whats a filling loop to top up your boiler pressure
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What is a filling loop?

Do you want to know exactly what a filling loop valve is?

Your filling loop is commonly used to top up your boiler's system water pressure, so you may want to know where it is and how to use this.

Whatever the case is, we have you covered with a detailed look at filling loops and their valves in this easy guide.

We've also included a very easy to follow and quick video guide from our boiler expert.

so, what is a filling loop? we hear the term quite a lot, you may of heard about topping up your boiler pressure and to find the filling loop,

A filling loop is a device that is used to repressurise your boiler and top up the water when you drain or bleed your system. They usually come as an additional attachment with the boiler.

On all pressurised systems you will see a filling loop, they can be next to an unvented cylinder or under a boiler. usually a silver hose shaped like a loop or sometimes directly installed to the boiler which go from the cold main to the return side of the heating pipework.


watch this video and see exactly what is a filling loop


Different types of filling loops

external filling loop

Generally, this type of filling loop features a braided hose that comes with compression fittings on either end.

There’ll be a minimum of one-stop valve present on this type of filling loop. The stop valve is a means of controlling water flow through the loop.

Beyond that, you have the double check valve which gets its name from the fact that it comprises two spring-loaded mechanisms. This valve with these two anti-backflow mechanisms is used to stop water back flowing into the mains.

When an external boiler filling loop is being fitted, it must be installed in the correct way. Failure to do so will stop the system from filling up as it should.

Internal Filling Loops
we have the internal version of boiler filling loops. These filling loops are less common. These filling loops have a more complex structure but work in essentially the same way.

Internal filling loops and their valves sometimes appear on combi boilers, Combi boilers come with a direct mains connection meaning that the filling loop might tee off from this connection.

all property owners should educate themselves to know what they look like and where to find them.


most properties now have a combi boiler pressurised system, when or if the pressure becomes low on the boiler it will require topping up, this can be done by the home owner and will save money on ringing your local heating engineer.

regular topping up and filling of your combi boiler pressure can cause bad corrosion

Of course if your boiler is losing pressure more than once every three months then you should call your local heating engineer as there may be an underlined issue which needs to be rectified to make sure your combi boiler lasts as long as possible and don't breakdown.

Topping up your boiler pressure more than twice per year can have catastrophic results for your boiler, did you know that every time you top up your combi boiler you are adding fresh water to the system which , when mixed with the dissimilar metals on your system can cause severe system corrosion, this corrosion blocks up the boiler with rust and the inside of your radiators can rust, flake away and get blocked up.

This means you may need to replace your radiators and boiler far sooner than you would expect, these pictures below highlight severe corrosion caused by over filling of your combi boiler.

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Adam Dilger
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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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