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Guide To Boiler Filling Loops


A boiler filling loop is a device for repressurising and filling heating systems. It works by establishing a temporary connection between the main water system and your boiler system. Most of the time, boiler filling loops are flexible, stainless steel, and about the length of a hose.

Just as the name suggests, “boiler filling” involves topping the boiler with mains cold water to increase its pressure.

When do I need to repressurise my boiler?

There are several instances that will require you to repressurise your boiler. Nonetheless, low pressure is the most common reason.

  • If your boiler pressure is below 1 bar.
  • After bleeding your boiler.
  • After sealing a leak.
  • After solving a boiler fault.

As hinted earlier, the knowledge of boiler filling loops is necessary for maintaining boiler pressure. If the pressure is too low, it might cause permanent damage to your boiler unit.

How to check for low boiler pressure

We normally advise our clients to first check their boiler’s pressure whenever they’re experiencing difficulties with their unit. Specifically, you’ll have to locate and read your boiler’s pressure gauge.

Indeed, we have different types of boilers by different manufacturers. Therefore, there are many types of boiler gauges. Nonetheless, we have two main types:

  • Hydraulic gauge: It’s the most common type (especially in old houses). It consists of a dial behind a glass or plastic screen. Its scale usually has two red sections that indicate extreme pressures (Too high and too low). If the arrow is the red zone on the left-hand side, then your boiler’s pressure is too low. The opposite is true for high pressure.
  • Digital gauge: They are slowly becoming more prominent (especially with newer models). Most of the time, the gauge will display a warning sign for low and high pressure.

What is the ideal pressure for my boiler?

To get it 100% right, you’ll have to consult your boiler’s user manual. Nonetheless, most manufacturers have stated 1.4 bar as the optimum pressure for their boilers. Anything between one and two bars is alright.

If your boiler’s pressure is below 1 bar then its pressure is too low. Likewise, something is wrong if the pressure is above 2.75 bars. That is dangerously high.

Reasons for low boiler pressure

Bleeding radiators

This mostly occurs after the “bleeding process.”

“Bleeding” refers to a procedure where some of the boiler’s air is let free. It’s usually done to reduce the boiler’s pressure. When done incorrectly, it might lead to low pressure.


Your boiler uses pipes to pass water and air. If any of the pipes are leaking, it might cause a pressure drop.

To locate the leaks, you’ll have to inspect the pipes and look for damp patches, especially near the radiators. Nevertheless, we advise you not to mess with the boiler unit. To avoid creating more problems, we advise you to leave it to a professional engineer.

Reasons for high boiler pressure

High boiler pressure is mostly caused when the heating system is overfilled with water. Additionally, it can also be a result of faulty pressure-releasing valves.

High-pressure problems are more difficult and dangerous to solve than low-pressure issues. We advise you to leave it to a certified heat engineer.

How to use boiler filling loops

Your boiler’s pressure has now dropped below “healthy” levels. As a result, you might have noticed that your boiler isn’t as efficient as it’s supposed to be. In the worst-case scenario, your boiler might even fail.

Fortunately, you don’t need expert technical skills to use boiler filling loops. They are easy to understand and use. On top of that, you’ll be relieved to know that a boiler filling loop is all you need for this operation.

The first step is to locate your unit’s boiler filling loop. If your boiler was recently installed, then its filling tool might be attached with blanking caps.

To be sure, we advise you to find your boiler’s user guide. Most manufacturers provide an image with all the parts labelled. In some cases, the technicians usually give the boiler filling loops to the owners.

Can’t find it? Then you might have to buy a new one. Don’t worry, they are relatively cheap and easy to acquire on the market. However, you must make sure that the new boiler filling loop is compatible with your unit. You might have to research a bit. To make things easier, work with your boiler’s brand and model details.

Basically, there are two main types of boiler filling loops that also affect their mode of use. We have:

  • External filling loops.
  • Internal filling loops.

Using an external boiler filling loop

  1. First and foremost, switch off your boiler and give it some time to cool.
  2. Inspect your boiler filling loop and make sure it’s in good condition. You’ll also want to make sure its valves are tightly closed.
  3. Connect the filling loop to the boiler system and open both valves. You should hear the mains cold water flowing into the system.
  4. Let the water flow for some time as you monitor the pressure reading. Leave the valves open until the boiler’s pressure is at 1.4 bars.
  5. Once the pressure has reached its ideal amount, close the valves one at a time.
  6. You can now turn on your boiler. Additionally, you’ll also have to press the “reset button,” if your model has one.
  7. Finally, you can now detach the boiler filling loop from the system. You might notice some water coming from the loop. Don’t worry, that is completely normal.

Is it safe to leave the boiler filling loop attached to the boiler system?

