30 November 1998
STANDARD HVAC CONTROL SYSTEMS
a. The complete HVAC control system design will include a series of drawings for each HVAC
control system, based on the standard control systems presented in this chapter.
b. Variations to these control systems are shown in chapter 5.
c. Air delivery systems can be designed with or without return fans. For example, selected systems
are shown with return fans. These HVAC systems are the multizone system, the bypass multizone
system, the dual-duct system, and the VAV system.
d. Where required, the control of a return fan can be incorporated into any air delivery HVAC system
shown in this manual by appropriately modifying the HVAC control system drawings.
e. The decision to use only a supply fan for HVAC systems versus the use of both a supply and a
return fan is based on the following: lower first cost, lower maintenance cost, lower energy costs, and
less complex control systems. The advantages of the supply / return fan system include positive control
of the outside air, return air, and relief air quantities under varying modes of operation. Return fans are
sized smaller than the supply fan by the amount of air necessary for minimum outside air, space
f. In the case of the VAV system, the control loop necessary to allow the return air flow to vary as
the supply air flow changes is known as the return fan volume control loop. This loop is costly, complex,
and, in some applications, difficult to control with stability. A return fan should be used in a VAV system
only when absolutely necessary. The need for a return fan can be reduced by keeping return duct losses
to a minimum using a plenum or making the return duct as short and as large as possible.
g. The transmitter ranges shown are standard. These ranges match corresponding EMCS ranges
where appropriate, and will not be changed.
h. The setpoints, timeclock schedules, etc. used throughout this chapter are used for example
purposes only. The actual values to be used in contract documents must be determined by the designer
as appropriate for each project and location.
2. IDENTIFICATION OF CONTROL SYSTEM DEVICES. Control system devices will be numbered in
accordance with the convention described for the unique identifiers shown in Section II of the Glossary.
3. PROJECT SPECIFIC DRAWINGS. To make the standard HVAC control system drawings project
a. Add or delete loops as required.
b. Use a common time clock and common control panel where appropriate for HVAC systems (as
described in chapter 5) unless the HVAC system is extended to EMCS.
c. If the project has EMCS, the time clock may be deleted and replaced with EMCS contacts.