Boiler filling loop: the 2 different kinds and how to use them

The boiler filling loop is a boiler component which provides a temporary connection to the mains in order to fill and pressurise the central heating system with water. It also allows the system to be topped up where necessary, for example, after bleeding radiators. Regularly topping up the system usually indicates a leak, and doing so will damage the system in the long-term.

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.

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.

Internal filling loops

Some boilers, such as the 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.

How to use the filling loop

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

  1. Firstly, identify the pressure gauge on your boiler. Most combi and system boilers require a pressure of 1 – 1.5 bar in order to operate.
  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.
  3. When the needle indicates the target pressure, close the filling loop.
  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. Congratulations – you 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.

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