ASHRAE Journal - December 2013 - 18

technical feature

or heating the outdoor air.3 In fact, Section 6.4.3.9
of ASHRAE Standard 90.1-2010 requires DCV in any
zone with a design occupancy greater than 40 people
per 1,000 ft2 (100 m2); and the 2013 edition lowers
this threshold to any zone with 25 or more people per
1,000 ft2 (100 m2).
Section 6.2.7 of ASHRAE Standard 62.1-2010 explicitly
permits the use of DCV, and lists some common DCV
technologies (such as CO2 sensors and occupancy sensors). This section also includes requirements for how
DCV is to be implemented. For example, while the controls can reduce the people component of the ventilation
rate (Rp × Pz) during periods of reduced population, the
outdoor airflow delivered to a zone cannot be any lower
than the building component of the ventilation rate (Ra
× Az).
"6.2.7.1.2. The breathing zone outdoor airflow (Vbz) shall
be reset in response to current occupancy and shall be no
less than the building component (Ra × Az) of the DCV
zone. Note: Examples of reset methods or devices include
population counters, carbon dioxide (CO2) sensors, timers,
occupancy schedules or occupancy sensors."
Section 8.3 clarifies that this applies whenever a
zone is "expected" to be occupied. That is, the standard does not require this building component of
the ventilation rate to be delivered to every zone 24

Cold Vs. Neutral Air Delivery
Many dedicated OA systems are designed to dehumidify the outdoor air-dry enough to offset all or
most of the indoor latent loads-and then reheat it to
approximately zone temperature (neutral). Delivering the dehumidified outdoor air at a neutral drybulb temperature can simplify control because it has
no impact on the zone sensible cooling or heating
loads.
However, when a chilled-water or direct-expansion
(DX) cooling coil is used for dehumidification, a byproduct of that process is that the dry-bulb temperature
of the air leaving the coil is colder than the zone. If the
dehumidified outdoor air is reheated to neutral, most of
the sensible cooling performed by the dedicated OA unit
is wasted.
If the dedicated OA system delivers air directly to
each zone (Figure 4), the dehumidified outdoor air can
be delivered cold, rather than reheated to neutral for
much of the year. The cold dry-bulb temperature of the
conditioned outdoor air (CA) offsets part of the sensible
cooling load in the zone, reducing the energy used by
the local terminal units. In addition, at design conditions, this may allow the terminal units to be smaller-
sized for less airflow and less cooling capacity-than in a
neutral-air system.1,2

Table 1 Comparison of ventilation control methods used with variable speed terminal fan control.
VenTilaTion
ConTrol MeThod

adVanTages

drawbaCks

* Less expensive and less maintenance than
direct flow measurement
* Well-suited for use with automatic, two-speed fan control.

* Less accurate (more over-ventilation) than proportional
control when variable speed fan control is used.
* Requires two OA damper position settings by installer.

Proportional Control
of OA Damper

* Less expensive and less maintenance than direct flow measurement.
* More accurate (less over-ventilation) than two-position damper
control when variable speed fan control is used.

* Less accurate (more over-ventilation) than direct flow
measurement.
* Requires two OA damper position settings by installer.

Direct OA Flow
Measurement
And Control

* More accurate (less over-ventilation) than two-position or
proportional control.
* Affords the opportunity to document outdoor airflow delivered.

* More expensive, and may require more space, than twoposition or proportional control.
* Added maintenance of airflow-measurement device.

Deliver OA Directly
To Each Zone
With a DOAS

* Outdoor airflow not impacted by variable speed terminal fan control.
* Affords opportunity to dehumidify OA centrally and deliver it
cold (rather than reheated to neutral).

* May require installation of pressure-independent dampers to
enable shutting off airflow to a zone or DCV.

