What is draft?
Draft is important to combustion efficiency and safe combustion. If draft isn’t adequate, chimneys can backdraft or spill. We measure draft between between two places in a combustion heating system. For example, we measure draft between the a combustion appliance or its vent and the mechanical room.
Types of draft
The word draft creates confusion because there are two common types of draft and a technician seldom identifies which type. The 2 types of draft are: overfire draft and vent draft. Overfire draft is the draft over the fire inside the combustion appliance and vent draft is the draft measurement in the vent. We use the term vent as a general term meaning: vertical chimney, vent connector, or horizontal vent. You measure both types of draft with reference to the mechanical room.
Many ambiguous terms describe draft and the associated combustion equipment. Atmospheric or natural draft occurs in an appliance or vent that has no fan to move the combustion gases. In these simple heaters, the flame’s heat moves the gases through the appliance and out the vent, which is usually a vertical chimney. Technicians usually measure overfire draft at 0.02 inches of water or 5 pascals. Vent draft—atmospheric or natural—varies with the height of the chimney, the chimney’s cross-sectional area, and the exhaust-gas temperature.
Fan powered draft
A variety of fans either force or induce a draft in an appliance and/or its vent. For example, forced-draft gives the appliance or its vent a positive pressure. Induced draft gives the appliance or vent a negative pressure, with reference to the mechanical room.
Fans may move exhaust gases through a combustion appliance or through its venting system or both. If we measure a negative draft in the heat exchanger or the venting system, we use the terms: draft-induced, draft inducer, induced draft, or power venter. The appliance or vent connects to the suction side of its fan or draft inducer. This type of fan sucks combustion gases through a heat exchanger or the vent and we call it a “draft inducer”. We call an appliance that has a fan downstream of the heat exchanger “draft induced.” Also, a venting system with a fan at the end of the venting system is also “draft induced” or more commonly we call it: power-vented by a power venter (the fan).
An appliance that has a fan in its burner (upstream of the heat exchanger) doesn’t have a generic name, but its burner is called a power burner. This “power burner” pressurizes the heat exchanger and provides combustion air and moves the exhaust gases through the heat exchanger. The appliance with a power burner usually vents into an atmospheric chimney. Thus, the power burner’s fan doesn’t create the chimney draft. Instead, the power burner’s heat creates the chimney draft.
Learn all about draft and venting with Saturn’s Combustion Testing and Combustion Venting (Short Course)