Combustion, Chimneys & Ventilation: Chimney Systems and Operating Principles

Vocabulary practice flashcards covering terminology, classifications, physical components, dimensions, and testing standards from the Centrica Gas Engineering Apprenticeship Chimneys Learning Guide.

BS 5440 Part 1

The British Standard code of practice specifying requirements for the installation and maintenance of flues and chimneys serving gas appliances with a maximum rated heat input of $70\,\text{kW (net)}$.

BS 5440 Part 2

The British Standard code of practice specifying installation and maintenance requirements for the ventilation of gas appliances with a maximum rated heat input of $70\,\text{kW (net)}$.

Aeromotive Force

The natural upward pull or flue draught generated within a chimney system, produced by the difference in density and temperature between hot combustion products ($170^\circ\text{C}$ to $250^\circ\text{C}$) and cooler ambient air.

Primary Flue Pipe

The initial section of flue pipe connected directly to an open-flued appliance spigot that establishes the starting draught for combustion products; it must never contain any bends.

Down Draught Diverter (DDD)

A device positioned in the same location as an open-flued appliance designed to divert down draughts from interrupting combustion, It pulls surrounding room air into the flue system alongside the exhaust gases. This dilutes the products of combustion and helps cool down the exhaust temperature as it travels up the chimney. It prevents the natural upward "pull" or draft of a tall chimney from drawing too much air through the appliance's burner. By creating a physical break, it ensures the burner operates under stable, constant conditions regardless of how strongly the chimney is drawing.

Secondary Flue

The flue conduit extending upwards from the draught diverter to the termination point, providing a safe evacuation pathway for diluted combustion gases; bends must not be installed within $600\,\text{mm}$ of the down draught diverter.

Type $B_{11}$ Gas Appliance

An open-flued appliance fitted with a down draught diverter that relies entirely upon natural draught to convey combustion products safely to the outside atmosphere.

Type $B_{12}$ Gas Appliance

An open-flued gas appliance with a draught diverter that incorporates a fan positioned downstream of the heat exchanger to assist in expelling products of combustion.

Type $B_{13}$ Gas Appliance

An open-flued gas appliance fitted with a draught diverter where the fan is positioned upstream of the heat exchanger to supply combustion air.

Firestop Spacer

A sheet metal plate installed where a chimney passes through an internal ceiling, floor, or wall to maintain fire compartmentation and guarantee a mandatory $25\,\text{mm}$ annular clearance from combustible materials.

Asbestos Cement Flue Joint

A vintage flue connection constructed with an upward-facing socket containing an internal rope seal and finished flush with an external cement seal to prevent flue gas leakage.

Selkirk Insta-Lock System

A prefabricated twin-wall gas vent system featuring an inner aluminium liner and a zalutite outer casing, secured together using bayonet locators and dimpled LockTab fixings.

Flexible Metallic Flue Liner

A continuous, corrugated stainless steel or aluminium conduit lowered down an existing masonry chimney to create an unbroken flueway, having an expected operational lifespan of $10$ to $15\,\text{years}$.

Precast Chimney Blocks

Specially moulded concrete masonry units incorporated into cavity or partition walls to create an integral flueway for compatible gas fires, usually transitioning into a twin-wall metal pipe in the roof space.

Transition Block

An adaptor block in a precast concrete flue system featuring a rectangular aperture at the base to seat onto plain blocks and a circular outlet at the top to connect to a metal twin-wall chimney.

Pre-2000 Precast Flue Block Standards

Regulatory specifications mandating that precast concrete flue blocks installed prior to the year 2000 must possess a minimum cross-sectional area of $12,000\,\text{mm}^2$ and a minimum wall thickness of $63\,\text{mm}$.

Post-2000 Precast Flue Block Standards

Regulatory specifications mandating that precast concrete flue blocks installed after the year 2000 must possess a minimum cross-sectional area of $16,500\,\text{mm}^2$ and a minimum wall thickness of $90\,\text{mm}$.

Smoke Pellet Test Standard

A standardized flue flow testing specification requiring a smoke pellet to generate at least $5\,\text{m}^3$ of pale grey smoke over a burn duration of $30\,\text{seconds}$.

Type C11 Appliance - Natural Draught Balanced Flue

A room-sealed horizontal balanced flue, natural draught appliance. Combustion air is aupplied from outside and products of combustion are discharged outside through the balanced flue system. No fan is used.

Type $C_{13}$ Appliance - Positive Pressure Fanned Flue

A room-sealed horizontal balanced flue, fan-assisted appliance which the fan is located upstream of the combustion chamber or heat exchanger, placing the combustion chamber under positive pressure.

