Method of venting a furnace

An improved furnace is provided which can be field modified to change the pressure drop across the inducer. The pressure drop is modified to accommodate vertical or horizontal venting and adequately lined and inadequately lined chimneys. A first rate of flow of combustion byproducts into the vent system is used when the chimney is adequately lined and a second relatively higher rate of flow is used when the chimney is inadequately lined. The rate of flow is changed by inserting or removing an obstruction in the flow of combustion byproducts. The obstruction could be a tab or choke placed at the inducer outlet or a similar device in the inducer inlet. When the tab is in place the flow of combustion byproducts is reduced. When the tab is removed, the flow rate increases. Alternatively, the inducer inlet could be adjusted to increase or decrease the amount of air flow through the inducer.