge’ for those familiar with the industry, it’s helpful to know what a Boiler Filling Loop looks like. They play an important role in making sure your boiler pressure is at a reasonable level (around 1.4 bar). If you don’t have the required knowledge, your boiler could be permanently damaged. This is even more frustrating if it’s solved simply by repressurising the system.

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

  • Your boiler pressure has dropped below 1 bar
  • You’ve recently bled your radiators
  • You’ve got a small leak
  • There’s a fault with the boiler itself

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:

How does a boiler filling loop work?

A boiler filling loop is essentially 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.

In most cases, the filling loop looks like a small braided hose with connections on either end. While there are different types used by different manufacturers, they allow you to connect to the central heating system and the mains supply to help you repressurise your boiler.

The boiler filling loop can also be detachable, requiring you to fix it into place when using it.

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!

Not a nice thought at all!

I can’t find it!

Boiler Filling Loops are usually located on the underside of the boiler fixed to the pipework. As mentioned before, it’s a short braided hose with connections on either end. Bear in mind that some boilers, such as the Viessmann Vitodens 100-W have an integral filling loop built into the system. Using the example below however, you can see how filling loops appear when attached to your pipework.

Boiler Filling Loop location

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.

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 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

  • Using your hands, fix the ends of the boiler filling loop to the boiler valves
  • Repeat for the other end
  • With the filling loop fixed into place, turn one valve so that it matches the direction of the pipe
  • Slowly turn the other valve. Water will start coming through the mains and into your system

boiler filling loop on

  • Water should start to come through the system. As this happens, your boiler’s pressure displayed on the gauge should rise
  • 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 boiler 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 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.

Why should they be used?

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 problem such as a leak somewhere in your system. There are a variety of areas that could be affected. It could be a pipe or radiator, or even another part of your heating system such as 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 because if gone unattended, your boiler will suffer 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 might be worth having a heating expert asses the issue directly.

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