There are four types of automatic control systems:
1. Electric - Controls using line voltage (usually 110V or 24V) and usually only control on/off functionality. The most popular type of electric controller is a timer.
2. Pneumatic - These systems use compressed air to operate diaphragms and mechanical relays to position dampers and valves. Once installed, it is difficult to change the sequence of operations. By the end of the 1990's, pneumatic controls were replaced with direct digital controls (DDC) with the possible exception of very large dampers and valves.
3. Electronic - Most of the first generation electronic controls included relays that switched between fixed set-points, and were not very popular. New DDC technology with micro-electronics can contain a complex operating sequence on site, or be connected to a central point.
4. Self Contained - These controls differ from the above in that they do not use an external power source. For example, A valve with a bellows diaphragm has enough pressure contained in the bladder to close it if the operating pressure drops.
There are two control categories:
1. Operating Control - This is when the room conditions are measured and adjusted to meet system set-points.
2. Safety Control - Operating limits are placed on equipment such that if the conditions exceed the limits, the controls interrupt the operating controls. For example, a freeze stat will close the fresh air damper if conditions at the heating coil reach freezing temperatures.