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THE USE OF BLOWER-DOOR DATA(1)
Max Sherman Energy Performance of Buildings Group
Environmental Energy Technologies Division
Lawrence Berkeley LaboratoryUniversity
of CaliforniaBerkeley, California
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Table of Contents
The role
of ventilation in the housing stock is to provide fresh air and to dilute internally-generated
pollutants in order to assure adequate indoor air quality. Blower doors are used
to measure the air tightness and air leakage of building envelopes. As existing dwellings
in the United States are ventilated primarily through leaks in the building shell
(i.e., infiltration) rather than by whole-house mechanical ventilation systems, accurate
understanding of the uses of blower-door data is critical. Blower doors can be used
to answer the following questions:.
- What is the Construction Quality of the Building Envelope?
- Where are the Air Leakage Pathways?
- How Tight is the Building?
- How Much Ventilation Does the Air Leakage Supply?
- How Much Energy Does the Air Leakage Lose?
- Is this Building Too Tight?
- Is this Building Too Loose?
- When Should Mechanical Ventilation be Considered?
Various ASHRAE Standards (e.g., 62, 119, and 136) are used to determine acceptable
ventilation levels and energy requirements
Keywords: Infiltration, Ventilation, Air Leakage, Indoor Air Quality, Energy,
Blower Door
Virtually all knowledge about the air tightness of buildings comes from field measurements
using Blower Door technology. Blower Doors measure air tightness which, in
turn, is the prime building factor in determining infiltration and air leakage. Blower
Doors can be used in a variety of ways for a variety of purposes that span the range
of energy, air quality, comfort and safety. This report summarizes what is and what
can be done with Blower-Door data in helping to answer these kinds of questions.
This report does not intend to cover issues related to the (fan pressurization) measurements
themselves. As the AIVC (1990) has shown, there exist many measurement standards
throughout the world, but the two used by the ASHRAE Standards discussed below are
the ASTM (1991) Standard (E779-87) and the CGSB (1986) Standard (149). Issues of
measurement uncertainty as described by Sherman and Palmiter (1994), and reproducibility,
as shown by Murphy et al (1991), while important, will not be discussed. Both technical
(e.g. ASTM 1990) and popular (e.g. Nissan 1985 and Meier 1994) articles are available
to familiarize the reader with some of the relevant issues.
This report focuses on single-zone buildings. While Blower Doors are sometimes used
for component or multizone leakage measurements, the vast majority of measurements
have been made for whole-building, single-zone situations, such as single-family
homes. Similarly, the simplified models and consensus standards have focussed on
these types of buildings.
"Blower Door" is the popular name for a device that is capable of
pressurizing or depressurizing a building and measuring the resultant air flow and
pressure. The name comes from the fact that in the common utilization of the technology
there is a fan (i.e. blower) mounted in a door; the generic term is "Fan Pressurization".
Blower-Door technology was first used in Sweden around 1977 as a window-mounted fan
(as reported by Kronvall, 1980) and to test the tightness of building envelopes (Blomsterberg,
1977). That same technology was being pursued by Caffey (1979) in Texas (again as
a window unit) and by Harrje, Blomsterberg and Persily (1979) at Princeton University
(in the form of a Blower Door) to help find and fix the leaks.
During this period the diagnostic potentials of Blower Doors began to become apparent.
Blower Doors helped Harrje, Dutt and Beya (1979) to uncover hidden bypasses
that accounted for a much greater percentage of building leakage than did the presumed
culprits of window, door, and electrical outlet leakage. The use of Blower Doors
as part of retrofitting and weatherization became known as House Doctoring
both by Harrje and Dutt (1981) and the east coast and Diamond et al. (1982) on the
west coast. This in turn led Harrje (1981) to the creation of instrumented audits
and Sonderegger et al. (1981) to computerized optimizations.
While it was well understood that Blower Doors could be used to measure air tightness,
the use of Blower-Door data could not be generally used to estimate real-time air
flows under natural conditions or to estimate the behavior of complex ventilation
systems. When compared with tracer-gas measurements, early modeling work by Caffey
(1979) was found wanting. There was a rule of thumb, which Sherman (1987) attributes
to Kronvall and Persily that seemed to relate Blower-Door data to seasonal air change
data in spite of its simplicity:
(EQ 1)
That is, the seasonal amount of natural air exchange could be related to air flow
necessary to pressurize the building to 50 Pascals.
