APPENDIX C (Continued)
supply air temperature might be appropriate. Also there are
cases in dry climates where it would be permissible to vary
supply air temperature.
Do not use a return air fan in a VAV system. Measuring
and control requirements for tracking the return air fan with the
supply air fan are too demanding to work in a Naval VAV system.
In most cases a return fan is unnecessary to return air to the
HVAC unit. In cases where the pressure drop in the return duct
exceeds the drop for outdoor air, the mixing box may be at a
lower suction pressure. Select outside air and return dampers
with this in mind.
Economizers should be used when significant benefits
can be shown. (Clearly dry or cold climates are cases where
economizers can be effective.) Feedback: Economizers in Navy
VAV systems fail in a majority of cases due to complex
arrangements. Keep economizer systems simple and use only
outdoor air dry bulb sensors for changeover. The economizing
feature can save a lot of energy in many applications but the
designer must confer with the appropriate NAVFACENGCOM EFD or EFA
when using this design.
a) Economizers should have outdoor air dry bulb type
changeover instead of outdoor air versus return air comparators
or enthalpy type changeovers.
b) Economizers should only be used when the system can
be designed with gravity relief. Return or relief fans should
not be used.
Maintaining Ventilation Air. In most systems there are
circumstances under which satisfying the cooling load will not
adequately ventilate the space. Unless it can be shown that this
is not the case, the designer shall design a positive means of
maintaining ventilation rates during minimum flow conditions, to
maintain IAQ. Select minimum positions of VAV terminal units to
meet this requirement. Note: It is impossible with a VAV system
to absolutely maintain a minimum ventilation airflow to any
space. The best that can be done is to maintain a constant
ventilation airflow on a per air handling unit basis and
recognize that ventilation will be improved in any zone that is
shorted by mixing between rooms (especially in zones where
fan-powered boxes are used) and shorted rooms tend to be those
where people and lights are not present, and thus the ventilation
requirements are lower anyway. Use ASHRAE Standard 62 to