Two-Position OA
Damper Control

18

ASHRAE JouRnAl

ashrae.org

D ecem ber 2013



ASHRAE Journal - December 2013

Table of Contents for the Digital Edition of ASHRAE Journal - December 2013

Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Ventilation Control in Terminal Units With Variable Speed Fan Control
Cooling Load Calculations for Radiant Systems
Botanical Research Laboratory
Strategies for Sustainability
Technical vs. Process Commissioning: Preparing a Cx Plan
Standing Columns
Engineer's Notebook
Building Sciences
The Performance Gap
Refrigeration Applications
New Product Preview
Energy Modeling
Emerging Technologies
HVAC Applications
Data Centers
Products
2013 Indices
Classified Advertising
Advertisers Index
ASHRAE Journal - December 2013 - Intro
ASHRAE Journal - December 2013 - Cover1
ASHRAE Journal - December 2013 - Cover2
ASHRAE Journal - December 2013 - 1
ASHRAE Journal - December 2013 - 2
ASHRAE Journal - December 2013 - Contents
ASHRAE Journal - December 2013 - Commentary
ASHRAE Journal - December 2013 - 5
ASHRAE Journal - December 2013 - Industry News
ASHRAE Journal - December 2013 - 7
ASHRAE Journal - December 2013 - Letters
ASHRAE Journal - December 2013 - 9
ASHRAE Journal - December 2013 - Meetings and Shows
ASHRAE Journal - December 2013 - 11
ASHRAE Journal - December 2013 - Ventilation Control in Terminal Units With Variable Speed Fan Control
ASHRAE Journal - December 2013 - 13
ASHRAE Journal - December 2013 - 14
ASHRAE Journal - December 2013 - 15
ASHRAE Journal - December 2013 - 16
ASHRAE Journal - December 2013 - 17
ASHRAE Journal - December 2013 - 18
ASHRAE Journal - December 2013 - 19
ASHRAE Journal - December 2013 - Cooling Load Calculations for Radiant Systems
ASHRAE Journal - December 2013 - 21
ASHRAE Journal - December 2013 - 22
ASHRAE Journal - December 2013 - 23
ASHRAE Journal - December 2013 - 24
ASHRAE Journal - December 2013 - 25
ASHRAE Journal - December 2013 - 26
ASHRAE Journal - December 2013 - 27
ASHRAE Journal - December 2013 - Botanical Research Laboratory
ASHRAE Journal - December 2013 - 29
ASHRAE Journal - December 2013 - 30
ASHRAE Journal - December 2013 - 31
ASHRAE Journal - December 2013 - 32
ASHRAE Journal - December 2013 - 33
ASHRAE Journal - December 2013 - Strategies for Sustainability
ASHRAE Journal - December 2013 - 35
ASHRAE Journal - December 2013 - 36
ASHRAE Journal - December 2013 - 37
ASHRAE Journal - December 2013 - 38
ASHRAE Journal - December 2013 - 39
ASHRAE Journal - December 2013 - Technical vs. Process Commissioning: Preparing a Cx Plan
ASHRAE Journal - December 2013 - 41
ASHRAE Journal - December 2013 - 42
ASHRAE Journal - December 2013 - 43
ASHRAE Journal - December 2013 - 44
ASHRAE Journal - December 2013 - 45
ASHRAE Journal - December 2013 - 46
ASHRAE Journal - December 2013 - 47
ASHRAE Journal - December 2013 - Engineer's Notebook
ASHRAE Journal - December 2013 - 49
ASHRAE Journal - December 2013 - 50
ASHRAE Journal - December 2013 - 51
ASHRAE Journal - December 2013 - Building Sciences
ASHRAE Journal - December 2013 - 53
ASHRAE Journal - December 2013 - 54
ASHRAE Journal - December 2013 - 55
ASHRAE Journal - December 2013 - 56
ASHRAE Journal - December 2013 - 57
ASHRAE Journal - December 2013 - 58
ASHRAE Journal - December 2013 - 59
ASHRAE Journal - December 2013 - The Performance Gap
ASHRAE Journal - December 2013 - 61
ASHRAE Journal - December 2013 - 62
ASHRAE Journal - December 2013 - 63
ASHRAE Journal - December 2013 - Refrigeration Applications
