Users Guide to Direct Digital Control of Heating, Ventilating, and Air Conditioning Equipment,

Heating, ventilating, and air conditioning HVAC systems are now under going the most significant change in control technology in the past 100 years. This change is the introduction of direct digital control DDC to HVAC systems. Computer control has been introduced to many things over the past few years, from automobile engines to home appliances to most industrial processes, but only recently have small computer control systems been introduced into the space conditioning industry. To understand the implications of this new HVAC control technology, it is necessary to consider several questions What is DDC and what are its capabilities Under what conditions is DDC a cost effective alternative to conventional control What constitutes good system design and installation practice This report addresses these questions by presenting information which describes DDC, presents its advantages and disadvantages, and gives applications guidance. Direct digital control uses a programmable digital computer to process information for the purpose of determining the correct control action. The input information for the computer comes from analog sensors such as temperature sensors and digital sensors such as switches. This information is used as variable data in a predetermined set of instructions called the control program.