Page 109

NORA Oilheat Technicians Manual

Chapter 6 Draft and Venting Hot Air Causes Lower Weight (Pressure) at Bottom of Chimney than Cold Air Figure 6-1: Chimney draft Hot Chimney Cold Chimney Chapter 6—Draft and Venting 6-3 Introduction Air is needed to burn oil cleanly and efficiently. We must understand how to supply air to the burner and how to ensure that all of the gases created in burning the fuel are vented to the outside. Additionally, for non-condensing appliances, we must make sure the water vapor created during combustion is vented to the outside prior to condensing. What is draft? Draft is a current of air in an enclosed area that is created by a difference in pressure. In practical terms, draft is a force that “pulls” or “sucks” the exhaust gases out of the heating unit and sends them up the chimney. During the combustion process hot gases rise through the heating appliance to the flue pipe and travel up the chimney, creating negative pressure or suction, also known as “negative draft” at the bottom of the chimney. The pressure difference is created because: • When the burner is off and the chimney is cold, the air inside the combustion area, heat exchanger, flue pipe, and chimney is at atmospheric pressure. • When the burner starts, the burner fan creates “static pressure” as it pushes air into the combustion area where it combines with the fuel to create a fire. • When the air and fuel burn, the temperature rises dramatically and the combustion gases expand to more than double their volume. This expansive pressure adds to the pressure created by the burner fan and pushes the combustion gases through the heat exchanger. • As the hot combustion gases travel up the chimney they create a pressure drop behind them that sucks the combustion gases out of the heat exchanger, Figure 6-1. Draft is the total effect of the positive pressures of the burner fan, the expansive pressure of the flame, and the negative pressure of the hot gases escaping the top of the chimney. Chapter 6 Draft and Venting


NORA Oilheat Technicians Manual
To see the actual publication please follow the link above