Understanding Different Types of Boiler Flue

Different types of boiler flues ensure boilers can be fitted safely and effectively - even in the most challenges of conditions - but before getting into the nitty gritty about the different types available, let’s start by looking at what a boiler flue is and how it works.

Essentially, boilers need to burn fuels such as gas or hydrogen to heat water, which is then circulated around a central heating system and a hot water cylinder for system and heat-only boilers. This is where boiler flues come into play. Boiler flues are pieces of pipework attached to gas boilers that allow waste gases produced during the heating process to be safely expelled. The two main types of flue systems are concentric systems and twin boiler flue systems.

Concentric and twin flue boilers: What’s the difference?

A concentric flue is a one-piece system that takes care of both the flow of air supply into the appliance and the removal of flue gases from the appliance. It’s a pipe within a pipe, with waste gases channelled away through the inner flue (the boiler waste pipe), and the air used in the combustion process arriving through the outer flue.

A twin-flue boiler system essentially performs the same task; however, it has a separate air intake and exhaust flue pipe. This means that as the flue runs aren’t bound together concentrically, twin flues provide greater flexibility, ensuring installers can meet regulatory requirements.

There are many advantages to choosing a twin-flue boiler system. For example, boiler flue position regulations often specify flues must be positioned a certain distance away from windows and doors. A twin flue boiler system may allow you to meet these regulations without needing to relocate your appliance altogether if it’s an unusual site such as a basement or plant.

Open and closed flues

Flues generally fall into two categories: open (also known as a conventional flue) or closed. An open flue has one tube that removes the by-products of combustion away from the appliance through an external wall using a draught diverter to pull the air through the flue. Here, the combustion air supply is drawn directly from the boiler’s surroundings rather than a fresh outside air supply, so there must be suitable ventilation around the appliance to ensure adequate airflow.

A closed flue is used with room-sealed appliances and is available in two types: balanced flues and fanned flues, with the latter being more common in modern boiler systems. Balanced flues use two pipes; one for air supply, and the other to expel products of combustion. Here, Oxygen is pulled into the boiler from the outside to facilitate combustion and uses the natural draught created by the flue gases to expel exhaust through an external wall. Fanned flues work in a similar way to balanced flues although, as the name suggests, this type of flue uses a fan to power air into the boiler and to also power the removal of waste gases. This allows for longer flue runs as the air and waste gases can be propelled greater distances.

How do horizontal and vertical flues work?

The only real difference between horizontal and vertical flues is the way they are positioned on a building. Horizontal flues (typically the most common type) are installed so the flue exits out of the external wall behind the boiler. In more complicated installations (such as where there isn’t an external wall, or installing a flue in that position would be in breach of regulation), a vertical flue could solve the problem. Vertical flues exit out of the roof, allowing waste gases to be expelled safely.

What combi boiler flue do I need?

Combi boilers, such as the C30 and C35twin flue combi boilers from Keston are normally room sealed, so they generally use a balanced or fanned flue system. However, often the choice comes down to the location of the boiler. For example, a more challenging installation could benefit from a twin-flue boiler system.

About boiler flue extensions

Boiler flue extensions essentially lengthen the flue run, providing greater flexibility when it comes to selecting a termination point. However, when it comes to extensions, it’s important to use the same flue manufacturer.

Boiler flue guards

Lastly, when it comes to positioning a boiler flue, if it’s not possible to keep the flue over 2m off the ground, it’s important to install a flue guard. This will not only protect the boiler against extraneous items entering or blocking the system, but it is also an important safety consideration.

To find out how a Keston twin flue boiler system can help simplify installation and provide a flexible solution visit here.