Type $C_{12}$ Appliance - Negative Pressure Fanned Flue

A room-sealed horizontal balanced flue, fan-assisted appliance where the fan is located downstream of the combustion chamber or heat exchanger, maintaining negative presure within the combustion chamber casing.

SE Duct

A shared vertical flue duct developed by the South Eastern Gas Board that allows multiple balanced-flue appliances to connect into a single common shaft fed by fresh air from low-level horizontal ducts.

U Duct

A shared flue system for multiple room-sealed appliances where combustion air enters from roof level down one shaft of a U-tube, while combustion gases discharge out an adjacent parallel upward shaft.

Closed Chimney System

A chimney configuration operating without a draught diverter (such as Type $B_{22}$ or $B_{23}$), primarily utilised in commercial installations and requiring specific manufacturer instructions for spillage testing.

Flue Terminal Guard

A protective cage providing a minimum $50\,\text{mm}$ clearance around any balanced or room-sealed terminal located less than $2\,\text{m}$ above finished ground, roof, or balcony levels.

Terminal Heat Shield

A protective metal deflector measuring at least $1\,\text{m}$ long with a $5\,\text{mm}$ air gap, required when a natural draught room-sealed terminal terminates within $1\,\text{m}$ of plastic guttering or within $500\,\text{mm}$ of painted eaves.

Flues in Voids Inspection Hatch

An access panel measuring at least $300\,\text{mm} \times 300\,\text{mm}$ situated within $1.5\,\text{m}$ of each joint along a concealed ceiling void, enabling engineers to visually verify flue integrity.

COSSVM

Carbon Monoxide Safety Shut-Off Void Monitoring System; an automated electronic sensor system installed in ceiling voids to isolate a boiler upon detecting carbon monoxide from a concealed flue.

Covered Passageway (Ginnell)

A covered alleyway between buildings where no new flue terminals may terminate, and existing installations must be assessed for risk of carbon monoxide accumulation or structural infiltration.

Flaunching

A smooth, contoured cement mortar fillet applied to the head of a masonry chimney stack that sheds rainwater away and mechanically seals a flue liner terminal or chimney pot.

Installation requirments of open chimmey system

1. No bends in primary flue.

2. Diameter of the chimney must be the same as the appliance.

3. DDD must be installed in the same location as the appliance.

4. The secondary flue should have no bends within 600mm of the down draught diverter (DDD).

5. If the chimney is running in an external location of over 3m it should be constructed of a factory insulated chimney pipe.

6. The chimney pipe should be supported every 1.8m unless stated otherwise by MIs.

7. There should be a 25mm clearance between the chimney pipe and any combustible material. Should the chimney pass through an internal ceiling or floor a firestop plate should be installed.

Matallic Alloy Flue and Terminal

The matallic flues can come in 3 varying constructions; single wall (usually with an external finish), twin wall (within a 25mm air gap between inner and outer flue walls and the seam is always at the top of the flue) and factory insulated (to be unsed when there is an external run of flue over 3m). There are two main manufacturer’s of matallic flue with differing methods of joining the components. The twin wall joins by twisting the upper length onto the ridged indent and twisting a quater of a turn, no further sealing of the join is required,

Terminals and their specifications

1. Must be approved for use with gas

2. Can be made of clay or metal.

3. Openings must be between 6mm and 16mm.

4. Terminal location critial to safe operation (reference MIs and GasTech for specifications).

5. Some chinmney sysytems don’t require a terminal.

6. Examples of terminals; ridge terminal, Terracota terminal, GC1 terminal, Twin wall terminal, chinmney liner terminal.

Testing Open Chimney Systems

1. Preliminary checks (visual checks on the condition of the chimney)

2. Flue Flow and Continuity Checks (test to ensure the chimney is pulling and that it’s mechanically sound and continuous.

3. Appliance and Flue Conbination Check (spillage test to ensure that both the chimney and the appliance can evacuate the products of combustion when the appliance is turned on).

Shunt Duct

A common vertical chimney system serving multiple natural draught open-flued appliances on different floors of a building, requiring a minimum cross-sectional area of 40,000 mm240,000mm2 and subsidiary flues rising at least 1.2 m1.2m (3 m3m for gas fires) before entering the main stack.

Vertex Chimney System

A hybrid flue arrangement where a fan-assisted room-sealed boiler in a building's interior connects via a concentric flue to a draught break in the ventilated loft space, converting into an open, natural draught chimney through the roof.

Balanced Compartment

A purpose-built sealed compartment housing an open-flued boiler (35 kW35kW to 70 kW70kW net) with an insulated twin-wall flue and dedicated air duct descending from within 150 mm150mm of the roof terminal to provide the operational safety benefits of a balanced flue.