ASHRAE Journal - December 2013 - New Product Preview
ASHRAE Journal - December 2013 - 66
ASHRAE Journal - December 2013 - 67
ASHRAE Journal - December 2013 - 68
ASHRAE Journal - December 2013 - 69
ASHRAE Journal - December 2013 - 70
ASHRAE Journal - December 2013 - 71
ASHRAE Journal - December 2013 - 72
ASHRAE Journal - December 2013 - 73
ASHRAE Journal - December 2013 - 74
ASHRAE Journal - December 2013 - 75
ASHRAE Journal - December 2013 - 76
ASHRAE Journal - December 2013 - 77
ASHRAE Journal - December 2013 - 78
ASHRAE Journal - December 2013 - 79
ASHRAE Journal - December 2013 - 80
ASHRAE Journal - December 2013 - 81
ASHRAE Journal - December 2013 - 82
ASHRAE Journal - December 2013 - 83
ASHRAE Journal - December 2013 - 84
ASHRAE Journal - December 2013 - 85
ASHRAE Journal - December 2013 - 86
ASHRAE Journal - December 2013 - 87
ASHRAE Journal - December 2013 - 88
ASHRAE Journal - December 2013 - 89
ASHRAE Journal - December 2013 - 90
ASHRAE Journal - December 2013 - 91
ASHRAE Journal - December 2013 - 92
ASHRAE Journal - December 2013 - 93
ASHRAE Journal - December 2013 - 94
ASHRAE Journal - December 2013 - 95
ASHRAE Journal - December 2013 - 96
ASHRAE Journal - December 2013 - 97
ASHRAE Journal - December 2013 - 98
ASHRAE Journal - December 2013 - 99
ASHRAE Journal - December 2013 - 100
ASHRAE Journal - December 2013 - 101
ASHRAE Journal - December 2013 - 102
ASHRAE Journal - December 2013 - 103
ASHRAE Journal - December 2013 - 104
ASHRAE Journal - December 2013 - 105
ASHRAE Journal - December 2013 - 106
ASHRAE Journal - December 2013 - 107
ASHRAE Journal - December 2013 - 108
ASHRAE Journal - December 2013 - 109
ASHRAE Journal - December 2013 - 110
ASHRAE Journal - December 2013 - 111
ASHRAE Journal - December 2013 - 112
ASHRAE Journal - December 2013 - 113
ASHRAE Journal - December 2013 - 114
ASHRAE Journal - December 2013 - 115
ASHRAE Journal - December 2013 - 116
ASHRAE Journal - December 2013 - 117
ASHRAE Journal - December 2013 - 118
ASHRAE Journal - December 2013 - 119
ASHRAE Journal - December 2013 - 120
ASHRAE Journal - December 2013 - 121
ASHRAE Journal - December 2013 - 122
ASHRAE Journal - December 2013 - 123
ASHRAE Journal - December 2013 - 124
ASHRAE Journal - December 2013 - 125
ASHRAE Journal - December 2013 - 126
ASHRAE Journal - December 2013 - 127
ASHRAE Journal - December 2013 - 128
ASHRAE Journal - December 2013 - 129
ASHRAE Journal - December 2013 - 130
ASHRAE Journal - December 2013 - Energy Modeling
ASHRAE Journal - December 2013 - Emerging Technologies
ASHRAE Journal - December 2013 - 133
ASHRAE Journal - December 2013 - 134
ASHRAE Journal - December 2013 - 135
ASHRAE Journal - December 2013 - 136
ASHRAE Journal - December 2013 - 137
ASHRAE Journal - December 2013 - HVAC Applications
ASHRAE Journal - December 2013 - 139
ASHRAE Journal - December 2013 - 140
ASHRAE Journal - December 2013 - 141
ASHRAE Journal - December 2013 - Data Centers
ASHRAE Journal - December 2013 - 143
ASHRAE Journal - December 2013 - 144
ASHRAE Journal - December 2013 - Products
ASHRAE Journal - December 2013 - 146
ASHRAE Journal - December 2013 - 147
ASHRAE Journal - December 2013 - 2013 Indices
ASHRAE Journal - December 2013 - 149
ASHRAE Journal - December 2013 - 150
ASHRAE Journal - December 2013 - Classified Advertising
ASHRAE Journal - December 2013 - Advertisers Index
ASHRAE Journal - December 2013 - Cover3
ASHRAE Journal - December 2013 - Cover4
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