ASHRAE Journal Supplement - AMCA InMotion - September 2019 - 27

Perimeter diffusers. Air outlets in a perimeter
zone need to be close enough that a 150-fpm throw at the
heating airﬂow rate will project down a window. Ideally,
slot diffusers will be located a couple feet from the window
and have two-way discharge. ADPI does not really work
for heating. As a result, there are proposals to develop
rating methods based on recent research. It is important
that the responsibility for adjusting linear-slot-diffuser
pattern controllers be clearly identiﬁed at the design stage
and veriﬁed after construction. This is best done by the
installing contractor.

Terminal-Unit Selection

PHOTO 6. Fan-powered terminal unit.

A number of codes affect air-distribution-system design.

Photo courtesy of Krueger

Some reference ASHRAE standards, while others include
them. All of the standards can be considered "acceptable
standards of care"; therefore, it is best to understand what
they require.

of "pressure-independent" VAV boxes.
The inlet-ﬂow probe on VAV boxes has been improved so
that VAV boxes now are able to provide both good airﬂow

ASHRAE standards 55, 62.1, and 90.1. ANSI/

and effective temperature control. Today's VAV boxes

ASHRAE Standard 62.1 and ANSI/ASHRAE/IES 90.1, Energy

feature a magniﬁed probe. In a test of different probes,5

Standard for Buildings Except Low-Rise Residential Buildings, are

all models exhibited linear response (to the square root of

referenced in most codes. ANSI/ASHRAE 55 is referenced in

the signal) down to ﬂows unlikely to be reproducible with

many codes, but seldom speciﬁed. ANSI/ASHRAE Standard

a direct digital controller. Consequently, the transducer

62.1 and ANSI/ASHRAE/IES 90.1 are prerequisites in the

on controllers is the device that sets low-ﬂow limits today.

Leadership in Energy and Environmental Design (LEED)

Some manufacturers who used to recommend a minimum

green-building rating system and, thus, must be met, while

signal of 0.03 in. wg (7.47 Pa) now recommend one of 0.01

compliance with ANSI/ASHRAE 55 is optional and worth

in. wg (2.49 Pa).

up to three LEED points. Though ASHRAE Handbook-

Hot-water coils have limited reheat capabilities for

Fundamentals3 suggests that, if discharge temperature is

reasons related to ﬂow, turbulence, and circuitry, and their

high, excessive room stratiﬁcation is likely, ANSI/ASHRAE/

selection often is challenged by low building loads from

IES 90.1 limits the quantity of air that may be reheated,

improved thermal resistance at the perimeter of buildings.

making comfort difﬁcult to maintain with single-duct

This is compounded by lower-temperature entering hot

reheat systems in northern climates. The solution is the

water and restrictions on return-water temperature with

fan-powered terminal.

some boiler designs. Some new designs show enlarged water

Single-duct VAV reheat terminals. The single-duct
VAV box (Photo 5) has been in common use for almost 50
years. In moderate climates or buildings with supplemental

coils to reduce operating air-pressure drop.
In recent years, the industry has seen a move toward
fan-assisted VAV units.

perimeter heat, it can meet most occupants' comfort needs.

Fan-powered terminals. The fan-powered VAV

Whereas the turndown of VAV boxes once was limited by

device (Photo 6) has been in common use since the mid-

the ability of pneumatic velocity controllers, modern direct

1970s. There are two types: parallel and series.

digital controllers have greatly extended the usable range

w w w. a m c a .o r g

A parallel box induces plenum air on a call for heating

20 19 A M CA i n m o t i o n

27
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ASHRAE Journal Supplement - AMCA InMotion - September 2019

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