A boiler is a very precise system. Gas or other fuels feed a fire. This fire in turn heats a reservoir of water or hydronic fluid. This fluid, laden with the fire’s heat, then rockets from the boiler via a series of pipes to provide heat or drive engines. A number of things, including an incorrect boiler fire, low water levels and incorrect piping, can thwart the best efforts of a boiler. Perhaps most important-especially in commercial and residential heating boilers-are the vents carrying exhaust air from the boiler. Exhaust air trapped in the boiler causes a number of problems. A lack of fresh air can snuff the fire. Stale exhaust air can also be hazardous to humans, causing serious health problems if not properly vented.
Boiler technology, dating all the way back to the 1700s, has utilized a number of methods to vent this exhaust air and ensure optimal boiler performance. Power venting, for example, vents combustion gasses directly to the outdoors via a sidewall vent. For a basement boiler, this would mean a short length of duct would connect the boiler directly to the outdoors. As its name suggests, power venting employs use of a motor and fan to forcibly remove combustion gasses from the boiler.
Another method of boiler venting is called direct venting. Similar to the power vent option, a direct line connects the boiler to the outside and a blower sends the exhaust out. The difference is in how the boiler takes in new air. Power-vented boilers take in new air from the room the boiler sits in. Direct-venting boilers, however, have an intake pipe, as well, that brings in fresh air from the outside to fuel the boiler’s fire. This means that the boiler isn’t wasting any of the heat energy being put into the indoor air by reusing that air for combustion. Instead, all heated air is used for keeping the home comfortable.
The third major method of boiler venting is called chimney venting. Where direct venting and power venting both use shorter lengths of horizontal ducting connecting the boiler to the nearby outdoors, chimney venting uses a very long vertical duct-a chimney-to pull the flue gasses up and out of the house. Chimney venting offers a distinct advantage in that it takes advantage of the thermodynamic properties of gas. It is just as the old axiom says: “Warm air rises.” Chimney vented systems, then, are themselves a sort of powered exhaust, using warm air’s levitating tendencies to push exhaust air from the boiler.