The COP2 Constant Operating Pressure Control includes a VFD that will modulate the speed of an approved Tjernlund exhaust fan to maintain a user adjustable negative pressure set point. The set point is adjusted through dip switches mounted on the COP2 circuit board. The operating range of the COP2 control is -0.05” W.C. to -0.53” W.C. in 0.01” W.C. increments. Pressure is measured in a chase, duct or vent at the farthest point from the exhaust fan with the pressure transducer and sensing tube kit. As exhaust volume increases within the duct/chase/vent the resulting reduction in measured pressure causes the COP2 control VFD to speed the fan up to handle the additional exhaust volume and slow the fan down when the exhaust volume is reduced to maintain a constant exhaust pressure.