Flue Systems

What is a flue system?

• APart of the boiler only
• BA passage for removing combustion products from an appliance to outside✓
• CA heating unit
• DA duct that brings fresh air into the room for combustion

Answer: Flue systems safely extract combustion products to the outdoor environment.

What are the main types of flues?

• AOnly plastic
• BOpen flues, room-sealed, balanced flues, and fan-assisted✓
• CJust chimneys
• DThe types vary between each manufacturer

Answer: Flue types include open chimney, room-sealed, balanced flue, and fan-assisted designs.

What is a natural draught flue?

• AAn indoor duct that carries warm air from the appliance through to the other rooms
• BAn electronic control that varies the speed of the fan to match the burner rate
• CA flue relying on the natural pressure difference (buoyancy) to exhaust gases✓
• DA flue in which a fan is used to force the products of combustion out of the terminal

Answer: Natural draught flues rely on hot gas buoyancy to drive gases out without a fan.

What determines the required termination height of a chimney or flue above a roof?

• AThe height is set by the local authority when building control approves the installation, and does not depend on the flue type
• BIt depends on the flue/terminal type and its position relative to the roof and nearby openings, per Building Regulations/BS 5440 and the manufacturer's instructions✓
• CLevel with roof
• D300mm minimum

Answer: There is no single fixed height that applies to every chimney or flue. The required termination height depends on the terminal type (open flue, room-sealed, fanned draught, etc.) and its position relative to the ridge and any nearby openings, as set out in Building Regulations Part J / BS 5440 clearance tables and the appliance manufacturer's installation instructions.

What is a balanced flue terminal?

• AA draught diverter
• BAn ordinary wall ventilator that allows fresh air into the room in which the appliance stands
• CA fitting that safely exhaust gases while drawing in combustion air through concentric ducts✓
• DA condensate drain

Answer: Balanced flue terminals use concentric ducts for safe outside air intake and gas exhaust.

Where can a balanced flue terminal be positioned?

• AOn walls, roofs, or chimneys with proper clearance from windows and openings✓
• BAnywhere
• COnly horizontally through an external wall, never vertically through a roof
• DOnly on the roof

Answer: Balanced flue terminals can be vertical (roof) or horizontal (wall) with required safety clearances.

What is a fan-assisted flue?

• AAn open flue fitted with a rotating cowl that spins in the wind to boost draught
• BA flue that relies on the boiler pump to move gases
• CJust a ventilation fan fitted in the room to extract stale air to outside
• DA flue using a motor-driven fan to ensure gases are expelled reliably✓

Answer: Fan-assisted flues use electric fans to ensure consistent, reliable gas exhaustion.

What happens if a chimney is too short?

• AInsufficient draught, causing incomplete combustion and potential CO spillage✓
• BOnly a cosmetic issue affecting the look of the roofline as seen from the street
• CIt causes no trouble
• DFaster heating

Answer: Short chimneys have insufficient draught, risking dangerous combustion gas spillage.

What is flue blockage risk from?

• ADebris, bird nests, soot, or structural damage✓
• BOnly from external damage
• CIt arises from nothing in particular
• DA sealed flue cannot become blocked along its length

Answer: Flues can be blocked by debris, nests, soot, or internal damage, causing dangerous spillage.

What is a flexible flue liner?

• AA component that is no longer fitted
• BA rigid steel pipe that was built into the chimney breast at the time the property was constructed
• CA flexible metal or plastic tube inserted into an existing chimney to improve draught and safety✓
• DA temporary duct

Answer: Flue liners improve draught in old chimneys and provide a gas-safe sealed passage.

What is the purpose of a flue terminal?

• AAn optional feature that may be left off if the flue exits above the roof
• BA cosmetic fitting chosen so that the flue outlet blends into the wall
• CTo increase the draught by narrowing the outlet at the end of the flue
• DTo safely disperse gases outside while preventing rain or debris entry✓

Answer: Flue terminals safely exit gases, protect against weather, and prevent downwind drafting.

What is downdraught in a flue system?

• ACold air that settles at the base of the chimney during the summer months each year
• BThe speed of the wind, measured at the flue terminal while a spillage test is done
• CWind pressure reversing normal draught, causing gases to flow back into the home✓
• DThe natural upward flow of gases in a working flue

Answer: Downdraught occurs when wind pressure reverses flue draught, risking dangerous gas spillage.

What clearance must be maintained from flue terminations to windows and doors?