Balanced Compartment Door Notice

A mandatory warning sign affixed to the self-closing entrance door of a balanced compartment stating that the door must be kept securely closed whenever the heater is in operation.

Condensate Pipework Sizing

Mildly acidic wastewater disposal piping that requires a minimum internal diameter of 19 mm19mm inside the dwelling and a minimum internal diameter of 30 mm30mm when run externally, where it must also be insulated.

Condensate Soakaway

A dedicated ground absorption point comprising a 100mm plastic tube with two rows of three 12mm holes on 25mm centres situated 50mm from the bottom facing away from the dwelling, backfilled with limestone chippings to a depth of 400mm

$100\,\text{mm}$ diameter sealed-base plastic tube with two rows of three $12\,\text{mm}$ holes on $25\,\text{mm}$ centres facing away from the building, bedded in limestone chippings to a depth of $400\,\text{mm}$.</p>" style="display: block; width: 50%; margin-left: auto; margin-right: auto;">

Communal Flue System (CFS)

A shared collective flue system designed for apartments to serve multiple room-sealed condensing or non-condensing boilers, featuring a common condensate collector at the base and prohibiting individual boiler condensate discharge into the main stack.

Room Sealed Appliance

An appliance that is entirely sealed from the room or installation space, utilizing a flue system with inner and outer ducts to supply combustion air and discharge products of combustion (POC).

Products of Combustion (POC)

The gaseous exhaust products created during fuel combustion that are conveyed and safely discharged outdoors through the inner duct of a room-sealed flue.

Condensing Boiler Legislation

Government statutory regulations mandating that all new boilers must be condensing appliances to ensure minimum operational efficiency standards and reduce carbon dioxide (CO2CO2​) emissions.

'Plume' Effect

The visible white cloud of condensed water vapour discharged from the chimney or flue terminal of a high-efficiency condensing appliance, which can sometimes cause a nuisance.

Plume Management Kit

A plastic flue terminal attachment kit used to redirect and divert the visible flue plume away from windows, walkways, or neighboring properties to a more desirable location.

Closed Chimney System

An open chimney/flue style installation designed for specialist appliances with no down draught diverter, where flue termination route and materials are defined strictly by manufacturer instructions.

Type B22​ Appliance

A closed chimney system appliance in which the fan is located downstream of the heat exchanger.

Type B23​ Appliance

A closed chimney system appliance in which the fan is located upstream of the heat exchanger.

Shared Chimney System

A chimney or flue system that serves two or more appliances, commonly installed in multi-occupancy buildings or industrial units, requiring natural draught appliances fitted with a draught diverter, flame supervision device (FSD), and safety shut-off controls.

Laminar Flow Effect

The fluid dynamics principle utilized in an SE-duct where parallel air movement keeps cooler combustion air along the outer duct perimeter (up to 50 mm50mm), preventing it from mixing with hot flue gases rising through the centre due to density differences.

Chimney Break (Vertex System)

A component in the loft space of a Vertex installation that transitions the appliance fan-assisted duct to an open chimney assembly, allows combustion air intake from the ventilated loft, and provides a location for flue flow and spillage testing.

Balanced Compartment Electrical Switch Safeguard

An electrical interlock switch fitted to the self-closing door of a balanced compartment designed to cut off appliance operation as soon as the door is opened.

Condensate Pipework Fall

The minimum mandatory gradient for running condensate drainage pipework, specified as at least 2.5∘2.5∘ to the horizontal or 50 mm50mm per metre towards the disposal point.

External Condensate Air Break

A tundish or open air gap installed in 32 mm/43 mm pipework outside the building between the condensate line and a rainwater pipe that connects to a combined sewer to prevent back-flooding in freezing conditions.

Non-Return Valve (NRV)

A mechanical ball or flap-type valve fitted between a boiler exhaust outlet and a fan-assisted communal flue system (CFS) to prevent combustion products from back-circulating into non-operating appliances.

Type C4 Appliances (C42 / C43)

Room-sealed, fan-assisted condensing appliances certified for connection to a concentric communal flue system (CFS), where C42 features a fan downstream of the heat exchanger and C43 features a fan upstream.

Type C8 Appliances (C82 / C83)

Room-sealed, fan-assisted condensing appliances certified for connection to an exhaust-only communal flue system (CFS-EO) with separate combustion air intake, where C82 features a fan downstream of the heat exchanger and C83 features a fan upstream.

Shared Chimney Label

A mandatory warning notice affixed to any gas appliance connected to a shunt duct, SE-duct, U-duct, or CFS stating that the appliance shall not be replaced or removed without the permission of the building owner.