To overcome the physical limitations of such rules of thumb, it is necessary to model
the situation physically which, in this case, means separating the leakage characteristics
of the building from the (weather) driving forces. As the early versions of the ASTM
Standard show, leakage is described conventionally as a power law, Equation
7, which was found to be valid empirically but without theoretical substantiation
(recent work by Sherman (1992a) has provided the theoretical basis for the expression).
Using orifice flow (Equation 8) as a physical
model, the Blower-Door data can be used to estimate the Effective Leakage Area (ELA)
Equation 9.
Using this orifice-flow paradigm, Sherman and Grimsrud (1980) developed the LBL Infiltration
model (Equation 14) which was then validated
by Sherman and Modera (1984) and incorporated into the ASHRAE Handbook of Fundamentals
(1989). Much of the subsequent work on quantifying infiltration is based on that
model, including ASHRAE (1988) Standard 119 and ASHRAE (1993) Standard 136. The important
equations are summarized in "APPENDIX: MODELING
TOOLS".
Blower Doors are still used to find and fix the leaks, but more often the values
generated by the measurements are used to estimate infiltration for both indoor air
quality and energy consumption estimates. These estimates in turn are used for comparison
to standards or to provide program or policy decisions. Each specific purpose has
a different set of associated blower-door issues.
Compliance with standards, for example, requires that the measurement protocols be
clear and easily reproducible, even if this reduces accuracy. Public policy analyses
are more concerned with getting accurate aggregate answers than reproducible individual
results. Measurements that might result in costly actions are usually analyzed conservatively,
but "conservatively" for IAQ is diametrically opposed to "conservatively"
for energy conservation.
Complicating any analysis is the fact that infiltration, being weather dependent,
is not constant. Because of the non-linearities involved, the equivalent constant
infiltration rate is not simply related to the average of the instantaneous values.
Sherman and Wilson (1986) have determined that the equivalent constant infiltration
rate is generally higher than the average for energy-related purposes and lower for
indoor air quality purposes, indicating that infiltration is not a particularly efficient
ventilation strategy. As shown in the appendix special purpose quantities are required
to take these effects into account.
To clarify the importance of these issues as well as provide operational guidance
to those wishing to use Blower-Door data, we have posed and then provided the means
to answer a set of questions commonly addressed with Blower Doors:
As mentioned earlier this semi-quantitative function was the original use for blower-door
technology. The goal here is to assure that the envelope is of sufficiently good
(i.e. tight) construction that leakage is not an important liability in energy, comfort
or air flow; that is, prime consideration was devoted to reducing drafts and uncontrolled
air movement. As such intentional openings are normally sealed in the test method
and the test may even be done prior to the completion of construction to, for example,
find penetrations in vapor barriers.
As the tightness value is an indicator only, and is not intended to be used in further
calculations, a single, simple measurement is appropriate: usually air changes at
50 Pascals. Examples of this kind can be found in the standards from Sweden (1989)
and Norway (1987) among others. For these types of standards it is sufficient to
assure that the fabric of the envelope is tight (e.g. below 3 air changes at 50 Pa)
and that ventilation must be provided through some other (i.e. mechanical) mechanism.
This question often follows the first when the building envelope is found not to
be sufficiently tight either because of excessive energy complaints or, more likely,
because of discomfort due to draft. The Blower Door is used as a means of inducing
flow through the leaks which can be detected by a variety of means as indicated by
an ASTM (1991) standard (E116) including smoke movement, sound propagation, and thermography.
The flow measuring part of the Blower Door is not needed.
Supertight construction makes use of these detection means to reduce or eliminate
leakage paths during the construction phase. As described by Gettings (1989), House
Doctoring makes use of these detection means to retrofit existing buildings. Many
types of air leakage paths, such as bypasses, can only be identified this
way.
While this question may appear to be similar to the first question, there are several
significant difference. This question seeks to quantify the air tightness
in such a way that it can be used to calculate the contribution of air leakage/infiltration
towards ventilation and energy requirements. Thus it needs to be more quantitative
and to reflect the leakage in normal operating conditions; that is, it must reflect
accurately that amount of air leakage through all leakage paths exposed to environmental
driving forces.
