What is a Central Heating Filling Loop?

If their marketing material is to be believed, you’ll use or come into contact with a 3M product 100 times in a single day, without even knowing it. Those guys make everything from post it notes, to traffic signs, to PPE, to domestic cleaning products. Similarly, if you’ve ever had to refill the water in your boiler, there’s another item which you will have used without knowing it - the central heating filling loop.

Essential Equipment

While not particularly flashy or exotic, the central heating filling loop is an essential part of your central heating system, which you literally can’t be without.

Your filling loop is what connects the boiler to the incoming supply of water and can come in the form of a temporary hose fitting, or, as is often the case with combi boilers, be integrated into the boiler itself.

Whichever is the case, the filling loop will have a valve on either end, enabling you to control flow into the boiler.

Types of Filling Loop

There are several types of filling loop.

Braided Hose

This consists of a flexible braided hose, akin to those used to connect taps and other fittings to water supplies throughout the household.

It has a valve at each end for easy connection to the supply and the boiler.

These are one of the more common types of filling loop used here in the UK.




As mentioned earlier, some modern boilers, combi types in particular, have an integrated filling loop.

These are usually accessed through a dedicated filling loop port or connection on the boiler, and often will not require additional loops, hoses or valves.

The end connected to the mains is typically opened or closed using a dedicated key.


These are usually a flexible hose, but do not have the valves as traditionally seen - and instead use a keyless connection at either end.


These filling loops use a tee-piece fitting that is installed into the fabric of the central heating pipework.

The valves used to turn on and off the water supply are incorporated into the tee-piece at either end.

These allow easy connection of a hose in order to fill or top up the central heating system.


Some filling loops combine different types of the connections listed above, for example a braided hose and a keyless connection, or tee-piece and braided hose.

These combination filling loops may be utilised in various boiler or system setups.


How To Use Your Filling Loop

From time to time, you may need to top up your central heating system, for example if you notice that the water pressure has dropped, resulting in stone cold radiators.

All you’re required to do is essentially turn on a tap – however, it’s important to observe certain precautions to avoid damaging your central heating system.

Here’s how to use your filling loop.

Locate the filling loop. It’s usually connected to the central heating system near the boiler or otherwise in a generally accessible area. The most common type will be a flexible braided hose with a valve at each end.
Confirm the boiler pressure. Before using the filling loop, observe the current water pressure as shown on the pressure gauge on your boiler, or the pressure indicator located somewhere on the system. Most pressure gauges have an obvious red zone and green range. The ideal pressure is normally between 1 to 1.5 bar.
Open the valves. Start with the one connected to the mains, then the valve connected to your system. Gradually turn them (anti-clockwise) so as to not shock or hammer the pipework.
Watch the pressure. You’ll likely hear the whoosh of water as it starts to fill the pipework. Keep an eye on the pressure gauge, you don’t want to over pressurise the system.
Close the isolating valves. Close the one at the system end of the filling loop first. Make sure you close off the valve to the supply too.
Leak check. Once you’ve finished, check for any leaks. Look around the filling loop and its connections. If you note any leaks, call a boiler engineer as soon as possible.



Central Heating Filling Loop Precautions

As we mentioned earlier, using the filling loop is essentially a case of opening and closing a pair of taps.

When it comes to plumbing however, even the simplest task can end in issues or headaches if the basics are not taken care of.

Improper use of the central heating filling loop can cause a variety of issues and even damage your boiler, for example:

Over Pressurisation

Remember when we said that you should fill your boiler to about 1 to 1.5 bar of pressure?

This is for good reason.

While the “green” range on your dial may go as high as 2 or even 3 bar, this is to account for the rise in pressure as the water heats up (it expands with heat, thus increasing pressure).

If you over-pressurise the system when it is cold, it will cause dangerous builds up of pressure that can cause parts of the boiler to rupture or burst.

Under Pressurisation

If the pressure is too low, the boiler may work inefficiently or shut down altogether.

The likelihood of this happening is low as chances are you’ll be using the filling loop due to low water pressure in the first palace but…never say never.

Pressure Changes

Fast or frequent changes in water pressure as a result of regular use of the filling loop can cause additional stress on the boiler.

This can lead to early wear and tear, diminished efficiency, and failure of system components.



No, not a Welshmans favourite vegetable but the unfortunate side effect of improper use of the central heating filling loop.

The filling loop may be poorly installed, or the valves themselves may leak.

This can cause damage to surrounding components and lead to corrosion and damage over time.

Reduced Efficiency

If improperly pressurised, your central heating system may operate inefficiently, leaving you with a colder house, increased energy consumption, and…higher energy bills.

Safety Risks

If improperly used, your filling loop may present safety risks which start out as mere water leaks.

However these can result in property damage around the boiler, or damage to electrical systems.

Avoiding these issues is simple…use your filling loop wisely!


Loopy Last Word

What’s a central heating filling loop? Basically a bendy pipe with a tap on each end. Use of our flexible friend is straightforward, however, don’t be shy if the range of issues resulting from improper use is driving you loopy…just get in touch and we’ll get your boiler back onto the straight and narrow in no time.

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Adam Dilger
Managing Director
We offer ground braking reliable heating solutions across the Northwest. With 20 years experience our engineers install, service and maintain your central heating systems. Connect with Adam on LinkedIn >>
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