Here we look at the types of flue systems available and which types are suitable for Hamworthy boilers and water heaters. You can choose from the flue types below or a bespoke boiler flue system can be designed by our flue partner, Midtherm.

What is an open flue system?

In an open flue system combustion air is drawn directly from the room, and flue gases may be discharged through a flue system using either an existing chimney or ducted flue system. The plant room requires ventilation, to include the combustion air supply, in accordance with BS5440 and BS6644.

The flue system will typically rise to the top of the building and discharge above roof level, although some modern low output commercial gas boilers may have an open flue system terminating at low level (in accordance with flue discharge requirements).

What is a room sealed flue system?

In a room sealed flue system combustion air supply is drawn via ducts from an outside air supply. Flue gases may be discharged through either a concentric or twin duct flue system. The plant room requires ventilation in accordance with BS5440 and BS6644. Plant rooms will require less ventilation than open flue systems, and with reduced air flow, warmer room temperatures are possible – a useful point when designing tighter buildings.

Room sealed appliances reduce the risk of flue gases spilling back into the plantroom due to poor flue operating conditions. In certain buildings, commercial boilers can be installed within occupied spaces where a dedicated plant room may not exist.

What is a concentric flue system?

A concentric flue system provides a very compact room sealed flue, where the air supply and flue gases are managed within a one piece concentric duct system. Typically using a duct within a duct, flue gases are expelled through the inner duct with combustion air being drawn in via the outer annulus. 

A concentric flue system will have the option for the terminal to be positioned horizontally for exiting the building through the wall (subject to IGEM UP10 regulations) or vertically through the roof. The need to penetrate the building with a single hole is an advantage.


Coping with condensate 

When changing commercial boilers from atmospheric boilers to condensing boilers the existing flues on this type of system are not able to deal with the modern pressurised and wet system where condensate will form. Modern condensing boilers generate condensate from the flue gases at a rate of around 13 litres per hour for a 100kW boiler, therefore flue systems must be water tight and pressure tight, and designed to drain the condensate back into the boiler. You may be able to make use of the existing chimney with a liner to enable it to cope with condensing operation.

See how the Carlton Hotel made use of their existing flue system with a liner for their boiler refurbishment projects.

Flue classifications and diagrams

Open flue type B
An appliance intended to be connected to a flue that evacuates the products of combustion to the outside of the room containing the compliance. The combustion air is drawn directly from the room.

Open flue type B2
A type B appliance without a draught diverter.


B23 flue system
Open flue type B23
A type B2 appliance incorporating a fan upstream of the combustion chamber / heat exchanger.


Room sealed type C
An appliance in which the combustion circuit (air supply, combustion chamber, heat exchanger and evacuation of the products of combustion) is sealed with respect to the room in which the product is installed.

Room sealed type C1
A type C appliance that is designed for connection via its ducts to a horizontal terminal, which at the same time admits fresh air to the burner and discharges the products of combustion to the outside through openings that are either concentric or close enough to come under similar wind conditions.

Room sealed type C3
A type C appliance that is designed for connection via its ducts to a vertical terminal, which at the same time admits fresh air to the burner and discharges the products of combustion to the outside through openings that are either concentric or close enough to come under similar wind conditions.

Room sealed type C5
A type C appliance connected to separate ducts for the supply of combustion air and the evacuation of the products of combustion. These ducts may terminate in zones of different pressures.

C13 flue system
Room sealed type C13
A type C1 appliance incorporating a fan upstream of the combustion chamber / heat exchanger. 
C33 flue system
Room sealed type C33
A type C3 appliance incorporating a fan upstream of the combustion chamber / heat exchanger.
C53 flue system
Room sealed type C53
A type C5 appliance incorporating a fan upstream of the combustion chamber / heat exchanger.

Flue options for Hamworthy products

The table below shows which boiler flue system and water heater flue system is compatible with which Hamworthy commercial boilers and water heaters. Full details about the flue systems can be found in the technical sales brochures.

Flue options available for each of Hamworthy's commercial heating products.

Need help choosing the right flue system?

Get in touch with our technical department who will help you choose the correct boiler flue or water heater flue system for your project.

Contact us