A measurement of heat intensity

Terminal Velocity

(fluid mechanics) The velocity with which a body moves relative to a fluid when the resultant force acting on it (due to friction, gravity, and so forth) is zero. In context of this class: Air begins to fall vertically when it slows to a certain speed. (usually at a velocity 50-75 feet per minute)

Thermal Boundary

The layer in a building enclosure that controls the transfer of energy (heat) between the interior and the exterior. It is a component of the building enclosure and it may, but does not have to align with the pressure boundary.

Thermal Bridge

A material with higher thermal conductivity transferring heat through an assembly faster than one with substantially lower thermal conductivity. For example, a steel stud in a wall will transfer more heat than the surrounding insulation, reducing the overall thermal performance of the system.


Is the distance (measured in feet) that the air stream travels from the outlet to the point of terminal velocity. Throw is measured vertically from perimeter diffusers and horizontally from registers and ceiling diffusers.


Heat pumps and air conditioners are generally sized in tons. Typical sizes for single family residences are between two and five tons. Each ton equals 12,000 Btuh. It is important to note that actual capacity is not constant and will change based on outdoor or indoor temperatures. The published capacity rating of air conditioners and heat pumps is based on performance at the ARI standard temperature levels of 95 F outside, 80 F inside.

Transfer Grille

An intentional opening used to relieve HVAC supply pressure, preventing unintentional pressurization or depressurization of spaces.