How Long Can Boiler Flues be in Terraced Houses?

Moving into a new home is exciting. Alongside this excitement however, practical questions arise—such as those around boiler systems.

In this article, we’ll answer one of these many questions: “How Long Can Boiler Flues Be In Terraced Houses?”

Let’s first start with the basics and define a boiler flue.

Understanding What A Boiler Flue Is?

A boiler flue is an essential component for all gas boilers, allowing gas fumes, water vapour, and heat from the boiler to escape safely. Improper boiler flue installation can lead to harmful gases entering your home, so it’s crucial to follow strict installation criteria.

All gas boilers need one for those unsure whether to install a new boiler flue. If your home’s primary fuel is gas, you need a flue installed, whether a combination (combi), regular, or system boiler.

Note: that boilers like back and electric ones don’t require flues. Electric boilers, which use electricity to generate heat rather than burning fuel, don’t need one.

Before discussing the maximum length of boiler flues, let’s look at the types available in the UK.

Types Of Boiler Flues Explained

There are multiple types of boiler flues. Currently, there are three types of boiler flues:

Balanced Flues

A balanced flue system is a closed system with two pipes. One pipe supplies your boiler with air, and the other expels waste gases and water vapour.

The system uses natural airflow to pull in oxygen for the boiler’s combustion process and expel the waste via the other.

Balanced flues are typically used for combi boilers installed on an exterior wall.

Open Flues

An open flue removes the waste gases and water vapour from the boiler through an external wall and is diverted through a draught diverter.

The draught diverter pulls the air through the flue and directs the combustion air supply from the boiler outside. A boiler using an open flue system uses the air surrounding it for the combustion process.

This means if your boiler uses an open flue system, it needs to be installed in an area with good ventilation.

Open flues are usually not used with modern systems because they are considered less safe than a balanced system.

Fan Flues

As the name suggests, this flue system uses a fan to drive oxygen to and away from the boiler. Fan flues are used when a boiler is installed with a more extended pipe network.

Most of these flue systems will have their flues installed in two directions, horizontally and vertically.

The most common direction in which a Gas Safe registered engineer will install a flue is horizontal. The engineer usually uses black or white round pipe piping.

Horizontal flues are more common because the engineer usually fits the boiler to an external wall and, where suitable, has it go through the wall.

Modern boiler flues are generally as short as possible, with as few bends as possible. These flues are as short as possible to prevent waste gases from accumulating in the piping.

When installing a flue vertically, it’s usually because there is no option other than to go up and through the roof. Also, flues will be installed vertically because they are within two metres of an opening in the neighbour’s property.

Over the last 15 years, round flue pipes have generally been used. They are regulated to be 22mm wide and must meet other requirements.

For a more detailed look, you can review the British Flue & Chimney Manufacturers Association’s (BFCMA) guide to choosing and using flues for domestic gas-burning appliances.

In the past, square flues were used to vent away the waste gases. Many older square-shaped flues used piping that was only 15mm wide. If this is true in your new terraced house, you will need it replaced.

Also, only a Gas Safe registered engineer can install a new boiler flue.

Numerous regulations and your manufacturer’s instructions will determine the length of your boiler’s flue if you’ve just moved into a terraced house or anywhere in the UK.

Boiler Flue Regulations

The only document you must review to understand all boiler flue regulations in England and Wales is The Building Regulations Approved Document J.

But let’s go through some basic dos and don’ts that the BFCMA have laid out for us to follow.

Do:

If you plan on fitting a new boiler to an existing flue system, ensure that your Gas Safe registered engineer inspects it to ensure it works. This check will require the engineer to examine all the joints to ensure they are securely fastened and allow for the most efficient flow of gases and water vapour.

A layperson can quickly check to ensure that the flue pipe connection to your boiler or any gas appliance rises vertically for at least 600mm before it changes direction. This requirement is due to the higher temperature of the gases released from the boiler, which can restrict its flow.

As a rule of thumb, always avoid bends in the flue, and none can be angled more than 45° from the vertical.

Always ensure the flue diameter is not less than the exhaust outlet of the appliance.

Don’t:

Never use uninsulated flue systems externally because the pipe can freeze. Always use BS EN-certified flue systems.

