Tell room-sealed, open-flued and flueless appliances apart by where air comes from and products go, and know what each type makes you check.
Identify the type before any other check by asking two questions: where does the combustion air come from, and where do the products go? In the diagram, blue dashed arrows show air for combustion and amber arrows show products of combustion.
Room-sealed
The figure shows a sealed combustion circuit: air enters from outside and products discharge outside, without using air from the room containing the appliance. Some room-sealed designs obtain air from a ventilated uninhabited space within the building; the combustion circuit still has to be sealed from the room. Many appliances use a concentric flue, with air entering through the outer passage and products leaving through the inner one. A balanced terminal places intake and discharge close together so wind effects are substantially balanced. Check the case seals, flue and terminal.
Open-flued
Air from the room, products up a flue or chimney to outside. A wall vent may replenish the room air, but it is not a sealed intake duct to the burner. A draught diverter often identifies this arrangement. It depends on adequate room ventilation and a clear, working flue; the prescribed spillage test checks that products clear under the test conditions.
Flueless
No flue: air from the room and products released into the room. Permitted only for suitable appliance types in suitable locations; room size and ventilation matter, alongside correct combustion and safe functioning.
Fans
Fan assistance is how air or products are moved, not a fourth type; a room-sealed appliance can be fan-assisted. A fanned-draught appliance needs an interlock that stops it firing if the fan or flue flow fails, or products could be released indoors.
The terminal
A flue carries the products outside, and its terminal is the outside end, where they discharge to open air.
- Keep it clear of windows, doors, air vents and air inlets so products cannot re-enter. Take clearances from BS 5440-1 and the manufacturer's instructions: they differ for natural- and fanned-draught terminals.
- Fit a terminal guard where it is low or beside a path, protecting the terminal and people from the hot outlet without blocking the discharge.
- Never discharge into an enclosed space, such as a covered passageway between buildings. A terminal below a carport roof is allowed if it keeps its clearances: at least 200 mm below the roof for a fanned-draught terminal (600 mm for natural draught) and 1,200 mm from any opening from the carport into the building.
- Keep ivy and shrubs clear: growth restricts the discharge and lets products collect and re-enter.
Where a terminal must not discharge
Products need open air to disperse. In an enclosed or covered space they collect instead of clearing and can be drawn back into the building. Never site a new terminal in a covered passageway between buildings. An existing one there has to be risk-assessed under the unsafe situations procedure: Immediately Dangerous if products are getting into the building, At Risk if they could. A terminal that now discharges into a conservatory built over it is Immediately Dangerous.