As described in the Appendix (Equation 10),
Blower-Door data can be reduced to an Effective Leakage Area (ELA) and a flow
exponent. The ELA quantifies the equivalent amount of holes in the (given configuration
of the) building and can be used with the LBL model to estimate the flow rate of
infiltrating air (Equation 13).
For most purposes it is desirable to normalize leakage (and ventilation) by the size
of the building, either for comparison or standardization purposes. Standard 119
defines the Normalized Leakage (NL, in Equation
11) for this purpose and also uses NL to define leakage classes. (See Table
1, "CHARACTERIZATION BY BUILDING LEAKAGE,".)
It is interesting to note that for a typical single-story house the normalized leakage
is simply related to the air changes at 50 Pascals using (the climate-independent)
approximation of Equation 12:
(EQ 2)
Sherman's (1987) correction factors must be applied if the leakage exponent or building
height are different from the default assumption.
If we are concerned about the pollutant-dilution capabilities of infiltration it
is important to take into account when and how varied the instantaneous infiltration
is as indicated by Sherman and Wilson (1986). As is done for Standard 136 these details
can be incorporated into a (annual) weather factor, w, to describe the ventilation
potential of each climate(2). Equation
17 describes the equivalent amount of air exchange derived from infiltration:
(EQ 3)
The factor 1.44w ranges between two-thirds and unity for most climates in
the U.S. and slightly higher in Canada. In addition to weather variations, w
is a function of height, leakage distribution and wind sheltering. (See Equation
14.) When combined with Equation 2 this
expression comes close to approximating the "divide by 20" rule.
This air change rate is a good estimate of the equivalent amount of ventilation
produced by infiltration, but it is not a good estimate of the average air change
rate or an air change rate suitable for making energy estimates.
If we are concerned about the thermal loads imposed by infiltration, it is important
to take into account when infiltration occurs (e.g. the energy impact, as well as
the driving forces, for infiltration are larger when the outdoor temperatures is
at 0oC than at 15oC). Infiltration-related climate can
be expressed using the concept of Infiltration Degree-Days (IDDs) as developed
by Sherman (1986b). In the units of kJ/m2, Equation
22 approximates the infiltration load (per unit floor area) and can be explained
as
(EQ 4) .
Typical values of IDD run between 2000oC-day and 7000oC-day
according to ASHRAE Standard 119, which makes a particular assumption about heating
and cooling limits. Standard 119 contains a table of IDD values for many cities as
well as a calculation method.
Although Standard 119 makes a certain set of assumptions about degree-days, they
can be recalculated for different purposes (e.g. heating-only) and the equation still
applies.
This question has embedded in it an assumption about the definition of "too
tight". For our purposes we will define it as meaning too tight to meet ASHRAE's
(1989) Ventilation Standard (62)of 0.35 ach using Equation
17 and assuming no significant contribution from mechanical ventilation. Thus
the building is too tight if
(EQ 5)
The building may be too tight for other considerations. For example, Dumont and Snodgrass
(1990) have shown that buildings with naturally-aspirated fossil-fuel appliances
may backdraft if there is insufficient air leakage. Although air leakage can
be an important factor in backdrafting, other factors such as the characteristic
of the combustion appliances and the amount of mechanical exhaust must also be considered.
Such considerations are beyond the scope of this report.
The implied definition here is to minimize drafts and energy consumption, which will
depend on climate. If we use Equation 23 as
an approximation to Standard 119, then the building will be too loose if
(EQ 6)
In the background to the formation of Standard 119 Sherman (1986a) shows that this
criterion is based on 150 MJ/m2 as the maximum allowed infiltration
load, which is a value believed to cut off the highest energy users without undue
hardship for the typical building under typical efficiency and fuel cost assumptions.
As the desire for energy conservation increases, energy standards may wish to strengthen
this requirement.
Other looseness considerations include draft which can lead to poor thermal comfort
as per ASHRAE's (1992) Thermal Comfort Standard (55) and moisture accumulation which
can lead to material problems.
The decision when and how to use mechanical ventilation depends somewhat on climate,
but it depends primarily on building tightness. If we use the leakage classification
of Standard 119 and apply our criteria for the range of weather factors found in
Standard 136 we can summarize the need for mechanical ventilation in Table 1 using
the guidance Standard 62 and Equation 3.