• AThe distances set by the manufacturer and BS 5440-1, which vary by flue type✓
• BVaries by region
• CJust visible distance
• DFitting a cowl to the terminal removes any clearance requirement

Answer: Flue terminal clearances to opening windows and doors depend on the flue type and appliance. Follow the manufacturer's instructions and BS 5440-1, so that combustion products cannot re-enter the building.

What is a conventional flue?

• AAny type of flue, since the term covers every design in current use
• BA modern flue of the type fitted to condensing boilers today
• CAn open flue that uses natural draught to carry products up a chimney✓
• DA sealed flue that takes its air from outside the building

Answer: A conventional flue is an open flue that relies on natural draught to take combustion products up a chimney or flue pipe to outside. The appliance takes its combustion air from the room.

What is flue gas temperature monitoring?

• AChecking the air temperature in the room near the appliance's controls
• BMeasuring exhaust temperature to verify efficient combustion✓
• CA check of the gas meter's temperature
• DA detail of no importance

Answer: Temperature measurement confirms proper combustion and helps diagnose problems.

What is the purpose of a draught stabilizer?

• AA cap fitted to the terminal to keep rain out of the flue
• BA device that is fitted to increase the draught in a flue which is running weakly
• CA device that reduces the draught by closing off part of the flue way permanently
• DTo maintain constant draft by drawing in room air if draft becomes too strong✓

Answer: Draught stabilizers automatically draw in room air if flue draft becomes excessive.

What should be checked during a flue inspection?

• AInspections follow no set list, so the engineer decides
• BJust visual appearance
• COnly the entrance
• DIntegrity, leaks, proper height, terminal condition, blockage, and draught✓

Answer: Flue inspections check for damage, blockage, proper height, draught, and terminal condition.

What is the risk of a damaged or leaking flue?

• AIt presents no risk
• BJust aesthetic
• CCarbon monoxide spillage into living spaces✓
• DHigher efficiency

Answer: Leaking flues can cause dangerous CO to enter the home instead of exhausting outside.

What is a condensing flue terminal?

• AAn uncommon fitting
• BJust a regular terminal
• CA terminal designed for condensing boilers, handling water discharge safely✓
• DA terminal rated to withstand high flue-gas temperatures

Answer: Condensing terminals handle the water vapour from condensing boilers with proper drainage.

What is a shared flue?

• AA flue serving one appliance that is split into two outlets at the terminal
• BA flue from multiple appliances, which can be risky without proper design✓
• CA flue that runs through a party wall into the neighbouring property
• DA large-diameter flue, defined by its size rather than its use

Answer: Shared flues from multiple appliances require careful design to prevent dangerous interactions.

What is the risk of a flue too large for the appliance?

• AA larger flue removes gases more quickly, so there is no disadvantage
• BBetter draught
• CWeak draught, poor combustion, and potential CO production✓
• DJust cost saving

Answer: Over-sized flues result in insufficient draught, causing poor combustion and CO production.

What is a fan-diluted flue?

• AA sealed flue
• BOptional equipment
• CA term that carries no real meaning in gas work at all
• DA flue where a fan draws room air into the flue to reduce exit temperature✓

Answer: Fan-diluted flues use fans to introduce room air, reducing exit gas temperature.

What is the purpose of a flue cap or cowl?

• ADecoration only, so that the flue outlet matches the roof covering
• BTo increase the temperature of the gases as they leave the flue
• CUnnecessary on any flue that terminates above the ridge of the roof
• DTo prevent rain, debris, birds, and wind from entering the flue✓

Answer: Flue caps protect against rain, debris, animals, and help prevent downdraught.

A gas fire is to be fitted to a brick chimney last used for an open coal fire. What must happen to the chimney first?

• AIt can be used as it is if the chimney pot is in place
• BIt must be capped at the top to stop downdraughts
• CIt must be swept, then checked to be sound and clear✓
• DIt must be lined, whatever its condition or size

Answer: BS 5440-1, the standard for flues serving gas appliances, requires an existing chimney to be swept before a gas appliance is connected unless the previous appliance burned gas, because soot and debris left by solid fuel can block the flueway or corrode a metal liner. The flue must also be sound and clear, and once the appliance is fitted a flue flow (smoke) test and a spillage test confirm the products of combustion are carried safely to the outside. Capping the top would block the flue, and a liner is needed only where the chimney is unsuitable as it stands.

What testing is required for a flue installation?

• AA check of the terminal position from outside the property
• BJust a pressure test of the flue carried out with a manometer at the base
• CSpillage test, draught measurement, and flue gas analysis✓
• DA visual inspection only, looking along the flue for obvious damage

Answer: Flue installations require spillage, draught, and gas analysis testing to verify safety.