Boiler Filling Loop: Installation, Repair & Replacement

Boiler Filling Loop: Installation, Repair & Replacement

Stuck or unsure about your boiler filling loop? Call PHS today and we can get an expert out to you ASAP to help find you a solution!

What is a filling loop and how does it work?

A boiler filling loop is a key element when it comes to your boiler. We use it to fill and pressurise the system with water and top it up when it is needed.

It is connected to the mains water supply and can be internal, built into the boiler (see the image above) or something we externally fit – as a piece of pipe or hose.

It can be made up with valves to stop water from going the wrong way through the system and is usually made using a stop valve at one end and a double check valve at the other end.

These are integral to ensure there is no backflow of water into the mains water supply, which could result in contamination.

What can go wrong?

If your boiler is in need of regular topping up, this can be indicative of a leak, so it is best to keep an eye on it to prevent any long term damage.

With this in mind, it is important to ensure the installation, repair and replacement of your boiler is done in a timely manner.

Using the filling loop can often be a good first step to take. There are some quick fixes you can do yourself, just check out the FAQs below.

If in the case of an emergency or you are in need of further help than the questions below provide, make sure you call PHS about your radiators not working or any other central heating problems. 


Why do we need a filling loop?

Most boilers need a filling loop or alternative built-in component in order to keep our boilers pressure at the optimum level of 1-1.5 bar.

If you don’t re-pressurise a boiler several problems can occur. If the pressure is low, there could be a leak anywhere in your property’s pipework, a leaky valve or even air in your system.

Meanwhile, high pressure could be easily fixed by simply bleeding your radiators (if you have a UFH system this guide on how to bleed your underfloor heating will be helpful too), but if left for a long period of time, can cause serious problems.

Without a filling loop, there would be no way to refill your system safely after bleeding your radiators.

Similarly, if there was a leak there would be no way to top the system up once the leak is located and fixed. 

Where is a filling loop?

Depending on your heating system, filling loops can look different and go in different places. Generally, you are looking for a flexible, stainless steel, braided length of hose.

But sometimes you can be looking for a smaller rigid loop. Quite often these are left with you by your boiler installer to keep somewhere safe, but with newer boilers, these can also be built in.

External Filling Loops

This will be a braided hose which goes from the incoming cold pipe onto the central heating return pipe. When installing external filling loops it is important to check what valves the loop has and ensure you are installing it the right way round, to avoid any water flowing the wrong way. If you’re unsure on the valve set up of your boiler, call a PHS engineer before doing any work.

Internal Filling Loops

Some boilers, namely combi boilers, actually have the filling loop integrated into the system. Mechanically, these function in the same way as external filling loops, but they can be found beneath the boiler in the form of a coloured lever.

Why is my filling loop not working?

As filling loops are quite cheap, we usually recommend you replace your filling loop if it doesn’t seem to be doing the job.

There may be problems with internal valves which are difficult to fix individually and it is much quicker to try a new filling loop first.

If this fails, then you may need to replace something internally in your boiler and this could be a more complex job.

Give a member of the PHS team a call today so we can help you locate and diagnose the problem.

How do I change my filling loop on a combi boiler?

Combi boilers tend to have flexible filling loops. These look like short, braided pieces of stainless steel and they go into two taps that are beneath your boiler and connected to copper pipes.

They are called flexible because of the way it is bendable/hose-like, rather than short and solid. In the event that you need to replace a filling loop that is built onto the side or beneath your boiler, you must follow three simple steps.

  1. Always ensure both taps are off.
  2. Undo the nuts that hold the hose on.
  3. Replace with the new hose and do the nuts up.

Can I re-pressurise a Baxi Boiler with and without a filling loop?

If the pressure gets too low in your boiler it is easy to re-pressurise the system and is something we recommend homeowners knowing anyway.

We are using the Baxi Boiler as an example as it has 3 main ways of re-pressurising depending on your model. Instructions do vary depending on the type of boiler you have and also the type of filling loop it is fitted with.