TABLE 1. CHARACTERIZATION BY BUILDING LEAKAGE
------------------------------------------------------------------------
LEAKAGE Minimum Maximum Typical Ventilation Recommended
CLASS NL NL ACH50 Requirement Ventilation Type
------------------------------------------------------------------------
A 0 0.10 1 Full Balanced Only
B 0.1 0.14 2 Yes Balanced
C 0.14 0.20 3 Yes Either
D 0.20 0.28 5 Some Either
E 0.28 0.40 7 Likely Unbalanced
F 0.40 0.57 10 Possible Unbalanced Only
G 0.57 0.80 14 Unlikely Unbalanced Only
H 0.80 1.13 20 None None
I 1.13 1.60 27 Buildings in this range may
J 1.60 be too loose and should be
tightened.
------------------------------------------------------------------------
Table 1 summarizes the need for mechanical ventilation for different building leakages.
It contains recommendations about which leakage classes require some sort of whole-house
mechanical ventilation and recommends the type. The flow addition principles described
by Sherman (1992b) indicate that balanced ventilation (e.g. an air-to-air heat exchanger)
is best for the tighter classes because it does not affect the internal pressure
and unbalanced (e.g. exhaust fan) systems are best for the looser classes because
they minimize variations in total ventilation. For those buildings in which large
depressurizations will not cause problems, unbalanced systems can be used regardless
of tightness.
This table can be used either as a guide to selecting ventilation for a new or existing
house whose tightness is known. Alternatively it can be used to guide construction
quality for a house where the ventilation system (or lack thereof) has been designed.
Equation 3 can be used to estimate the impact
that leakage will have towards meeting the 0.35 ach requirement of ASHRAE Standard
62, but the equations in Standard 136 must be used to combine both infiltration and
mechanical ventilation.
A building of Leakage Class A is sufficiently tight that no credit can be taken for
infiltration towards meeting a ventilation requirement; such a house should be considered
airtight and all ventilation and pressure relief must be designed through
the mechanical system. Classes B and C represent looser, but still quite tight construction.
While infiltration may be non-negligible for energy concerns in some climates, its
contribution towards ventilation will be too small to count on and there is still
a ventilation system requirement. Classes D and E begin to be leaky enough that the
infiltration may become a significant part of the ventilation requirement. It may
be possible to meet the requirement with natural ventilation or intermittent mechanical
ventilation. Leakage Classes F and G will usually be sufficiently leaky that in all
but sheltered and mild climates explicit mechanical ventilation is probably not needed.
Leakage Classes H and above would not be expected to require purpose-provided ventilation
and usually represent opportunities for cost-effective tightening.
Equation 2 through Equation
6 have a set of default assumptions embedded in them regarding some of the details
of the buildings. In the aggregate we would expect these assumptions to lead to reasonable
averages, but for a single structure the details can be important. Thus, for the
purposes such as setting energy standards we might use Equation
4 to get a robust estimate of the impacts of certain options.
An aggregate analysis by Sherman and Matson (1993) using existing databases has estimated
the loads associated with residential infiltration for the U.S. stock, and shows
that the requirements for the current stock to meet the ASHRAE ventilation requirement
through ventilation are about 3EJ, but that about 2EJ could be saved if those houses
were tightened to meet ASHRAE Standard 119.
It may not always be possible to meet both standards through infiltration. In more
extreme climates there may be no airtightness level that would simultaneously allow
that, or the allowed range of tightness values would be so narrow as to preclude
designing for it.
Using over 200 weather sites, we have generated a map of the continental U.S. (See
Figure 1, "Air Tightness Levels".)
showing four different zones regarding air tightness requirements and the range of
air tightness levels that can meet energy and ventilation standards.
FIGURE 1. Air Tightness Levels. Each of the four zones represents
an increasingly larger range of airtightness that would be meet both ASHRAE Standard
119 and ASHRAE Standard 62. Zone 1 buildings cannot meet both standards; Zone 2 and
3 buildings can. Zone 4 (not labeled) has a large acceptance range, but requires
very leaky construction.
Zone 1 represents the severe climates of the Northern tier in which designing
to meet the air tightness standards for energy conservation would make it practically
impossible to reliably get sufficient ventilation from infiltration to meet the ventilation
standard. Thus in Zone 1 good design should include mechanical ventilation.
