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 www.mcquay.com. 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
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
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
40¡ these schools, the classroom load profiles
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
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 www.mcquay.com.
13600 Industrial Park Boulevard
Minneapolis MN 55441