Building Design and Construction Handbook - Merritt - Ventech!

13.90 SECTION THIRTEEN
system may require cooling of the interior and heating of the exterior zones at the
same time. Thus, it is impossible to do a heating and cooling job with a single
system in such structures.
Where heating and cooling are to be done with the same duct system, the airhandling
equipment should be arranged to service individual zones—one or more
units for the exposed zones, and one or more units for the interior zones.
When a heating system is already present and an air-conditioning system is
added, a heating coil may be used to temper the outside air to room temperature.
A duct-type thermostat may be placed in the discharge of the unit to control the
steam or hot-water valve. When a room thermostat is used, the spare coil capacity
may be used for quick morning pickup. Later in the day, the system may be used
for cooling the premises with outside air if the building-heating system or other
internal heat sources overheat the premises. In buildings with large window areas,
it is advisable to place under the windows for use in cold weather some radiation
to supply heat in addition to that from the heating coil, to counteract the down draft
from the cold glass surfaces.
The heating-coil size should be such that its face area is about equal to the
cooling-coil face area. The number of rows deep should be checked with manufacturers’
ratings. When the unit is to be used for tempering only, the coil need be
sized for the fresh-air load only [Eq. (13.30)].
When large quantities of outside air are used, it is usual practice to install a
preheat coil in the fresh-air duct and a reheat coil in the air-conditioning unit. Install
the necessary filters before the preheat coil to prevent clogging.
13.35 CONTROL OF COMPUTERIZED HVAC
SYSTEMS
Control of HVAC systems ranges from simple thermostat ‘‘on-off’’ control to control
by sophisticated electronic and computerized systems. Many variations of electronic
and computerized systems are available for system control and energy management.
Programmable Thermostats. These are available for control of the total environment.
They provide energy savings and improved comfort levels and are widely
utilized to control many types of commercial unitary products. Different programmable
types are available for heat pumps and for single- and two-stage heating and
cooling equipment.
Programmable thermostats conserve energy by automatically raising or lowering
the space temperature to preprogrammed settings several times each day. Building
temperature adjustment is performed during occupied or unoccupied hours, again
conserving energy.
Other types of thermostats are available that produce energy savings:
• Reduction of overshoot on recovery from the setback temperature, limiting the
rate of temperature rise
• Selective recovery from the setback temperature by utilizing lower stages (singlestage
and multistage only)
• Selective recovery from the setback temperature using a heat pump in lieu of an
auxiliary heat source (heat-pump thermostat)