Thursday, March 29, 2007

Analog vs DDC HVAC Control

To be sure DDC control through central control rooms have a great advantage for the operators of large facilities. DDC allows you to interface with the control systems, it allows the operators to change the set-points and operating characteristics of terminal boxes, fans etc depending on the needs of the moment. This may be a major advantage in large complicated buildings with a variety of different types of usage's.

DDC control however, may not be required in smaller facilities with very simple operating requirements. In these types of facilities many times it is less expensive to install stand alone analog VAV control. These will include a controller and space thermostat that will operate respective terminal boxes, the combination may be stand alone analog while DDC may still be utilized for central fans, chillers etc.

For example, in a large facility say with 2000 terminal boxes, if stand alone analog is utilized for control of the terminal boxes in lieu of DDC the savings may be considerable. The cost of savings may be as great as that shown below.

Stand-alone Analog Terminal box Control:
Controller, Thermostat & Wiring = $225

DDC Terminal Box Control:
Controller, Thermostat & Wiring = $400

The difference for the Terminal box control can be as great as $175 per terminal box. So, for the 2000 box example the total savings to go stand alone analog may be as great as $350,000. Of course there are DDC short cuts that can be taken that will reduce this cost difference, so the designer will have to evaluate all possibilities.

Remember that DDC will allow interactive communication between the item being controlled and the central control computer. Stand alone sacrifices this communication mode to produce a reduction in price, however, all other control characteristics between the two methods are basically equal, additionally with many stand alone systems wireless or LAN modes can be utilized to tie the systems together.