APPENDIX C (Continued)
a) For a mild or moderate heating situation when the
peak heating load on the exterior wall is less than 400 Btuh per
linear foot of wall, the following arrangement is recommended.
Each zone should be served by the series fan-powered VAV box with
a hot water heating coil and overhead supply diffusers. The
thermostat should modulate the VAV damper closed to the
mechanically field set minimum position at the cooling setpoint.
On the further drop in space temperature to the heating setpoint,
the hot water valve should open (using a modulating electric
spring-return actuator) to allow control of heating. An
alternate arrangement uses a parallel fan-powered VAV box with
heating coil. The VAV box fan motor should be started when the
VAV damper closes to a supply rate of 4 air changes per hour.
b) Since the use of fan-powered boxes adds several
hundred dollars per zone of first cost, the zoned baseboard heat
option (described for very cold climates) may be considered, even
in mild climates.
c) In perimeter zones there will be a need to setback
the temperature in many buildings when there is no occupancy.
This is generally accomplished by a night setback thermostat
which can bring the heating system back on line to maintain the
In perimeter zones with a VAV cooling system and a
separate perimeter heating system, design controls so that
occupants cannot adjust thermostats for simultaneous heating and
cooling. This can be done by using a thermostat with cooling and
heating setpoints integrated so that ranges of possible
adjustment do not overlap.
Systems that are to be shut down or setback during
unoccupied periods can present special problems. The greatest
load will occur during start-up. The supply air or coil shall be
capable of bringing the temperature back to design in a
reasonable time without ill effects. For instance, the designer
must be careful that the temperature of supply air during
start-up is not too far below the room dew point temperature to
Specify a method for building warm-up after setback.
Many systems contain totally interior zones that have been
designed without heat. The space, after setback over a weekend,
can require an unacceptably long period of time to come up to
temperature. This warm-up sequence could be accomplished by