Boiler Filling Loop: What It Is and How to Use It

A central heating system is one of the basic and most important mechanisms of UK homes. A boiler is the main part of this entire system, which produces hot water for heating the rooms and also for direct supply in hot water taps and showers. So, there would hardly be any homes in the UK without a boiler. 

Whether it is a gas-fired or electric boiler, the consistency in supply of hot water is largely dependent on one basic phenomenon: the correct water pressure. Modern boilers in the UK are designed to operate continuously, but they are also equipped with basic safety features. One of these safety mechanisms is directly associated with correct water pressure. If, for any reason, pressure drops below a preset level, the safety mechanism is automatically triggered, forcing the boiler to shut down. The instant solution to low pressure in a boiler is to repressurize it. Here, the "filling loop" comes into play, a simple yet essential part of the central heating system.

What a filling loop is, how it works, and how to use it correctly and safely are some of the most common questions. Keep reading this blog to know everything about a filling loop.     

Boiler Water Pressure – An Overview 

Before understanding the role of a filling loop in the boilers, it is important to get an awareness of what is meant by boiler water pressure and why it is so important. 

Boiler Water Pressure, commonly referred to as simply "Boiler Pressure," is a measurement of water circulating within the sealed central heating system, from boiler to radiators and back. It is sometimes confused with the term "Water Pressure", which is not correct. The term Water Pressure refers to the pressure of cold water directly coming from the main water supply and has nothing to do with the "Boiler Pressure". So, be clear about these two different terms. 

In simple words, Boiler Water Pressure or Boiler Pressure is the amount of water available in the heating system. Ideally, when the boiler is cold, the pressure should be around 1 to 1.5 bars, which is typically displayed on the boiler's front panel, either analog or digital. When the boiler is functional and producing hot water, this pressure increases as the heat expands the water. In this state, boiler pressure should range between 1.5 to 2.00 bars. 

As a result of continuous operation or due to some fault within the central heating system, including the boiler itself, the pressure often drops below the required level. In this scenario, there will either be no heating or inconsistent or intermittent heating. It may also result in no hot water in taps and showers. When it happens, the boiler would most likely be turned off automatically by its internal safety mechanism to avoid physical damage. On the other hand, when boiler pressure is too high, the safety mechanism comes into play, and the Pressure Release Valve (PRV) releases extra water to lower the Boiler Pressure.

The boiler is designed to work efficiently at the correct pressure. The correct pressure is important for the boilers to ensure:

• Perfect and efficient heating.
• Perfect circulation of hot water across the heating system.
• Lesser possibilities of developing errors or faults in the boilers. 

In any of the above scenarios, the first thing you need to note is the boiler pressure. If the boiler pressure reads below the above-mentioned ranges, it is time to repressurize it, using the Filling Loop. 

What Causes the Boiler Pressure to Drop? 

The role of a filling loop starts when the boiler pressure drops below the workable level, i.e., below 0.5 bar. There might be several reasons behind a low boiler pressure, which may include the following:

• Bleeding of Radiators. It is quite common to release air from the radiators. Although it does not necessarily lower the boiler pressure, it may sometimes. 
• Boiler pressure often drops due to unnoticed small leaks within the central heating system or even within the boiler itself. It may appear as small cracks in the pipes or as continuous dripping from the radiator valves. Sometimes, it even happens in the expansion vessel.   
• Poor or inadequate boiler maintenance may also result in low boiler pressure. 
• A faulty pressure release valve may also cause a considerable drop in boiler pressure. Sometimes it gets stuck in an open state, causing continuous water leakage from the system. 
• Even routine or natural evaporation of boiling water can lower the boiler pressure. 

When the pressure drops below the desired level, typically below 0.5 bar, the boiler often stops working and shows an error code on the display panel. In some cases, it will not ignite even if you try to do so, because the system safety mechanism locks it out to avoid further damage. 

In this scenario, the role of filling the loop starts.

The Filling Loop

The filling loop can either be external or internal. The purpose and function of this filling loop is to repressurize the boiler whenever the pressure drops below the desired level:

• External Filling Loop - In most of the modern boilers, such as a Combi Boiler, the filling loop is just a simple and small flexible interwoven hose, often referred to as an “external filling loop". It is usually available on the front side of the boilers, on the lower side, along the main water supply (cold water) pipe. An external filling loop is usually made up of braided metal. It has two isolation valves, often of red, black, or blue color. Per the UK Water Regulations requirements, sometimes it is fitted with a double check valve. 
• Internal Filling Loop – Some of the most modern boilers come with an internal filling loop. Contrary to the internal filling loop, it is a built-in filling key, instead of a flexible hose. In some boilers, it is in the form of a permanent auto-filling mechanism. These are also fitted on the front but below the main casing of the boiler. 

In any case, the role of a filling loop is to top up the boiler pressure. 

How to Repressurize the Boiler With an External Filling Loop?

Here is a step-by-step process of repressurizing the boiler, with the help of a filling loop:

• Turn off the Boiler – First of all, check the boiler pressure. If it has dropped below the desired level, typically below 0.5 bar, it needs repressurizing. If that is the case, switch off the boiler and wait a few minutes for it to cool down. 
• Locate the Filling Loop and the Two Valves – The filling loop (typically the external one) is often located beneath the main casing, along the main water pipes. It is fitted with a valve on each side of the filling loop. Each valve has a lever that rotates to 90 degrees. When aligned at a right angle to the water pipes, it is in the off position. To turn it on, you will need to rotate the lever to align it along the direction of the water pipe. 
• Turn Both Valves On – Turn both the valves slowly to the "on" position, one by one. When both are correctly opened, you can clearly hear water dropping inside the system. While the water continues to drop, keep watching the pressure gauge. 
• Turn the Valves Off – When the pressure is between 1.3 and 1.5 bar, turn off both valves. Turn them off, one after the other. Make sure that the pressure does not exceed 1.5 bar at this stage. Turn off both valves well before the pressure exceeds 1.5 bar. 
• Restart the Boiler – Once the pressure is topped up, wait for a few minutes before turning on the boiler. After that, ignite the boiler. If everything has been done perfectly, the ignition should happen just normally, and the boiler should start heating the water. 
• Remove the Filling Loop – When the pressure is topped up, remove the filling loop and place it back in its respective place. 

How to Repressurize the Boiler With an Internal Filling Loop?

If a boiler is fitted with an internal filling loop, repressurizing the boiler is quite simple, though some steps may vary by make and model:

• Insert the filling key.
• Rotate the key to the unlocked position.
• Turn on the valve.
• Wait for the pressure to top up to the desired level.
• Turn off the valve
• Remove the key.

Conclusion

The boilers are designed to work properly only when the boiler pressure is within a certain limit. When the boiler is in a cold state, the pressure must be between 1 and 1.5 bars, whereas it may rise up to 2 bars when it starts producing hot water. When the pressure drops below 1 bar, for any reason, the boiler often shuts down through an automated safety mechanism to prevent any physical damage to internal components. At this stage, the boiler needs repressurizing to 1.5 bars for smooth operation. This can be done easily by using the filling loop, either the internal or external filling loop, depending on the make and model of the boiler.