Zone 2 represents the moderate climates in which careful design and control of building
air tightness can allow buildings to be designed to simultaneously meet energy and
ventilation standards. Zone 3 represents the mild climates ranging from the Puget
Sound through Texas to the Southeast. In these climates there is a substantial range
of air tightness that would meet both standards.
In Zone 4, coastal California and some of the Southwest, there is a large range of
acceptable leakage, but the climate is so mild that it is necessary to have very
leaky houses to meet the ventilation standard, leakier in fact than new construction
tends be built. Mechanical ventilation may need to be considered in this zone (and
some of Zone 3) because of insufficiently low construction quality.
The issue of whether these standards are set at appropriate levels is a valid one,
but the expressions presented above can be used to help understand the implications
of a variety of standards and levels. The equations are at a degree of simplicity
that rivals the rule of thumb in Equation 1,
but contains significantly more usable information. It is interesting to note that
with the correct interpretations Equation 2
and Equation 3 can be combined to yield that
rule for certain circumstances.
Infiltration and ventilation in dwellings is conventionally believed to account for
1/3 to 1/2 of the space conditioning energy. As energy conservation improvements
to the thermal envelope continue, the fraction of energy consumed by the conditioning
of air may increase. Air-tightening programs, while decreasing energy requirements,
have the tendency to decrease ventilation and its associated energy penalty at the
possible expense of adequate indoor air quality. In this report we have demonstrated
how data collected from Blower Doors can be used to address these issues and have
indicated some of the limitations thereon.
- AIVC, "A Review of Building Airtightness and
Ventilation Standards", TN 30, Air Infiltration and Ventilation Centre, UK,
1990
- ASHRAE Standard 119, Air Leakage Performance for Detached
Single-Family Residential Buildings, American Society of Heating, Refrigerating and
Air conditioning Engineers, 1988.
- ASHRAE Standard 62, Air Leakage Performance for Detached
Single-Family Residential Buildings, American Society of Heating, Refrigerating and
Air conditioning Engineers, 1989.
- ASHRAE Handbook of Fundamentals, Chapter 24, American
Society of Heating, Refrigerating and Air conditioning Engineers 1989
- ASHRAE Standard 55, Thermal Environmental Conditions
for Human Occupancy, American Society of Heating, Refrigerating and Air conditioning
Engineers, 1992.
- ASHRAE Standard 136, A Method of Determining Air Change
Rates in Detached Dwellings, American Society of Heating, Refrigerating and Air conditioning
Engineers, 1993.
- ASTM STP 1067, Air Change Rate and Airtightness in
Buildings American Society of Testing and Materials, M.H. Sherman Ed., 1990
- ASTM Standard E779-87, "Test Method for Determining
Air Leakage by Fan Pressurization", ASTM Book of Standards, American Society
of Testing and Materials, Vol 04.07, 1991.
- ASTM Standard E1186-87, "Practices for Air Leakage
Site Detection in Building Envelopes", ASTM Book of Standards, American Society
of Testing and Materials, Vol 04.07, 1991.
- A. Blomsterberg, "Air Leakage in Dwellings",
Dept. Bldg. Constr. Report No. 15, Swedish Royal Institute of Technology, 1977.
- G.E. Caffey, "Residential Air Infiltration",
ASHRAE Trans, V85(9) pp41-57, 1979
- CGSB Standard 149, Determination of the Airtightness
of Building Envelopes by Fan Depressurization Method, Canadian General Standards
Board 1986
- R.C. Diamond, J.B. Dickinson, R.D. Lipschutz, B. O"Regan,
B. Schole, "The House Doctor's Manual:, Lawrence Berkeley Laboratory Report
PUB-3017, 1982.
- R.S. Dumont, L.J. Snodgrass, "Investigation of
Chimney Backflow Conditions: A case study in a Well-Sealed House," ASHRAE
Trans V96(I), 1990.
- M.B. Gettings, "Blower-Door Directed Infiltration
Reduction Procedure Description and Field Test", ASHRAE Trans. V.95(I),
1989.
