A side-by-side comparison of room-sealed, open-flued and flueless appliances: where the air comes from and where the products go.
The comparison places the three flue arrangements side by side and answers the same two questions for each: where does the combustion air come from, and where do the products of combustion go?
A room-sealed appliance takes air from outside and sends products outside through a sealed path. The combustion process is sealed off from the room around it.
An open-flued appliance takes its air from the room and sends the products up a flue to outside. It therefore depends on enough room air and a clear, working flue.
A flueless appliance takes air from the room and releases the products back into the room. This is only acceptable for certain limited appliance types and relies on ample room ventilation.
The colour key separates air for combustion (blue) from products of combustion (grey), so the path of each is easy to follow.
Recognising the type quickly tells you what to check: outside terminals and seals for room-sealed; room air and flue for open-flued; and ventilation for flueless.
Treat this as a concept comparison. The detailed rules for each type are in the current Approved Document J, BS 5440 and the manufacturer instructions.