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Last updated: 23rd May, 2024


What is a Boiler Filling Loop? In-Depth UK Guide

What is a Boiler Filling Loop? In-Depth UK Guide

Key takeaways

  • A boiler filling loop is a temporary connection used to re-pressurise your heating system.
  • Proper operation of the filling loop is crucial for maintaining correct boiler pressure.
  • Filling loops should be disconnected after use to comply with water regulations.

When it comes to Boilers, there can be some confusing jargon. Just what exactly is a Boiler Filling Loop?

A boiler filling loop is an incredibly useful tool that is often used during boiler installations and boiler fixes. 

In this guide, we’re going to be explaining exactly what a boiler filling loop  is, how it works and answering some of the most commonly asked questions about these handy tools.

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What is a Boiler Filling Loop? In-Depth UK Guide

Boiler Filling Loop

A boiler filling loop is a tool used to provide a temporary connection between the boiler and the mains water supply, allowing you to re-pressurise your boiler and fill your heating system with water.

How Does a Filling Loop Work?

Fortunately, even if you don’t have much experience fixing boilers, a boiler filling loop is a rather simple tool to use.

If your boiler pressure has dropped below the recommended bar, you will need to top up the pressure to ensure your boiler operates as it should. 

Anything below half a bar can have significant consequences on how your boiler operates and could lead to long term issues if left unfixed, so it’s absolutely essential to know how to top up the pressure if it does drop.

Topping Up Your Boiler Pressure Using a Filling Loop

  • Braided hose pipe 

  • Filling loop valves

  • Blanking caps

Topping Up Pressure With Boiler Filling Loop

Before attempting to re-pressurise your boiler, make sure to check that the filling loop valves are in the ‘off’ position before continuing (see image below).

How to Top Up Your Boiler Pressure Using a Filling Loop

  1. Attach the filling loop valves to either end of the braided hose pipe.

  2. Connect the boiler or the water mains with the filling loop valves.

  3. Using your hands, ensure the filling loop is fixed firmly in place onto the braided hose pipe.

  4. To turn the valves into the ‘on’ position, turn one valve so that it matches the direction of the pipe. This should allow water to start coming from the mains into your central heating system.

  5. As the water enters your central heating system, the reading on the boiler pressure gauge should start to rise.

  6. Once the gauge reaches the recommended bar for your boiler (usually around 1.4 bar), turn the valves back so they are once again closed.

  7. Make sure to disconnect the filling loop from your boiler once you have finished using it.

This step is incredibly important as, although it may seem like a good idea to leave it in place for future use, this is actually against regulations as it could cause irreparable damage if left connected. 

Filling Loop FAQs

What Causes a Boiler to Lose Pressure?

Boiler Losing Pressure

A boiler sometimes loses pressure by itself over time and it isn’t usually a cause for concern so long as it doesn’t keep on dropping.

Persistent low boiler pressure can be caused by two major problems: a leak or an issue with the boiler itself. 

If you think your central heating system may have a leak, or if you suspect there is an issue with your boiler that may be causing the pressure to constantly drop, don’t hesitate to call a Gas Safe registered engineer to come and take a look.

What Pressure Should My Boiler Be?

Different boiler brands and models have different boiler pressure recommendations, but most boilers are designed to be around 1 and 2 bar units.

Make sure to always check your manual before you make an adjustment to your boiler pressure. 

Taking the pressure too high or low, even for a second, can waste fuel and might end up with you losing control of your boiler entirely.

How to Tell if Your Boiler Pressure is Too Low

Engineer With Filling Loop

You can check to see if your boiler pressure is too high or low by taking a look at your boiler pressure gauge and ensuring it is within the recommended pressure range for your individual boiler model.

Certain electronic boiler models also use error codes to tell you about major problems, such as a change in boiler pressure. 

Here are some of the most common boiler brand error codes relating to boiler pressure.

Viessmann Boiler Pressure Error Codes:

  • Fault Code C1 = Differential air pressure.

  • Fault Codes B9, ED, F0, F-E5, F-EC, F-ED, F-EE, F-EF, F-FD, F-FF = Maximum pressure limiter issue.

  • Fault Code F5 = Faulty gas pressure switch.

  • Fault Codes EE and EF = Faulty air pressure switch.

  • Fault Code 0F = Service required.

Alpha Boiler Pressure Error Codes:

  • Fault Code 1 = General ignition failure.

  • Fault Code 2 = Low pressure in the primary system.

  • Fault Code 61 = Incorrect system pressure, air in the pump.

  • Fault Code 0A37 = Insufficient pressure and flow (switches the boiler off automatically).

  • Fault Code 0E37 - Insufficient system pressure.

Worcester Boiler Pressure Error Codes:

  • Fault Code E9 = Safety limiter.

  • Fault Code A1 = Dry pump.

  • Fault Code F0 = Major internal error.

  • Fault Code CE 207 = Low water pressure.