Sometimes they do not have a filling loop at all. Follow our step by step instructions below for both rigid and flexible filling loops and for boilers without filling loops at all:

Re-pressurising a Baxi Boiler with a flexible filling loop

  1. Locate the taps beneath the boiler.
  2. Make sure the right-hand tap is fully closed before beginning work.
  3. Locate the taps beneath the boiler and unscrew the two end caps.
  4. Locate your filling loop, which will be a flexible braided piece of hose.
  5. Connect the filling loop to each end of the tap, tightening them by hand.
  6. Open the first tap fully.
  7. Locate the pressure gauge.
  8. Slowly open the second tap.
  9. You will hear the sound of running water and you will see the pressure rise on the pressure gauge.
  10. When the gauge shows a pressure between 1-1.5 bar you should close the second tap fully first, followed by fully closing the first tap.
  11. Unscrew the left-hand nut of the hose. Water may come out of the hose.
  12. Unscrew the other end of the hose and remove the flexible filling loop.
  13. Replace both end caps to the taps.

Re-pressurising a Baxi Boiler with a rigid filling loop

  1. Locate the taps beneath the boiler and unscrew the two end caps.
  2. Check the rubber washers are secure on your rigid filling loop.
  3. Position the filling loop between the two taps and loosely screw on the first nut.
  4. Screw the second nut as tight as you can with your hand.
  5. Tighten up the first nut.
  6. Open the first tap fully.
  7. Locate the pressure gauge.
  8. Slowly open the second tap.
  10. When the gauge shows a pressure between 1-1.5 bar you should close the second tap fully first, followed by the first tap.
  11. Unscrew both nuts again and remove the filling loop from between the two taps.
  12. Some water may come out at this stage, but it should be a small amount.
  13. Replace both end caps and leave the boiler as you found it!

Re-pressurising a Baxi Boiler with the permanent filling link instead of a filling loop

This will show an error code E118 on your boiler and it means the water pressure is too low.

Beneath the boiler, you should be able to locate a green lever. Pull the lever until the pressure gauge shows 1-1.5 bar, when it reaches anywhere in this range, you can release the lever.

Do not let the pressure increase above this figure or you could cause your boiler/water system irreversible damage.

It is not uncommon for water to drip from the air break when you increase a system’s pressure, just clean this up with a cloth and keep an eye on the area to ensure the flow of water from there is not consistent. As a final check, look round your radiators to ensure they are not leaking any water.

If while you are re-pressurising your boiler, you exceed 2.5 bar of pressure, you must contact a service engineer, immediately. 

How do I disconnect a filling loop?

Similarly to replacing a filling loop, you must follow simple instructions.

Ensure the taps are fully off before starting, remove the nuts and do not turn the taps back on until either you have put end caps over the taps or you have replaced the filling loop – depending on what your boiler requires.

How to open a filling loop valve?

Sometimes a filling loop valve can get stuck and it can be quite stiff to open.

Unless you are very sure of the part you are working with, we recommend you never force anything when it comes to boilers and you call a professional instead.

There have been many instances when people have thought they are pulling a valve and in fact, they are pulling something completely different and it results in more damage and more cost for the customer when the expert is finally called.

How do I fit a combi straight filling loop?

For instructions on a combi straight filling loop please see instructions on flexible filling loops earlier in this guide. You can apply the same method.

If you are struggling to locate your filling loop or you are not sure if the problem is to do with the filling loop, call PHS immediately.


Why should you not just IGNORE it if your boiler loses pressure regularly?

Topping up your boiler may seem like a quick fix to pressure problems, but adding in water from the mains all the time will cause internal problems, such as the build-up of sludge.

The oxygen present in mains water can also speed up corrosion. Getting a problem fixed is the best long term solution.

Not to mention, if there is a leak somewhere it could cause your entire property severe damage in the long term, so it really is best not to ignore the signs.

Boiler Filling Loop: Installation, Repair & Replacement

Still stuck or unsure about your filling loop? Or are you still have pressure issues with your boiler despite re-pressurising it several times? Maybe it just isn’t fixing the issue at hand?

If you’re in London, Essex, Kent, Suffolk or Hertfordshire call PHS today and we can get an expert out to you as soon as possible to help find you a solution.