We know there are some who leave it for convenience purposes. However, we strongly advise against it. The filling loop should be disconnected after repressurizing the system. The practice isn’t safe and might permanently damage the boiler filling loop.

It’s also bad for your boiler. Kids might accidentally knock and activate the filling loop, allowing water into your boiler. The excess water will increase the boiler’s pressure levels. If you’re not careful, it can be dangerous.

The reverse can also happen, the boiler filling loop might allow water from the boiler to flow back into the mains. It might lead to contamination since the water you expect to come through your shower will be supplied to your taps for drinking.

Using an internal boiler filling loop

It’s important to note that there are two types of internal boiler filling loops:

  • Keyed filling loops.
  • Keyless filling loops.

Just as the name suggests, the internal boiler filling loop is in-built. You don’t have to connect anything to your boiler system. They are most common with modern boilers.

Keyed boiler filling loops

If your boiler has a keyed boiler filling loop, then there is a key that came with the unit. Most of the time, the engineer will give you the key after installation. If not, then you might want to check your user’s guide. Usually, it’s situated beneath the boiler, in a flap.

Next, look for the “keyhole” where the key fits. Once again, its position depends on your boiler’s brand and model. For example, the Worcester Boch model has the “keyhole” to the front right-hand side.

Insert the key and turn until it snaps open. Some brands have padlock icons or “on” and “off” markings. Once the key is in place, turn on the white tap slowly and monitor the pressure reading.

Close the tap once the pressure reads 1.4 bar.

Finally, turn the key back to its initial closed position and remove it.

Keyless internal boiler filling loops

As the name suggests, keyless boiler filling loops don’t use keys. They are the simplest and easiest to use. On top of that, they mostly come with modern boilers (especially combi boilers).

All you have to do is locate “the loop lever” usually located at the bottom of the boiler. Most of the time it’s blue.

Next, pull down the lever to activate the boiler’s filling loop.

As usual, you should pay attention to your boiler’s pressure gauge. You should disengage the lever or push it back into position once the pressure is at 1.4 bar.

How do I deal with my over-pressurised boiler?

Have you set the pressure too high? You might have made a mistake and ended up over-pressuring your boiler.

Also, while repressurising your boiler, you might have also introduced lots of air bubbles into the system. What do you do now?

Don’t worry, there’s a way you can reduce and stabilize your boiler’s pressure.

“Boiler bleeding” is the best way to solve this problem. As mentioned earlier, it involves allowing air or water to flow out of the boiler system.

You bleed a boiler by opening the radiator valve.

For this procedure, you’ll need a screwdriver or radiator key, a medium-sized container, and a large towel.

  1. Turn on all the radiators to their maximum settings and let them run for about 15 minutes.
  2. Next, switch off your boiler’s central heating system to allow the water and air to settle down. You should also give the system enough time to cool.
  3. Locate the radiator that is closest to the boiler unit. Make sure the radiator tap is closed before placing the container underneath the bleeding tap.
  4. Take your radiator key or screwdriver and slowly open the bleeding tap. You have to be careful, don’t rush and completely open the tap. You should only open until water comes out. If you completely open the bleeding tap, there’s a chance it might never close again.
  5. Close the bleeding tap once enough air and water is out of the system.
  6. Repeat the process for all your radiators. We advised you to bring a towel since the process is somewhat messy.
  7. Once you’re done, confirm that all bleeder taps are tightly closed before turning on your boiler.
  8. Check your boiler’s pressure levels. The pressure should be around 1.4 bar. If it’s low, then go ahead and repressurise your boiler to the required amount.

Is it normal to have to frequently repressurise my boiler?

No, it’s not normal.

Ideally, you should only have to pressurise your boiler, at most, twice a year. In such a case, repressurising is recommended and treated as a maintenance procedure to help improve your boiler’s lifespan.

More than twice a year is a red flag. It’s likely that your boiler system is suffering from internal faults. Leaks are the most common problem. However, you should also be on the lookout for component failures, especially those related to expansion and pressure release vessels. If not treated, these problems may damage your boiler.

Despite that, the act of frequently repressurising your boiler is in itself dangerous. As you frequently top your boiler, the oxygen in the mains water supply will accumulate in the pipes and radiators, and can ultimately lead to corrosion. Additionally, it will also insulate the components thus reducing the boiler’s efficiency. 


Boiler filling loops are easy to use once you understand how they work.

Nonetheless, before you touch anything, we advise you to go through your boiler’s user manual. It will give you invaluable information about the location of your boiler filling loop, its type, the recommended pressure levels, and the necessary safety precautions.

Is your boiler still bothering you? Call us today. We have a team of professional engineers ready to work on it.

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