ASHRAE Journal - ACMA Supplement - Fall 2013 - 17

Running spiral ducts to diffusers will minimize leakage and resistance to air flow.
spaces lowers the ventilation heating requirements on the perimeter,
and maximizes the benefit of free cooling for interior spaces.
VEntilation and indoor air quality

Air systems integrate ventilation and heating/cooling functions
in a single ducted delivery system. Wet systems require a separate
duct system to deliver ventilation air, in addition to the piping
required for heating and cooling. In schools and other high people
density applications, dedicated ventilation ductwork becomes
quite large. This “dual system” approach raises first cost, while
free cooling capacity is compromised. Of course, ventilation loads
are very impactful in any system, so controls to deliver only
enough outside air to satisfy the needs of occupants is critical—
with any system. VAV systems provide a means of throttling
ventilation to meet measured demand in each zone. Dedicated
ventilation used with piped systems does not generally incorporate
zone level damper controls, and therefore cannot take full advantage of demand based control of ventilation. Today’s VAV systems
can use demand-limiting ventilation based on ASHRAE Standard
62 requirements. This strategy provides the right amount of outside
air—based on changing occupancy rates—to eliminate overventilation, which wastes energy, or under-ventilation, which
compromises air quality and occupant health. This difference
results in significant energy savings compared to most wet systems
that use a constant, unregulated amount of outside air. VAV systems
also use an air handler or rooftop unit where higher levels of filtration are easily accomplished. UV protection of cooling coils,
MERV-13 filtration, and advanced photo-catalytic air cleaning
systems become feasible. Enthalpy recovery from exhaust is also
more cost effective, and practical in centralized systems using air
as their energy transport medium.
HEat rEcoVEry

In the early days, VAV systems used simple electric reheat to
condition perimeter zones in winter. This was a step up from
w w w. a m c a . o r g

reheat without reduced airflow—and it is still practiced—but
high-performance VAV systems eliminate reheat in favor of heat
recovery. Fan-powered VAV boxes recover heated ceiling plenum
air. If additional heat is needed, hot water or electric energy is
added to the recovered hot plenum air. The need for additional
perimeter heat can be eliminated by adding enough inexpensive
insulation on the perimeter so that heat loss is less than the
recovered heat gain of internal loads. Of course, cool primary
air from the HVAC system must be heated, but this load is limited
to meet ventilation requirements. This is no different for any
system. If it’s cold outside, ventilation air is going to have to be
heated in any perimeter system. There are nuances here. Is the
VAV box fan in series or parallel? Or, is there a dedicated heating
system with dual duct VAV boxes? Are separate air handler/
rooftop units dedicated to perimeter zones separated by exposure?
Is the supply air temperature reset upward as cooling loads
diminish? These differences drive first cost, and certainly impact
energy cost. Key here is this common attribute: high-performance
VAV uses heat recovery and does not reheat beyond that needed
in any system to temper adequate ventilation air.
EfficiEnt fans and capacity control

Three modern improvements in fan energy use are embraced in
the high-performance air system. All stand-alone fans now carry
an efficiency rating called a fan efficiency grade (FEG). ASHRAE
90.1 and model energy codes establish minimum FEG levels
and limit applications to assure efficient operation. This encourages system designers to choose efficient fans operating close
(within 10%) of their peak efficiency. Second is the use of variable speed capacity control for fans. Electrically commutated
(EC) motors with permanent magnets are cost effective at smaller
horsepower ratings, with variable frequency drives used in larger
sizes. The third improvement is a method of controlling fan
capacity that provides sufficient duct pressure to satisfy only the
most demanding zone. Hence, the high-performance air system

a m c a I n t e r nat I o na l

inmotion

Fall 2013

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ASHRAE Journal - ACMA Supplement - Fall 2013

Table of Contents for the Digital Edition of ASHRAE Journal - ACMA Supplement - Fall 2013