- D.T. Harrje, "Building Envelope Performance Testing,"
ASHRAE J., p39-41, March 1981
- D.T. Harrje, A. Blomsterberg, A. Persily, "Reduction
of Air Infiltration Due to Window and Door Retrofits", CU/CEES Report 85, Princeton
University, 1979
- D.T. Harrje, G.S. Dutt, J.E. Beya, "Locating
and Eliminating Obscure, but Major Energy Losses in Residential Housing," ASHRAE
Trans. V85(II), pp521-534, 1979
- D.T. Harrje, G.S. Dutt, "House Doctors Program:
Retrofits in Existing Buildings," Proc. 2nd AIVC Conference, p.61-72, 1981.
- J. Kronvall, "Air Tightness Measurements and
Measurement Methods," Swedish Council for Building Research, Stockholm, D8:1980.
- A.K. Meier, Home Energy, pp. 25-37, V.11 No.
1, A.K. Meier Ed., 1994
- Murphy W.E., Colliver D.G., Piercy L.R., Repeatability
and reproducibility of fan pressurization devices in measuring building air leakage,
ASHRAE Trans. V 97(II), 1991.
- N. Nissan, Energy Design Update, V.4 No. 4,
N. Nissan Ed., 1985
- Norway, "Thermal Insulation and Airtightness
Building Regulations", Royal Ministry of Local Government and Labour, Chapter
53. 27 May, 1987.
- L. E. Palmiter, T. Bond., "Modeled and Measured
Infiltration: A detailed Case Study of Four Electrically Heated Homes" Electric
Power Research Institute Contract Report RP 2034-40, 1991.
- M.H. Sherman, "EXEGESIS OF PROPOSED ASHRAE STANDARD
119: Air Leakage Performance for Detached Single-Family Residential Buildings."
Proc. BTECC/DOE Symposium on Guidelines for Air Infiltration, Ventilation, and Moisture
Transfer, Fort Worth, TX, December 2-4, 1986. Lawrence Berkeley Laboratory Report
No. LBL-21040, July 1986.
- M.H. Sherman, "Infiltration Degree-Days: A Statistic
for Infiltration-Related Climate," ASHRAE Trans. 92(II), 1986. Lawrence Berkeley
Laboratory Report, LBL-19237, April 1986.
- M.H. Sherman "Estimation of Infiltration from
Leakage and Climate Indicators", Energy and Buildings, 1987
- M.H. Sherman, "A Power Law Formulation of Laminar
Flow in Short Pipes," J Fluids Eng, Vol 114 No 4 pp 601-605, 1992
- M.H. Sherman, "Superposition in Infiltration
Modeling," Indoor Air V.2, pp. 101-114, 1992
- M.H. Sherman, D.T. Grimsrud, "The Measurement
of Infiltration using Fan Pressurization and Weather Data" Proceedings, First
International Air Infiltration Centre Conference, London, England. Lawrence Berkeley
Laboratory Report, LBL-10852, October 1980.
- M.H. Sherman, N.E. Matson, "Ventilation-Energy
Liabilities in U.S. Dwellings, Proc. 14th AIVC Conference pp 23-41, 1993, LBL Report
No. LBL-33890 (1994).
- M.H. Sherman, M.P. Modera, "Infiltration Using
the LBL Infiltration Model." Special Technical Publication No. 904, Measured
Air Leakage Performance of Buildings, pp. 325 - 347. ASTM, Philadelphia, PA, 1984;
Lawrence Berkeley Laboratory
- M.H. Sherman, L.E. Palmiter, "Uncertainties in
Fan Pressurization Measurements. Special Technical Publication of ASTM, Air Flow
Performance of Building Envelopes, Components and Systems, (In Press), LBL-32115
(1994)
- M.H. Sherman and D.J. Wilson, "Relating Actual
and Effective Ventilation in Determining Indoor Air Quality." Building and Environment,
21(3/4), pp. 135-144, 1986. Lawrence Berkeley Report No. 20424.