  • Fault Code H07 = Low water pressure that is impacting performance.

  • Fault Code 1065 B = Pressure sensor is defective.

  • Fault Code 1970 B = Sudden, rapid drop in pressure.

Is Low Boiler Pressure Dangerous?

If a high or low boiler pressure is left untreated for an excessive amount of time, you could end up destroying a large part of your heating system, especially if the boiler relies on the constant movement of water to function properly.

Why Aren’t Some Filling Loops Connected to the Boiler at All Times?

One main reason for this is because water regulations strictly prohibit used water from flowing back into the mains. 

This is because if you water that runs through your radiators accidentally comes out of the shower and taps then you could be exposed to a highly contaminated water source!

Where is the Filling Loop on My Boiler?

Typically, the engineer that installs your boiler will leave it nearby so you can easily access it should any issues arise later down the line.

However, if you are unable to find it, you can easily find a replacement online or at your local DIY shop. 

How Much Does a New Filling Loop Cost?

A new boiler filling loop costs between £7 and £20 - a worthwhile investment to ensure you have one on hand should you ever need it.

If your boiler is over 15 years old and you find that it keeps losing pressure, it may be time to get a new efficient boiler to replace it. 

At iHeat, we install a wide range of high quality A-rated boilers from top brands such as Worcester Bosch, Viessmann, Alpha and Ideal

Get a free, fixed quote today!

For more advice and information about your boiler, check out our handy boiler guides

iHeat Boiler Servicing

At iHeat all of our subcontracted engineers are Gas Safe registered and can service your boiler swiftly and safely. Prices of an expert boiler service with iHeat start from £99.These figures might fluctuate regionally and can also depend on the specific type of boiler you own, such as a combi, gas, or oil boiler. 

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iHeat New Boiler Costs-

Boilers can be a daunting purchase for many people as they’re an appliance with a lot of responsibility, providing heat for you and your family is something you want to get right. Boilers aren’t exactly a quickly disposable item either, potentially lasting you a decade.

Effectively, new boiler cost can be split into two segments: the first is the actual boiler itself (unit price), and the second is the cost of the boiler being installed (set up) in your property by an expert engineer. 

Here at iHeat, we want to remove all of this undue stress and make the decision making process of upgrading to a new central heating system, as easy as possible.

Boiler costs can vary depending on a number of factors including their brand, model, fuel, output, warranty, labour and installation type. Typically a new boiler will cost between £1,845 and £3,500, below is a list of average boiler installations offered by iHeat (guide only).

Installation Type

Price (inc VAT)

Combi to combi swap


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System to combi conversion


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New boiler install


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Back boiler to a combi


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System to system


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Frequently Asked Questions

What Happens If You Leave the Filling Loop Open on the Boiler?

Leaving the filling loop open can lead to continuous water flowing into the boiler's system, which might increase the pressure beyond recommended levels. Excessive pressure can cause the boiler to operate inefficiently or, worse, result in damage to the boiler and the heating system due to leaks or component failures.

Can You Repressurise a Boiler Without a Filling Loop?

It is generally necessary to have a filling loop to repressurise a modern boiler. However, if for some reason the filling loop is unavailable or malfunctioning, you would need to contact a professional for an alternative solution, such as installing a temporary external filling loop or repairing the existing one.

Should Filling Loop Be on Flow or Return?

The filling loop should be connected to the return side of the heating system. This is typically where the boiler system is designed to accept incoming cold water. Connecting to the return ensures that the new water does not cool the water already heated in the system too rapidly, which can affect efficiency.

How to Top Up a Boiler Using a Filling Loop?

To safely top up your boiler, start by ensuring it's turned off. Locate the filling loop, which is usually a silver or grey flexible hose beneath the boiler with a valve at each end. Slowly open the valves on the filling loop. You will hear water entering the system. Watch the pressure gauge on the front of the boiler. The typical operating pressure is between 1 and 1.5 bar. Once you reach the correct pressure, close both valves on the filling loop. After repressurising, check around the boiler for any leaks.

How Do I Know If My Filling Loop is Open or Closed?

The position of the handle on the filling loop’s valves indicates if it's open or closed. Typically, when the handle aligns with the hose (in line), the valve is open; if it’s perpendicular, the valve is closed. Ensure both valves are securely closed after topping up the boiler to prevent leaks.

Why is My Filling Loop Not Working?

If your filling loop isn't working, it could be due to several reasons such as the valves may be seized or stuck, which can happen if they're not operated regularly. There could be a blockage in the hose or valves, or the valves or the hose might be faulty or damaged and need replacing.

What Do I Do If My Boiler Won't Repressurise?

If your boiler won't repressurise despite correctly using the filling loop, ensure it is properly attached and both valves are functional. Check for leaks as a leak in the system can prevent pressure buildup. Refer to your boiler's manual for troubleshooting specific to your model. If you can't resolve the issue, it may be time to call a qualified heating engineer.

Stephen Day profile photo
Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.