What is a Boiler Filling Loop? How to Fix and Replace Guide

Last updated on October 15th, 2024

A boiler filling loop is needed on all pressurised central heating systems. If you have a combi boiler or system boiler you should have a filling loop.

The filling loop is used to repressurise the boiler by connecting the cold water to the heating system via a flexible or solid pipe and two valves. I have been servicing boilers and central heating systems for over a decade and have had many problems with filling loops.

Combi Boiler Filling Loop

A modern combi boiler filling loop will usually be a built-in filling loop on the bottom of the boiler with two handles to let the water into the heating system.

If it has an easy-fill filling link then this will be a single blue or green lever that is pulled to let the water in and released to stop.

If it does not have one built-in then a combi boiler filling loop will typically be right under the boiler on the pipes. It could also be hidden away in a kitchen cupboard somewhere or in a different room to the combi boiler but very rarely.

Central Heating Filling Loop

A central heating filling loop is an external filling loop somewhere on the central heating system. This is usually in the hot water cylinder cupboard as system boilers do not have a cold water supply at the boiler as combis do.

A central heating filling loop will be a flexible braided hose with a valve at either end. They should have a non-return valve at one end and one or two isolating valves.

Filling Loop Off Position

The filling loop in the off position, like all isolation valves, is across the pipe. The open position of a filling loop is in line with the pipe so the off position is always directly across the pipe on quarter-turn valves.

Boiler Filling Loop Not Working

Coming across a boiler filling loop not working on a job is quite common. They can become damaged or not close fully causing the boiler pressure to rise too high and leaving you needing to reduce the boiler pressure.

There are a few different reasons for the filling loop not working:

• Plastic handle snapped: If the plastic handle on the filling loop valve is turning but not letting the water through, the plastic underneath is probably snapped or damaged.
  • You can remove the handle with a screwdriver and replace it, or use some plumbers grips or pliers to turn the metal part of the valve under the handle to open and close it.
• Filling loop valve stuck: With a stuck valve, you must try forcing it open with a pair of pliers. Remove the plastic handle first as it will snap if you don’t.
• Water pressure too low: Very rarely I will come across a house where the pressure isn’t very high in the house so it doesn’t fill the boiler pressure to where you want.
  • If a house only has 0.5 bar of pressure and you want the boiler pressure to be at 1 bar, then this is impossible.
  • To fix this you will need to find out why the pressure of the house is so low and fix it.
  • There could be a pressure-reducing valve somewhere or you might need to upgrade the water main pipe from the street to the house.

No Filling Loop on Boiler

If there is no filling loop on the boiler then you probably need a key. Older Worcester boilers used to annoyingly use a big stupid plastic key to repressurise the boiler. Thankfully they don’t do that anymore as Worcester Bosch (and Baxi) have started using easy-fill filling links on their new boilers which is great.

If you have a pressurised heating system then you must have a filling loop to repressurise the boiler. If there is no filling loop on your boiler it must be somewhere else on the system. This could be anywhere. You will need to get one fitted if you can’t find one anywhere.

How to Disconnect Filling Loop

To disconnect a filling loop you must unscrew the ends of the hose from the filling loop valves. You will probably need to use some plumbers grips to start the turning anti-clockwise then unscrew it with your hands.

Make sure both filling loop valves are fully closed before doing this and screw on blanking caps if you have them. There will probably be a little bit of water in the filling loop when it is disconnected so put an old towel down.

How to Change Filling Loop Valve

Learning to change a filling loop valve depends on which valve you want to change.

Heating pipe valve

The easiest way to change the filling loop valve on the heating pipe:

1. Disconnect the filling loop from the cold water valve and leave it connected to the heating pipe valve.
2. Point the filling loop hose into a bucket and open the heating pipe valve to let the water out of the heating system.
3. When the water stops coming out, grip the valve with some plumbers’ grips and turn the nut with an adjustable spanner to remove it from the valve.
4. Fit the new valve and tighten it up with an adjustable spanner and grips.
5. Repressurise the boiler and bleed the radiators.

If the valve is a non-return valve or doesn’t open to let the water out, you can crack the nut on the valve to drain it into the bucket, or you can drain the central heating system from a drain valve elsewhere on the system.

Cold water valve

The easiest way to change the cold water filling loop valve:

1. Turn off the cold water mains at the stopcock.
2. Open a cold water tap to release the pressure.
3. Disconnect the filling loop from the cold water filling loop valve.
4. Place a bucket under the cold water valve to catch any water.
5. Grip the valve with some grips and turn the nut with an adjustable spanner.
6. Fit the new valve and tighten up the nut.
7. Connect the filling loop.
8. Repressurise the boiler and bleed the radiators.

Conclusion

Boiler filling loops are necessary on pressurised heating systems. They can be hard to find sometimes as they can be fitted in some strange places but mostly they are on or near the boiler. They can be a headache when they are not working.

Replacing them is quite easy most of the time but can also be a nightmare. Knowing how to bleed a radiator will certainly help as you will have to drop the pressure on the heating system.

If you have a boiler filling loop leaking, you will probably have to replace it. If it's only leaking when you open the valves you can leave it if you want.

Feel free to ask any questions in the comments below, and I’ll do my best to help. If you found this post helpful, please consider sharing it.

FAQs

What does a filling loop look like?

A standard filling loop looks like a braided hose with a valve at either end. There will be a non-return valve at one end and one or two isolating valves to let the water into the heating system.

On newer combi boilers the filling loop will be built in on the bottom of the boiler and is usually a small copper pipe with a lever at either end. If the combi boiler has a newer easy-fill filling link, then that will look like a brass and black plastic unit with a single green or blue handle.

What pipes does a filling loop go on?

A filling loop needs to go on the mains cold water pipe and a central heating return pipe. It’s best to be on the return pipe as this pipe is slightly colder than the flow but really they can go on any central heating pipe, flow or return anywhere in the property.

What is an R24 filling loop?

An R24 filling loop has a double-check valve to stop the dirty heating system water from going back into the cold water supply and two isolating valves for letting water into the central heating system. These valves are connected with a braided flexible hose and are pretty common.