Also, ensure that no combustible materials make contact with the surface of the boiler flue.

There are stipulated distances for boiler flues to abide by.

Boiler Flue Regulated Distances

Your home’s boiler flue will expel waste gases and water vapour, which can also be hot.

By law, you must install a flue to protect the property’s residents, your neighbours, and the general public.

Also, remember that your flue needs to be positioned a certain distance from doorways, windows, and other boiler flues.

These regulations are outlined by the National House Building Council (NHBC) and are updated regularly.

Factors that will affect your boiler’s flue position include:

• Where the exhaust is positioned (location)
• Whether it is a balanced flue system with a natural or fanned draught
• And if it is an open flue system with a natural or fanned draught. Generally, natural draught flues are not to be used.

Generally speaking, here are the minimum separation distances for gas outlets:

• Below an opening, you need your boiler flue outlet to be 300mm
  • For balanced flues with a natural draught, it depends on the appliance’s heat input rating, which means:
    • 0-7kW needs to be a minimum distance of 300mm
    • 7-14kW needs to be a minimum distance of 600mm
    • 14-32kW needs to be a minimum distance of 1,500mm
    • 32-70kW needs to be a minimum distance of 2,000mm
• Above an opening, you need your boiler flue outlet to be 300mm
  • For balanced flues with a natural draught, it depends on the appliance’s heat input rating, which means:
    • 0-32kW needs to be a minimum distance of 300mm
    • 32-74kW needs to be a minimum distance of 600mm
• Next to an opening, you need your boiler flue outlet to be 300mm
  • For balanced flues with a natural draught, it depends on the appliance’s heat input rating, which means:
    • 0-7kW needs to be a minimum distance of 300mm
    • 7-14kW needs to be a minimum distance of 400mm
    • 14-70kW needs to be a minimum distance of 400mm
• Below gutters, soil pipes or drain pipes, your boiler flue outlet should be 75mm
  • For balanced flues with a natural draught, it is 300mm
• Below eaves, your boiler flue outlet to be 200mm
  • For balanced flues with a natural draught, it is 300mm
• Below the balcony or carport roof, your boiler flue outlet to be 200mm
  • For balanced flues with a natural draught, it is 600mm
• From a vertical drainpipe or soil pipe, your boiler flue outlet should be 150mm
  • For balanced flues with a natural draught, it is 300mm
• From an internal or external corner or to a boundary alongside your boiler flue outlet to be:
  • For balanced flue with fanned draught, it is 300mm
  • For balanced flues with a natural draught, it is 600mm
  • For open flue with fanned draught it is 200mm
• For flues being above ground, roof, or balcony level it is 300mm for all flues.
• For flues facing a flat surface or boundary wall need to be 600mm
• For flues facing other flues your balanced flue with a natural draught needs to be 600mm apart.
  • For balanced flue with fanned draught it is 1,200mm
  • For open flue with fanned draught it is 1,200mm
• For multiple flues mounted on the same wall:
  • Vertically from one another all need to be at least 1,500mm apart
  • Horizontally from one another all need to be at least 300mm apart

This is a lot of information to take it. Honestly, this is why only Gas Safe registered engineers can repair, clean, and install boiler flues.

For those who want to know the basics of boiler flue regulation standards, remember the following:

Basics Of Boiler Flue Regulations

Whatever your boiler manufacturers instructions say, they need to be followed to the letter. This means your boiler’s manufacturer instructions must be easily available to reference in the future.

As mentioned, your boiler’s outlet needs to be positioned away from combustible products and heat-sensitive material, such as plastic guttering.

If your property backs onto a public space, like a pavement, you need to ensure that the flue is at least 2.1m off the ground.

Now to answer the main question.

How Long Can My Boiler Flues Be? In My Terraced House.

In a terraced home the length of the boiler’s flue will be down to safety regulations and manufacturer guidelines.

But to generalise, at the maximum allowable length your terraced house’s boiler flue could be as long as 10 metres, which is the limit for many modern boilers.

If you have discovered that your terraced house’s old boiler is in a problematic spot, you should consider taking our new boiler survey.

Select that you need your boiler moved, along with answering the other questions, and book a Gas Safe registered engineer to install it.

If you need some more questions answered, contact one of our experts.