- R.C. Sonderegger, J.Y. Garnier, J.D. Dixon, "Computerized,
Instrumented, Residential Audit (CIRA)" Lawrence Berkeley Lab Report No. PUB-425,
1981
- Sweden; BFS 1988:18, "Nybyggnadsregler",
Ch. 3,4; National Swedish Board of Physical Planning and Building, 1989
A stack coefficient [-]
Af building floor area [m2]
ACH air change rate (ach) [h-1]
ACH50 air change rate at 50 Pascals pressure difference (ach)
[h-1]
B wind coefficient [-]
C´ generalized shielding coefficient [-]
Cp heat capacity of air [1.022 kJ/kg-°K]
E annual load [kJ]
ELA effective leakage area [m2]
fs stack factor [(m/s)(°K)1/2]
fw wind factor [-]
g gravity [9.8 m/s2]
H building height [m]
HI inside enthalpy [kJ/kg]
HO outside enthalpy [kJ/kg]
IDD infiltration degree days [°C-day]
n power-law exponent [-]
N number of hours [h]
NL normalized leakage area [-]
P pressure [Pa]
Q air flow rate [m3/s]
R fraction of total leakage area in the floor and ceiling [-]
s specific infiltration [m/s]
so average specific infiltration [0.71 m/s]
ΔT inside-outside temperature difference [°C]
To absolute temperature used for reference [298 °K]
κ leakage coefficient [m3/s/Pan]
v measured wind speed [m/s]
X difference in ceiling/floor fractional leakage area [-]
w air change rate factor accounting for effect of local weather (m/s)(4)
ρ density of air [1.2 kg/m3]
[h] indicates hourly value
Blower doors can generate sets of fan flow, and house pressure pairs. Sherman (1992a)
has shown that these data can be expressed empirically as a power law:
(EQ 7)
where the subscript, f, relates to fan-induced pressure or flow. For ease
of use and understanding this two-parameter characterization of flow is reduced to
the one-parameter characterization of the effective leakage area of an orifice:
(EQ 8)
If we assume that these two expression characterize the flow at some reference pressure,,
then we calculate ELA from the blower door data:
(EQ 9)
which leads to
(EQ 10) .
While 10 Pa is sometimes used as the reference pressure in Canada, ASHRAE Standards
and Handbooks normally use 4 Pa for the reference pressure. Accordingly, 4 Pa has
been used as the reference pressure throughout this report.
The effective leakage area, ELA, quantifies the absolute size of the openings
in the building and for the LBL infiltration model is determined by summing the respective
component leakage areas of a specific building. (Blower doors directly measure the
total leakage.) A better measure of the relative tightness, however, is the normalized
leakage as defined in ASHRAE Standard 119:
(EQ 11)
If we combine this expression with Equation 10
for typical conditions found in a single-story house we find that
(EQ 12)
where
is the number of air changes through the house induced by a 50 Pascal pressure from
blower door operation. Note that as air tightness is independent of the driving forces,
there is no need for climate-dependent factors.
The fundamental relationship between the infiltration and the house and climate properties
is expressed by the LBL infiltration model as described in Sherman and Modera (1984),
which is incorporated into the ASHRAE (1989) Handbook of Fundamentals. While Palmiter
and Bond (1991) have suggested potentially valuable improvements to the model for
certain circumstances, the current version is widely used.
The LBL infiltration model is used to generate, on an hourly basis, specific infiltration
and air flow rates. The hourly infiltration rate is calculated using the following
relationship:
(EQ 13)
The LBL infiltration model calculates specific infiltration rate, s[h], as:
(EQ 14)
where the stack and wind factors (fs and fw
respectively) are a function of building properties
The stack factor is calculated as shown in Equation
15
(EQ 15)
where R and X are measures of leakage distribution, H is the
height of the building and To is the outside drybulb temperature.
The wind factor is calculated as shown in Equation
16.
(EQ 16)
where C' can be found from Table
2, "Shielding Parameters," as a function of shielding class.
Table 2: Shielding Parameters
--------------------------------------------
I II III IV V
Class None Light Moderate Heavy Very Heavy
C' 0.34 0.30 0.25 0.19 0.11
--------------------------------------------
A and B can be found from Table 3, "Terrain
Parameters," as a function of terrain class.
Table 3: Terrain Parameters
-------------------------------------------
I II III IV V
Class None Light Moderate Heavy Very Heavy
A 1.30 1.00 0.85 0.67 0.47
B 0.10 0.15 0.20 0.25 0.35
-------------------------------------------
The LBL model allows estimation of instantaneous air change rates. As shown by Sherman
and Wilson (1986) a simple average of these values has, unfortunately, little physical
significance. In order to use the hourly values to find out more physically interesting
information it is necessary use the appropriate type of weighted average over the
appropriate period.
