Dedicated Outdoor Air
Systems (DOAS) and
Building Pressurization
ASHRAE Annual Conference
Albuquerque Session 6
Monday June 28, 2010
Stanley A. Mumma, Ph.D., P.E.
Prof. Emeritus, Architectural Engineering
Penn State University, Univ. Park, PA
sam11@psu.edu
Web: http://doas-radiant.psu.edu
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Presentation Outline
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Importance of building pressurization.
Impact of TER on building pressurization.
Estimating building pressurization needs.
Ratio of pressurization air flow to total OA
for various occupancy categories.
Impact of unbalanced flow on TER
performance in DOAS applications.
New DOAS configuration?
Research needs.
Conclusions.
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Is building pressurization
important?
 Yes or no?
 Do you employ it?
 Why pressurize?
– Limit moisture migration through the
envelope, summer and winter?
– Limit hot or cold locations around the
perimeter—thermal comfort?
 How do you determine the required flow?
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Source of pressurization air?
 Is the OA specified by Std. 62.1
sufficient?
 Is toilet exhaust part of the
pressurization air requirement?
 Does the use to total energy recovery
have any bearing on the issue of
pressurization?
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An example of a pressurized
building without TER
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Example of the same building with
DOAS and TER
 Notice how the toilet exhaust is handled?
 Why, and how is it different than an all air
system—even with heat recovery.
 The TER flow is unbalanced!! Does it
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matter?
DOAS Defined for This Presentation
20%-70%
less OA,
than VAV
DOAS Unit
w/ Energy
Recovery
Cool/Dry
Supply
Parallel
Sensible
Cooling System
Building Pressurization
High
Induction
Diffuser
Building with
Sensible
and Latent
Cooling
Decoupled
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How much air flow is required for
building pressurization?
 Well, it depends--right? On What?
– Building tightness.
– Building use.
– Construction quality.
– Wind velocity and direction.
– Stack effect.
– Method of automatic control—if any.
 So how do you know what to design for?
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How much infiltration would you
expect if no pressurization—
excluding toilet exhaust?
Consider normal practice construction!
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It depends 
None
½ ACH
1 ACH
> 3 ACH
What is the basis for your opinion?
Is ACH a floor area or wall area concept?
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Published air tightness
recommendations. ref: given in paper
Note: Leakage is per unit area of exterior perimeter wall
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Do we attempt to control a
building pressure at 50 Pa?
 249.1 Pa = 1” H2O.
 So 50 Pa = 0.2” H2O—clearly much higher
than we attempt to pressurize buildings.
 Most buildings using pressurization
differential control have 0.03” H2O, or 7.5 Pa.
as the set point
 So the answer is no!
 Can we modify the published air leakage
rates to reflect 7.5 Pa rather than 50?
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Converting published 50 Pa
leakage rates to 7.5 Pa.
Applying the parabolic
relationship:
Bldg
type
Flow at Flow at
50 Pa 7.5 Pa
School 0.4922
0.1902
Offices 0.2735
0.1507
Flow, cfm/ft2 wall area
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Converting leakage rates at 7.5 Pa.
from wall area to floor area.
 Assume 15,000 sq ft building with 10 ft high
walls.
 Assume the length to width ratio is 2:1.
 Resulting wall area is 5,195 ft2
 Leakage for office = 0.1057*5,195=549 cfm
 Converting to a floor area bases:
549/15,000=0.037 cfm/ft2 floor area (office)
 Similarly for school:
Leakage is: 0.066 cfm/ft2 floor area.
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Is there another place that OA is
specified on a cfm/ft2 floor area basis?
 Is 0.037 and 0.066 cfm/ft2 floor area about the
same order of magnitude as other floor area
based sources?
 If expressed as an ACH, what would we get?
 Is it coincidence that the pressurization flow
is of the same order of magnitude as the floor
component of ASHRAE Std. 62.1?
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TER unbalanced flow for
different occupancy
categories when the
pressurization flow is
~1/2 ACH
 Does this surprise you?
 Have you observed, as I
have, TER jobs returning
only 30-40 % the flow
expected?
 How bad is 30% to 70%
of OA for pressurization?
Note: floor frac = Pressurization fraction
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Impact of unbalanced flow on
TER performance
For unbalanced flow, mOA= mRA + mPressurization
hOA
hSA
Supply air
Outdoor air m
OA
0 scfm Purge
or seal leakage
Wheel Rotation
Exhaust air
hEA
hRA Return air,
including toilet
mRA exhaust
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Brief heat exchanger tutorial
 h, efficiency ?
