Visit our office at Longacres Garden Center


A boiler filling loop is a component which provides a temporary connection from the mains water in order to fill and pressurise the central heating system.

It also allows the system to be topped up where necessary, for example, after bleeding radiators.

The filling loop itself consists of either a rigid section of pipe, or a braided hose. A filling loop can be either part of the boiler itself, or it can be fitted close by. Filling loops usually have a stop valve at one end and a double-check valve on the other.

How to use a filling loop

If the pressure is too low or keeps dropping, repressurising your boiler and central heating system is very easy:

1.     Firstly, find and identify the pressure gauge on your boiler. Most combi and system boilers require a pressure of 1 – 1.5 bar in order to operate correctly.

2.     Open the valve or valves on the filling loop. You should hear the sound of rushing water – the system is filling up. You should also see the needle on the pressure gauge rise. Some of the newer generations will only have a digital read out on its display, always consult your boilers operation manual.

3.     When the needle or digital display indicates the desired pressure of 1-1.5bar, close the filling loop on both ends.

4.     Use the controls on the front panel of your boiler to clear any low-pressure fault codes. This may or may not be necessary, depending on the make and model of your boiler.

5.     These faults codes could include but are not limited too; E110 E117 E118 E119 EA227 LC L2 FF F1 F2 F3 F4

6.     Once you’ve completed step 1-4– you will have heating and hot water again!

Why do I keep having to top up my boiler?

If you keep having to use the filling loop to top up your boiler, you probably have a leak somewhere. The leak will either be on your central heating (one of the joints or radiators), or one of the boiler’s components. Faulty expansion vessels and pressure release valves are likely suspects inside the boiler.

Either way, if your boiler pressure keeps dropping, you should get the leak fixed pronto. Regularly topping up the system with oxygen-rich mains water will encourage internal corrosion and the build-up of sludge. If left untreated, this will gradually ruin your central system from the inside out, destroying the boiler and causing your radiators to develop leaks.

Regularly topping up the system usually indicates a leak, and doing so will damage the system in the long-term.

Filling loop overview

In a sealed central heating system with a combi or system boiler, there is no feed and expansion cistern in the loft. Instead, the system is supplied by the mains.

The requirement that the connection is temporary is because Water Regulations prohibit the backflow of water into the mains, which would risk contaminating it. The risk of contamination from a central heating system is even greater because of the corrosion and sludge which can build up inside a central heating system, and/or the presence of anti-corrosion and biocidal chemicals. A permanent connection to a central heating system is therefore not permitted, and wouldn’t be possible anyway due to the fact that cold mains water is likely to be at a higher pressure than the pressure of, for example, the central heating system of a standard combi boiler when cold.

External filling loops

A generic external filling loop simply consists of a length of braided hose with compression fittings at each end. There will be at least one stop valve which is used to control the flow of water. There will also be a double check valve to prevent the backflow of water into the mains. A double check valve is so called because it contains not one but two internal spring-loaded anti-backflow mechanisms. When installing an external filling loop, care must be taken to ensure that it is fitted the right way round. Otherwise, valve will stop the system from filling up. See photo no.1

Internal filling loops

Some boilers, such as the Worcester Bosch Ideal Viessmann ideal Logic Combi range, feature integrated filling loops. They may look more complicated, but still function on exactly the same principle. Since all combi boilers have a direct connection to the mains anyway (mains water is heated by the boiler for the hot taps), the filling loop may tee off from this pipe.  See photo 2 internal filling loop

How to keep your boiler from breaking down in the future:

Regular as by way of an annual boiler service is the best way to avoid potentially costly and even dangerous boiler faults. In fact, you should have all gas appliances in your home safely checked and serviced on an annual basis by a Gas Safe registered engineer. See our boiler service price comparison here.

Picture of RBHM


Leave a Replay

About RBHM

Based in Longacres Bagshot, RBHM is the premium central heating and boiler service company in South England

Recent Posts

Follow Us

Sign up for our Newsletter

Instant Boiler Quote

Boiler Quote

Installation Details



Typically replies within an hour

I will be back soon

Hey there 👋
How can we help?