Room-sealed and open-flued appliances
Room-sealed appliances take combustion air from outside and send flue gases outside through a sealed case. Open-flued appliances take their air from the room.
Most modern boilers are room-sealed. Open-flued appliances rely on the room for combustion air and on the flue to carry gases away, so ventilation and flue performance checks matter more.
Which type an appliance is changes the checks on a service or gas safety check.
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A plain-language explanation, not a technical procedure or legal advice. Follow the current regulations, industry procedures and manufacturer’s instructions.