The Tjernlund Duct Airstat senses positive pressure in a forced air heating and/or cooling system. The single pole, single throw, snap-acting switch is activated by a diaphragm with integral pressure probe inserted directly into the duct or plenum.The Duct Airstat can be used to initiate duct boosters, or other auxiliary devices as duct boosters, electronic air cleaners, electrical duct heaters, and humidifiers. The electrical contacts of the switch are closed upon sensing positive air pressure generated by the blower.