ASHRAE Journal - January 2014 - 59

COLUMN HVAC APPLICATIONS
have a water coil located on the plenum inlet, caution
must be used to not overheat the downstream fan motor.
To be safe, the coil discharge temperatures should be
limited to no more than 110°F (43°C). Engineers should
take special care to ensure that both occupied heating
and morning warm-up situations are covered in the
design with proper stages of electric heat or proportional
water coil valves.
The need to rapidly warm a space following a night
setback has its own set of requirements. Air needs to be
heated as rapidly as possible, with a maximum of mixing,
without too much regard for occupant comfort.
While this requires both a highDT and a high airflow, the
physics of room air movement dictate that the hotter the
air temperature, the longer it will take to heat the room
for a given heat delivery rate due to stratification.
On the surface, reheating cold primary air seems to
be a wasteful practice in terms of optimum utilization
of energy resources; however, there are many situations
where it is not only necessary, but also beneficial to do so.
Providing Comfort with a Great Diversity of Loads.
When heating and cooling are required from a single
air-distribution system due to climate and building
design factors, reheat is often an economical solution.
Considering it is used in a few locations only part of the
time, the energy penalty for reheat is minimal.
Supplementing Baseboard Perimeter Heat.
Baseboard heating systems can be the most effective
means by which to offset perimeter heating demand
loads. At times, however, peak heating demand loads
may exceed the installed baseboard capacity and may
require supplemental overhead heating.
Maintaining Minimum Ventilation Rates. The ben-
efit of an installed reheat coil in a non-perimeter zone
becomes apparent when the minimum ventilation rate
exceeds cooling demand. This happens when the quantity
of supply air to a space required to provide proper
ventilation exceeds the quantity required to offset local
heat sources, such as when the ratio of occupants to
equipment (which requires little ventilation air) shifts
toward occupants, as in conference rooms. As was
observed in the recent ASHRAE research study RP-1515,
modern computers and flat panel monitors give off such
small amounts of heat that it resulted in interior systems
going into reheat at minimum VAV settings. The system
then maintained a heating setpoint several degrees
below the normal interior temperatures, as required
FIGURE 1 Standard 62.1 rule on overhead heating.
by California code. Occupant complaints were frequent
until VAV box minimums were lowered.
If ventilation minimums exceed the load or humid-
ity control requires a higher airflow, a slight amount of
controlled reheat can prevent subcooling. It is important
that the room setpoint be maintained at the cooling setpoint,
which for interior zones should be all the time.
The alternative is to reduce supply air temperature at the
air handler. This may result in other spaces that cannot
be cooled at design maximum airflows and may increase
the space's relative humidity.
Controlling Humidity. Both humidity and ventilation
can be better controlled using reheat coils. When
the local humidity is too high, drier, cooler air can be
added, then slightly reheated to avoid subcooling. Fan
coils may require both heating and cooling coils to be
operated at the same time to control space temperatures
and avoid sub-cooling. They also have to supply
ventilation air. For this, a dedicated outdoor air system
(DOAS) can be used to assist fan coils in dehumidifying
as well as ventilating a space. In the case of fan powered
VAV terminals, a sensible cooling coil on the induction
inlet can offer increased flexibility and countless
control options when coordinated with the ventilation
supply unit.
Water Coil Selection
Most manufacturers provide software to accurately
predict the performance of their products. In today's
environments, it is common to see lower heating inlet
water temperatures as well as higher water temperatures
in sensible cooling coil applications. It is recommended
that coil schedules be prepared using a program that
has inlet and installation effects dialed in, as meeting
the required Btu/h and gpm in a specification is often
impossible if mounting condition factors are not used in
the calculations. Unlike many air handlers, fin spacing,
circuiting, and tube spacing are fixed for most VAV fan
box and fan coil water coils.
JAN UARY 2014 ashrae.org A SHR A E J O U RNA L
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ASHRAE Journal - January 2014

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