A gas boiler needs to be vented to prevent combustible and toxic gases from building up inside a house. A vent damper is a simple device that can reduce heat loss when the boiler is not in operation.
Dampers need to be linked so that they open fully when the boiler fires. Venting is an important part of proper boiler operation and can extend the life and effectiveness of the boiler. If a boiler is not vented properly, it can cause pressure to build up in the system or exhaust gases to flow back into the house. Boilers present a confined space hazard when fired without proper venting. The leftover toxic gases can cause long-term health problems and even be fatal to humans and pets.
Furthermore, the gases that can build up only need an ignition source such as a spark or electrical short to cause an explosion. Several different venting designs are present on gas fired boilers. Older boilers require a masonry chimney to discharge the hot gas. Direct vent boilers use the leftover heat from combustion to reheat the system and then releases relatively cool air.
Closed or sealed system boilers draw air from outside on houses that are heavily weatherized. The last system is the most dangerous if improperly vented because it can draw air from the outside and release combustion gases inside.