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					Edition No. 5                                                                                                        October 2000

S       Y       S       T       E        M                S         O         L         U      T         I       O         N           S
                                             Four-pipe Unit Ventilator Performance
     n this issue, we describe the

 I   benefits of two-pipe unit
     ventilator system design for
 schools, and provide an approach
                                             With A Two-pipe Design
                                             Unit ventilators have been providing a
                                             healthy, comfortable environment that
                                                                                             4 Pipe System
                                                                                             A common system design for unit
 to achieve similar performance to           encourages learning in classrooms for over      ventilators is hot water heat and chilled
 four-pipe alternatives. More and            80 years. Unit ventilators were designed        water cooling. This design offers many
 more, modern construction is                specifically for the unique challenges          benefits. The school can be heated by
 making two-pipe unit ventilator             presented by school buildings and offer         natural gas and an efficient chiller can
 system design a viable, cost saving         many features that are important to school      accomplish the cooling. The chiller size can
                                             design:                                         be based on the block load and not the
 approach to school design.
                                             • Indoor air quality. Unit ventilators are      connected load (Diversity can be applied).
 For more than 80 years, school                the only system designed to provide up to     Having both hot water and chilled water
 buildings have relied on AAF®-                100% outdoor air directly into each           available allows simultaneous heating and
 HermanNelson® unit ventilators                classroom. ASHRAE Standard 62.1-              cooling throughout the school. It also allows
                                               1999 requires 15 cfm of outdoor air per       dehumidification by means of reheat within
 from McQuay to keep classrooms
                                               person in a classroom. Therefore, a           the classroom.
 comfortable and promote better                classroom with 30 students would require
 learning. Our second article                  a ventilation rate around 450 cfm. The        This is a good way to provide an excellent
 highlights some of the benefits that          typical supply air volume for a classroom     learning environment. However, the system
 make this system vital in today’s             is 750 to 2,000 cfm. While some               requires that two sets of pumps and two sets
 classroom.                                    centralized systems can accommodate           of insulated piping be installed throughout
                                               this high percentage of outdoor air, the      the school. Also, dehumidification by
 You can find this and other issues of         cost of doing so can be significantly         means of reheat can raise the operating
 Engineering System Solutions on               higher in terms of equipment, building        expenses for the school.
                                               materials, and operating expenses.
 our website at                                                              2 Pipe Alternative
 Also, check out our “What’s New”            • Decentralized approach. A unit                A two-pipe system reduces the first cost
 page for information and literature           ventilator system provides a dedicated        and operating cost of the system by
                                               unit for each classroom. This allows          eliminating the need for one set of
 on McQuay Unit Ventilators and
                                               some classrooms to be in heating while                               Continued on next page.
 other McQuay products.                        others are in cooling. When a classroom
 McQuay provides semi-custom                   is not in service, the outdoor air can be
                                               minimized to reduce operating costs. If a
 system solutions for commercial
 HVAC applications. This newsletter
                                               unit ventilator requires service, only one
                                               classroom is affected.
                                                                                             IN THIS ISSUE
 is written specifically for the HVAC
                                             • Flexibility. Unit ventilators can be either   Introduction. . . . . . . . . . . . . 1
 engineering community. We                     chilled water or DX cooled. The DX can
 welcome your comments, feedback,              be self-contained or remote. They can be      Four-pipe Ventilator Performance
 and article suggestions.                      electric, hot water or steam heated. Water    With A Two-pipe Design . . . 1
                                               source heat pump and ground source heat
 Hugh Crowther, P.E.                           pump versions are also available. The
 Director of Applications                      unit can be located in the exterior wall      Creating A Better Climate For
 McQuay International                          below the window or ceiling hung. Self-       Learning–AAF®-HermanNelson®
                                               contained units are floor only.               Unit Ventilators” . . . . . . . . . 3
Four-pipe Unit Ventilator Performance With A Two-pipe Design continued from page 1.

