The application of HVAC controls starts with an understanding of the building and HVAC systems, and the use of the spaces to be conditioned and controlled. The type of HVAC system determines the control sequence. Several types of control products such as pneumatic, electric, analog electronic, or electronic direct digital control (DDC) can then do the basic control sequence.

Background. At one time, draft dampers ( by thermostat control of the dampers) controlled heating. The use of mechanical stokers for coal firing required another step in the use of control. When oil burners were introduced, the concept of combustion safety control became necessary. This involved the sensing and proof-of-flame in the proper time sequence of introducing draft, fuel, and ignition.

The use of fans to deliver ventilation as well as heated air was controlled by dampers, which varied the source and volume of air. The typical control of unit ventilators was by pneumatic controls and included the following features: minimum outside air, discharge air, low-temperature lim, and thermostats with lower night settings activated by compressed supply pressure level. The increased usage of air conditioning led more complex control sequences in larger systems to central monitoring and control.