The appropriate period may be all of the occupied hours or it may be a heating or
cooling season. The appropriate type of weighted average depends on the physical
process involved.
In using the LBL model below, a default set of assumptions have been made about heights,
sheltering and leakage distribution. While believed appropriate for estimating impacts
of large populations, corrections for these affects could be significant in individual
cases.
The effective air change rate is defined as the constant air change rate which
would supply the same amount of pollution dilution (or, equivalently, the same average
pollution level) as the actual hourly time series under consideration. It can be
calculated by a process similar to that used in ASHRAE Standard 136-93:
(EQ 17)
where w is the equivalent value of s that would yield the same pollution
levels under constant conditions. If we are careful to assume a minimum value for
the specific infiltration, we can approximate the exact expression as follows:
(EQ 18)
This harmonic average can never be more than the normal arithmetic mean.
The energy used to condition air depends on the temperature or enthalpy difference
between the infiltrating and exfiltrating air. Since the driving forces for infiltration
also depend on the temperature difference, the relationship is non-linear.
A simplified method for treating this non-linearity is to create a statistic that
quantifies the infiltration-related climate. The Infiltration-Degree Day (IDD) method
of Sherman (1986b) creates such a statistic. During the heating season the IDDs can
be calculated by summing over each heating hour:
(EQ 19)
where TH is the indoor heating temperature setpoint (19° C), T[h]
is the outside drybulb temperature and so=0.71 m/s.
For the cooling season, as latent cooling loads may be quite important, both latent
and sensible cooling loads must be considered. The IDDs for each hour should be taken
as the larger of the two values:
(EQ 20)
where TC is the cooling setpoint temperature (25°C).
(EQ 21)
where HO is the enthalpy of the outside air and HI is the enthalpy
of the indoor air.
Hours of heating, cooling and ventilation are determined based on outside temperature
conditions. The total number of IDDs (both heating and cooling) is a good estimate
of the energy intensity of the climate with respect to infiltration. The annual energy
intensity, reflecting heating and cooling energy consumption, can be calculated from
the normalized leakage and the number of infiltration degree days:
(EQ 22)
where the coefficient 86.4 has the units of s/day.
Compliance is checked with the two relevant ASHRAE standards: Standard 119, the tightness
standard, and Standard 62, the ventilation standard.
ASHRAE Standard 119 relates normalized leakage to infiltration degree-days. The standard
can be expressed in the following form:
(EQ 23)
A building is considered to be in compliance with the tightness standard when the
above relationship is true. This expression only guarantees compliance if the definitions
and classes are used as defined, but it will be used herein as a reasonable approximation.
The effective air change rate, as calculated using Equation
17, is the value of the air change rate that should be used in determining compliance
with minimum ventilation requirements. ASHRAE Standard 62 sets minimum air change
rate requirements, for residences, of 0.35 air changes per hour. If we use Equation
17 to represent the effective minimum air change rate then the requirement becomes:
(EQ 24)
A building may be considered to be in compliance with the ventilation standard
when the above relationship is true. It should be noted, for smaller residences,
that the additional requirement of a minimum of 7.5 l/s per occupant must also be
met in order to meet compliance.
Equation 17 through Equation
24 are true assuming that infiltration is the only contributor to the total ventilation.
While this is true for most U.S. houses, incorporation of mechanical ventilation
must be considered as an option. To do so requires that be recalculated. The equations can be found
in Sherman (1992b). Similar considerations are required for combustion-induced ventilation.
Footnotes
- (1)
- This work was supported
by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of
Building Technology of the U.S. Department of Energy under contract no. DE-AC03-76SF00098.
- (2)
- ASHRAE has interpreted the method of standard 136 to be acceptable
for meeting the requirements of Standard 62. Since Standard 136 uses an annual evaluation,
one can infer that only long-term values are important.
- (3)
- The
term "mechanical ventilation" refers to whole-house, purpose-provided ventilation
systems operating for substantial parts of the day. Because of the normally low duty
cycles, local exhaust in kitchens and bathrooms are not generally included.
- (4)
- Note that in ASHRAE Standard 136 the units are expressed in air
changes per hour. For a single-story structure the conversion factor between ach
and m/s is 1.44.