How does it work ?
A basic oil-fired, forced-air heating system consists of a burner fed by heating oil from a storage tank usually located inside the house firing into a combustion chamber in the furnace. The combustion gases pass through the furnace, where they give up heat across a heat exchanger. They are then exhausted to the outside through a flue pipe and chimney. For most systems, a barometric damper, acting as a mixing valve in the flue pipe, downstream of the furnace proper, isolates the burner from changes in pressure at the chimney exit by pulling varying quantities of heated room air into the exhaust. A circulating fan passes cool house air from the cold air return ducts over the furnace heat exchanger, where it is warmed up, then moved into the hot air ducts, which distribute the heated air throughout the house.
Note that there are two entirely separate air movement paths within the furnace.
A third air path, external to the furnace, is the dilution air pulled through the barometric damper.
Avoid heat loss
In most houses, the quantity of dilution air drawn through the barometric damper is much greater than required for combustion and can represent from 3 to 15 percent of the total heat loss in the house. Thus, anything that reduces this dilution airflow without compromising the performance of the furnace will lead to increased fuel savings and efficiency.
Many oil furnaces can be sidewall-vented by using an additional induced draft fan, normally located downstream of the furnace and barometric damper, located on the inside wall of the house. Some of these have a long run time after the burner shuts off in order to purge the furnace system of any combustion gases. The use of the barometric damper and long purge time both tend to reduce efficiency.
Some newer furnaces might have an optional direct connection for outside air for combustion (sealed combustion) instead of using indoor air. Care must be taken if this approach is followed. On a cold winter day, if the air is not warmed somewhat before it reaches the burner, it could cool the fuel oil and cause start-up problems.