 Circuit with min.
m*Cp
 Tco if HTX infinitely
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long
Temperature profile for
infinitely long HTX
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Heat Exchanger effectiveness, e
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For unbalanced flow, mOA= mRA + mPressurization
hOA
hSA
Supply air
Outdoor air m
OA
0 scfm Purge
or seal leakage
Wheel Rotation
Exhaust air
hEA
hRA Return air,
including toilet
mRA exhaust
e = mOA(hOA-hSA)/mRA(hOA-hRA) = (hEA-hRA)/(hOA-hRA)
eapparent = e*mRA/mOA = (hOA-hSA)/ (hOA-hRA)
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100
1050
90
950
80
850
70
750
60
650
50
40
Recovered
MBH
based
Recovered
MBH
upon an 85F 140 gr OA
Eff.
condition, an 75F 50%
Eff
RAApp.
condition,
and a
130” Dia EW (519 sfpm
FV OA stream)
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20,000
Balanced
18,000
16,000
Lecture Conf. rm
550
Recovered heat, MBH
Eff and apparent eff
Impact of unbalanced flow on EW performance:
20,000 scfm of OA
450
14,000
Educ.
12,000
Return air flow: scfm
10,000
8,000
350
6,000
Office
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What does the need for
pressurization and the negative
impact of unbalanced flow on the
TER performance Suggest?
 A new product.
 With fully integrated pressurization
unit and a balanced flow DOAS.
 It might look like:
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EW heat recovery performance cases
1050
Unbal F HR
BFHR
BF HR, 500 sfpm FV
950
130”
Dia
At summer design conditions
Recovered Heat, MBH
850
750
124”
Dia
130”
Dia
117”
Dia
650
130”
Dia
112”
Dia
550
450
350
250
20,000
72”
Dia
18,000
16,000
14,000
12,000
Air Flow, scfm
10,000
8,000
6,000
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Design and Off design heat recovery
School
800
793
Design
750
713
700
669
Heat Recovery, MBH
650
1. 20000, 14000 85F 140 gr 130" wheel
2. 14000, 14000 85F 140 gr 130" wheel
3. 14000, 14000 85F 140 gr 112" wheel
600
550
500
450
4. 20000, 14000 75.45F, 90.25 gr 130" wheel
5. 14000, 14000 75.45F, 90.25 gr 130" wheel
6. 14000, 14000 75.45F, 90.25 gr 112" wheel
400
350
300
250
227
200
205
Off-Design
150
1
2
3
4
6 Cases, See details
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192
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First and Energy cost implications
 Cooling energy use increases a little.
 Fan energy use decreases due to EW flow
resistance removal from the pressurization
path.
 Net impact for 40o N. Latitude locations is
annual energy use is about the same for an
unbalanced flow unit and the integrated unit.
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First and Energy cost implications
 About a $ 6/scfm first cost savings for air
removed from the EW and processed through
the pressurization unit. In the example of an
office with an OA flow of 20,000 scfm, and a
pressurization requirement of 14,000, the first
cost savings amounts to about $84,000.
 Significantly, the low cost pressurization
component is a good place to provide reserve
capacity. Since about 20% reserve capacity is
often provided, a 20,000 scfm OA requirement
would provide another $24,000 first cost
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savings!
Important pressurization unit
control point: Maintain a constant
flow, rather than a DP!
 Assures balanced flow DOAS.
 Provides predictable basis for
equipment sizing.
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Advantages of integrated unit
1. Enables the first cost of the balanced flow
DOAS to be reduced by nearly the fraction of
pressurization flow.
2. Does not degrade the TER performance
resulting from unbalanced flow
3. Allows reduced fan energy use since less
combined supply air and purge air flow
occurs on both sides of the wheel. Important
since fan energy use is significant.
4. Allows lower operating cost, even though
the cooling/dehumidification energy use
may increase a little.
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Advantages of integrated unit
5. Eliminates the added installation first cost
for two systems (DOAS and Pressurization).
6. Allows energy use for pressurization to be
limited with flow measurement control.
7. Allows reserve capacity to be added to the
pressurization unit, at much lower first cost
than in the DOAS where expensive heat
recovery is used.
8. Simplifies controls by dividing the duties.
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Disadvantages of integrated unit
1. May use more cooling energy.
2. Energy use results are sensitive to
equipment selection, i.e. coil DP, fan h, TER
selections (DP & effectiveness), air required
for pressurization, cooling COP's.
3. May be falsely perceived as more complex.
4. Not very beneficial for unbalanced flows of
less than 10%.
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Research Needs
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Confirm the recommended limiting/
bounding flow rate for pressurization, i.e. 0.5
ACH (0.06 scfm/ft2 [0.3 L/s*m2]).
Confirm that the recommendation to employ
fixed measured pressurization flow control,
with the occasional few hours of moisture
migration through the envelope when
infiltration may occur, does not lead to IAQ or
comfort problems.
Establish a systematic method for
determining the appropriate reserve capacity.
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Conclusions
 Building pressurization important
 Adequate pressurization is achieved
with ~1/2 ACH, or about 0.06 cfm/ft2
floor area.
 Recommend an integrated balanced
flow DOAS with a pressurization unit.
 Can provide huge first cost savings
without an energy penalty.
 Now is the time for a new product!!
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