Do all boilers have a filling loop?

A filling loop serves a vital role in boiler systems – repressurising. Although filling and repressurising can be achieved through other means. Boilers can be seriously damaged by loss of pressure or having too much pressure. As such, a filling loop that has lost functionality should be addressed quickly to avoid wider heating issues. 

In this blog, we’ll cover how filling loops work, what they look like, and the boilers they can be found in. 

What does a filling loop do?

A filling loop is a boiler component that is designed to provide a temporary connection to mains water supply. It allows the system to be refilled and repressurised. This is important to maintain boiler efficiency and avoid issues affecting your central system.  

It’s often recommended to repressurise a boiler when: 

• Your radiators have recently been bled. 
• Soon after your boiler pressure drops below one bar. 
• The boiler has a small leak. 
• Your boiler has a fault that’s affecting its functionality. 

The filling loop typically consists of a length of solid or flexible piping, ending in two valves. One of these will be a stop valve and the other a double check valve. The former controls the flow of water as it passes through the loop and into the boiler. The latter gets its name from the fact it contains two spring-loaded mechanisms. These have an anti-backflow function – preventing water from flowing from the boiler into the mains.

Types of filling loop

While all boiler filling loops are intended to serve the same purpose, they can come in a wide variety of designs and models. There are also two different types, which vary based on how they feed into the boiler. Certain types of boilers will often contain one type of filling loop, which is good to know when it comes to repairing or replacing the part. 

External

External filling loops consist of a braided hose with compression fittings at each end. These are the stop valve and double check valve respectively. The former connects to the boiler at its designated terminal, and the latter connects to the mains water line. External loops are used when the mains connection port is located far away from the boiler. As a result, the length of an external filling loop will vary depending on the distance it has to reach. 

Internal

This type follows the same appearance and features as an external filling loop, except it is fully integrated with the boiler system. Due to this, many internal filling loops will be shorter and forgo braiding on the hose.  

Both internal and external filling loops can be purchased separately. External filling loops can usually be found in decent hardware shops, although the choice may be limited. On the other hand, internal designs are often harder to find as they have to sit within the boiler’s housing. NBS has a large range of filling loops of all types and designs. 

Which type of boilers have a filling loop?

Not all boilers have a filling loop, such as an old Worcester boiler. These older boilers sometimes have a hot water cylinder, which is a standalone tank that is designed for pressurisation. However, most boilers produced within the last 5-10 years will have a filling loop. The nature of the parts will vary across different boiler types and models. As a result, it’s important to know when sourcing boiler spares. 

Combi boiler

Most combi boilers have internal filling loops that are fully in-built behind the boiler cover. In some cases, these boilers will have an easy filling link instead. This commonly appears as a single green or blue lever on the underside of the system. Pulling it down causes water to flow into the boiler. 

Regular domestic boiler 

Household boilers are often positioned close to water pipes so they can access the mains supply. Regardless of whether the system has an internal or external filling loop, it can usually be found on the underside of the boiler. 

Centralised boiler plant

In apartment buildings there’s usually a main boiler that supplies hot water to different flats within the complex. Here, filling loops are used to fill and pressurise Heat Interface Units (HIUs). 

Using a boiler filling loop

Repressurising your boiler with a filling loop can be done without a heating engineer. However, this should be done with care as boiler systems can be damaged by pressure that’s too high. The pressure inside your boiler refers to the balance of air and water in the system. Some components are not equipped to function when there’s a sizable imbalance. Don’t worry though, we’ve put together quick guides for using both internal and external filling loops. 

Internal filling loop

• Turn off the electricity and the boiler. 
• Open the hatch and insert the filling key. 
• Activate the system following its normal process (this will vary between boiler brands) 
• Tighten the knob clockwise and turn the key so it’s in the open padlock position. 
• The pressure gauge should show the level increasing.  
• When it reaches the range between 1-1.5 bars, remove the key.

External filling loop

• Turn off the boiler and remove the isolation valve cap. 
• Fit the external filling loop (this will vary depending on how the boiler is set up) and tighten both ends. Make sure both valves are in the correct position. 
• Turn the lever slowly to allow water into the boiler. 
• Check that the pressure gauge is in the 1-1.5 bar range, after which, turn the lever to the closed position. 
• Disconnect the external filling loop and clean up any spillage.

Expertly remanufactured boiler parts

If you’re looking for a supplier to help with the repair of your boiler filling loop, look no further than NBS. We stock a large range of OEM parts and widely compatible boiler spares that can be used in domestic heating systems. With us, you’ll have a quick and effective solution to your boiler repair needs. Contact us today.