insulated pipes and one set of pumps. The                Choose The Right Boiler                         3. Best
equipment cost for the unit ventilator is                A high efficiency (over 90%) condensing         Use a variable speed drive on the pump with
also reduced because there is only one coil              boiler is the best choice. In addition to the   face and bypass. The unit ventilators will
required. Overall, the construction costs for            obvious benefit of lower operating costs,       need to be able to close the water valve
a two-pipe system can be as much as 25%                  condensing boilers can be selected in           when the air is in full bypass. As the unit
less than a four-pipe arrangement.                       modules to provide staging and redundancy.      ventilator valves close off, the variable speed
What Are The Differences?                                In a two-pipe system, the coil in the unit      drive will slow the pump to meet the
The first cost and operating cost savings                ventilator needs to be selected to meet the     required flow. In heating mode, the variable
are desirable, but some performance is                   more demanding role of cooling. As a            speed drive can be limited to half flow
sacrificed with the two-pipe concept. It is              result, the coil will be oversized for          maximum. For chilled water, use 44°F
no longer possible, on the same loop, to                 heating, which could lead to control            supply water temperature and 10°F
have one unit ventilator in mechanical                   problems and poor space temperature             temperature difference. For heating, use an
cooling (economizer cooling is available)                control. By using a condensing boiler, the      80°F water temperature reset upward from
while another is in heating. Also,                       heating loop can be operated at 80°F and        an outdoor air sensor.
dehumidification by means of reheat is not               modulated up with an outdoor air reset          ASHRAE Standard 90.1-1999 requires
possible.                                                controller. The lower temperature will          hydronic systems with 10 or more pump
How To Minimize The Impact                               allow good space temperature control and        horsepower to be variable flow. With face
It is possible to minimize the impact of the             improve the changeover time from heating        and bypass arrangements, end of cycle
two-pipe concept by modifying the system                 to cooling.                                     valves should be used to allow for pump
design. With a well-designed system, the                 Other benefits of condensing boilers            savings.
changeover from heating to cooling can be                include a smaller footprint and no need for     What About Dehumidification?
accomplished in under 20 minutes.                        circulating boiler pumps – both of which        In southern climates, dehumidification is
Modern Construction Is The First Step                    reduce the size requirement for the             critical to maintaining a good learning
The key issue surrounding two-pipe                       mechanical room. In addition, a                 environment. ASHRAE 62.1-1999
changeover systems is how well they                      condensing boiler allows quick changeover       recommends maintaining the relative
perform in moderate weather. What if the                 because it can accept low entering water        humidity between 30 and 60%. Without
classroom on the north side requires                     temperatures without damage.                    simultaneous heating and cooling, this may
heating while the classroom on the south                 Pump Design And Face And Bypass                 appear to be difficult.
side needs cooling?                                      Face and bypass offers the best space           Again face and bypass is the key. By using
Modern construction plays a key role in                  temperature and humidity control and the        face and bypass, all air that passes through
making the job easier for a two-pipe                     best protection against coil freeze up. The     the cooling coil gets dehumidified because
system. Today’s school buildings are more                flow through the coil is always constant        the coil surface is always below the dew
tightly sealed with heavy insulation and                 ensuring good dehumidification in the           point. When a control valve is used in lieu
good windows, reducing infiltration and                  summer and minimal freeze potential in the      of face and bypass, sensible temperature is
minimizing load swings. This leaves two                  winter. Face and bypass also simplifies the     maintained but dehumidification is given up.
loads for the unit ventilator system to                  pump design. The following are pump             Dehumidification is more effective with
address – outdoor air and internal heat                  design options for a two-pipe system:           face and bypass damper control.
gains – both of which are constant in all                1. Good                                         A draw through design also helps with
classrooms regardless of the weather, and                Use a single speed pump with face and           dehumidification. The air passes first
both of which can be addressed by a two-                 bypass. Design the chilled water for 44°F       through the coil, and then through the fan
pipe system.                                             supply water temperature and 10°F               creating an even flow over the entire coil
School layout and orientation may also                   temperature difference. The heating will        surface to improve dehumidification. This
play a key role in assessing how the school              then operate with 80°F water temperature        is the way central air handling stations are
will perform during moderate weather. The                reset upward from an outdoor air sensor at      designed.
orientation of a school may be such that                 the same flow as the chilled water rate.
                                                                                                         Lowering the supply air volume will
one block or wing of classrooms will                     2. Better                                       further enhance the dehumidification
behave differently than another block of                 Use a two speed pump with face and              capabilities of face and bypass. The colder
classrooms (i.e. classrooms facing East will             bypass. For chilled water, use the pump on      the air leaving the coil, the more
get morning sun while classrooms facing                  high speed with 44°F supply water               dehumidification the classroom receives.
South will get afternoon sun). Once the                  temperature and 10°F temperature                Advanced controls, such as McQuay’s
differences have been identified, then a                 difference. For heating, operate the pump on    Microtech‘ unit controller, are required to
solution can be sought. In many cases, the               low speed with 80°F water temperature reset     maintain the correct amount of outdoor air
solution may be as simple as having two                  upward from an outdoor air sensor. The          and the classroom temperature while
loops – one serving each block of                        two-speed pump will offer the school            operating at reduced supply air volumes.
classrooms.                                              excellent operating savings and not sacrifice
                                                         climate control.
                                                                                                                   deadband, all the classrooms will behave
                                                                                                                   approximately the same since two-thirds of
                                                                                                                   the load is the ouside air load and internal
                                                                                                                   heat gains. Finally, for the 30°F deadband,

                                                                                                                   resetting the chilled water up to 50°F and

                                                                                                                   the hot water down to 80°F will meet the

                                            65¡            60
                                                                                                  Dehumidifi-      requirement.

