Summary of definitions

adverse pressure: is a pressure that inhibits flow in the desired direction. The negative pressure in a basement is an adverse pressure because it works against chimney draft.

air changes per hour (ACH): the number of times in an hour that all of the air in a building is replaced with outdoor air.

backdraft: when the upward flow in a chimney fully reverses and 100% of the combustion gases from the appliance (if it is firing) and air in the chimney flow into the building.

building envelope: the surfaces, formed by all components of the building, that enclose the conditioned (heated or cooled) space.

chimney: a primarily vertical shaft enclosing at least one flue for conducting gases to the outdoors

combustion air supply: is air from outdoors supplied directly through a duct to the appliance combustion chamber.

combustion spillage: when some of the products of combustion are released into the building.

draft: the pressure difference which is available to drive the flow of air and/or combustion gases through an appliance and its venting system.

draft, natural: the pressure difference created in a venting system by the temperature difference between the air and/or combustion gases in the venting system and the outdoor air.

driving pressure: is a pressure that produces flow in the desired direction. Heat in a chimney produces draft which is a driving pressure. Wind blowing over the top of a chimney produces a driving pressure that assists in pulling exhaust gases from the chimney.

effective stack (height): refers to the relative performance of the stack (house or chimney), normally in standby mode, rather than specifically to its linear height, but is influenced by actual height as well as temperature difference.

energy momentum: the tendency of a venting system to continue to produce draft after combustion has stopped as the heat energy stored in the materials of the system is released into the flue.

equivalent leakage area (ELA): the size of hole you would get if all the leaks in a house could be gathered together in one place.

neutral pressure plane (NPP): the level between the high pressure zone at upper levels and the low pressure at lower levels in a house at which the pressure is equal to atmospheric pressure.

passive make-up air supply: is air from outdoors supplied indirectly in the form of a duct terminating in proximity to the combustion appliance; this is the 'hole in the wall' approach to air supply.

stack effect: the pressure difference created in a building by the temperature difference between the inside air and outdoor air.

venting: the action of air or gases escaping to the outdoors