                                    60¡                                                           cation is more   Conclusion
                              55¡                                                                 effective with   A two-pipe unit ventilator system design
                        50¡                                                                       face and         makes sense in schools built or remodeled
                                                                                                                   using modern construction techniques. In
                  45¡                                                          20%
                                                                                                  bypass damper
            40¡                                                                                                    these schools, the classroom load profiles
      35¡                                                                                         control.
                                                                                     10% R
                                                                                                                   will be dominated by outdoor air loads and
                                                                                                                   internal heat gains. Both can be
                                                                                                                   accommodated with a two-pipe unit
                                                                                                                   ventilator system that uses face and bypass
                                                                                                                   damper control and a draw through design.
ASHRAE Standard 90.1-1999 and                                      2. The system must be designed and              The result can be significant installed and
Changeover systems                                                    installed with controls that allow           operating cost savings versus central
Two-pipe changeover systems are energy                                operation in one mode for at least 4
                                                                                                                   systems or four-pipe valve control unit
efficient in that they cannot be heating and                          hours before changing back to the other
                                                                      mode.                                        ventilator systems, as well as the added
cooling at the same time. However, the                                                                             advantage of effective humidity control.
actual changeover requires changing the                            3. Reset controls are provided that allow       Add to this the inherent unit ventilator
fluid temperature, which does waste                                   the hot water and chilled water setpoints    advantage of bringing outside air directly
energy. To address this, ASHRAE STD                                   to be no more than 30°F apart at             into the classroom, and the two-pipe
90.1-1999 has three requirements:                                     changeover.
                                                                                                                   system may deliver the best of both worlds
1. There must be a deadband in the                                 The first requirement can be met with           – an environment that encourages learning
   changeover of at least 15°F outdoor air                         economizers. For example, a deadband of         for students and teachers at significantly
   temperature.                                                    15°F from 50°F ambient to 65°F ambient          lower cost for school administrators.
                                                                   meets the requirement. For the 4 hour

Creating A Better Climate For Learning–AAF®-
HermanNelson® Unit Ventilators
For more than 80 years, school buildings                           along with a longer school year that            design of AAF-HermanNelson unit
have relied on AAF®-HermanNelson® unit                             extends into both heating and cooling           ventilators provide precise, efficient
ventilators from McQuay to keep                                    months, has created the need for more           temperature and humidity control to
classrooms comfortable and promote                                 humidity control.                               maintain comfort conditions in the
better learning. From the beginning, AAF-                          While they have been improved over the          classroom. In addition, full-length filter
HermanNelson unit ventilators have been                            years to meet these and other challenges,       helps ensure that all air delivered into the
designed and built with the idea that                              the benefits that have made AAF-                classroom is filtered.
properly ventilated classrooms increase                            HermanNelson unit ventilators the               Because they reside in the classroom,
the mental and physical capabilities of                            classroom ventilation system of choice          AAF-HermanNelson unit ventilators are
students of all ages, and provide a better                         still apply:                                    designed to be exceptionally quiet. Recent
environment for learning.                                                                                          enhancements enable the units to achieve
                                                                   Promotes Good Classroom IAQ That
AAF-HermanNelson unit ventilators are as                           Encourages Learning                             “whisper quiet” operation while delivering
vital in today’s classroom as they were                            Unit ventilators are the only system            rated air. These include a new fan wheel
more than 80 years ago. New and                                    designed to provide up to 100% outdoor          design that incorporates fluid dynamics, an
remodeled schools are sealed tight to meet                         air directly into each classroom to help        expanded discharge opening that
energy efficiency codes, creating the need                         meet ASHRAE Standard 62.1-1999                  minimizes air resistance as air exits the
for more classroom ventilation to combat                           ventilation requirements. Face and bypass       unit, and a new rigid fan motor mount that
airborne pollutants – the causes of poor                           damper control and the draw through             minimizes vibration.
indoor air quality. The added ventilation,
                                                                                                                                            Continued on back page
Creating a Better Climate for Learning continued from page 3.

Economical                                                   service. Quality components, scratch and                     quality, environmentally friendly finish that
In addition to the economies of a                            dent resistant exterior, and impenetrable                    is available in architecturally pleasing
decentralized system approach, each unit                     interior withstand classroom bumps,                          colors designed to match any decor. A trim
ventilator can be operated for about the same                bruises, and curiosity. Easy access to unit                  profile requires minimum classroom space
cost as lighting two 100-watt light bulbs.                   interior and components encourages                           while delivering an environment that
Flexible                                                     regular maintenance to ensure peak                           encourages learning.
Added to the configuration flexibility of                    performance.                                                 Look to the school experts at McQuay for
the units are the benefits of individual                     MicroTech™ Direct Digital Controls ensure                    your new or remodeled school project. See
classroom control and 0 to 100% fresh air                    the correct operating sequence without                       how directly adding fresh air into your
operation that provide room to grow by                       human intervention while allowing for easy                   classrooms using AAF-HermanNelson unit
allowing an increase in class size to be                     adjustments and remote monitoring.                           ventilators can create a better climate for
accommodated without system changes.                         Aesthetically pleasing and                                   learning.
Reliable                                                     environmentally friendly
AAF-HermanNelson unit ventilators are                        For classroom aesthetics, AAF-
designed to provide years of reliable                        HermanNelson unit ventilators feature a

For comments or suggestions, please call or write:
Chris Sackrison, Editor
McQuay International
13600 Industrial Park Boulevard
Minneapolis, MN 55441
Phone: (763) 553-5419
For more information on McQuay products and services, or to speak with your local representative, call (800) 432-1342, or visit our web page at

 13600 Industrial Park Boulevard
 Minneapolis MN 55441

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