ASHRAE Journal - July 2014 - 70

Column EnErgy modEling

Improving Infiltration
In Energy Modeling
By LISA C. NG, Ph.D., ASSoCIATE MEMBEr AShrAE; ANDrEw K. PErSILy, Ph.D., FELLow AShrAE; STEVEN J. EMMErICh, MEMBEr AShrAE

As building envelope performance and HVAC equipment efficiencies continue to be
improved to reduce building energy use, a greater percentage of the total energy loss
of a building can occur through envelope leakage. Although the energy impacts of
unintended infiltration on a commercial building's energy use can be significant,1
current energy simulations and building designs generally do not accurately account
for envelope infiltration and the impacts of improved airtightness.
New strategies to incorporate airflow calculations
into building energy calculations have been developed,
which are more accurate than current approaches in
energy simulation software and easier to apply than
multizone airflow modeling.2 These new strategies are
based on relationships between infiltration rates calculated using multizone airflow models, weather conditions, and building characteristics, including envelope
airtightness and HVAC system operation.

Infiltration in Energy Modeling Now
Infiltration has long been recognized as a key component of heating and cooling loads. Various methods exist
to account for infiltration in load calculations and more
detailed energy analysis. EnergyPlus uses the following
empirical equation to calculate infiltration:
Infiltration = Idesign × Fschedule [A + B|ΔT| + C × Ws + D × Ws2] (1)
where Idesign is defined by EnergyPlus as the "design
infiltration rate," which is the airflow through the building envelope under design conditions. Its units are
selected by the user and can be h-1, m3/s·m2 or m3/s.
Fschedule is a factor between 0.0 and 1.0 that can be scheduled, typically to account for the impacts of fan operation on infiltration. |ΔT | is the absolute indoor-outdoor
temperature difference in °C, and Ws is the wind speed
in m/s. A, B, C and D are constants, for which values are
suggested in the EnergyPlus user manual.2 However,
those values are based on studies in low-rise residential
buildings and do not apply very well to taller buildings and mechanically ventilated buildings. Given the
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challenges in determining valid coefficients for a given
building, a common strategy used in EnergyPlus for
incorporating infiltration is to assume fixed infiltration rates, sometimes using a different constant value
depending on whether the HVAC system is on or off.
However, this strategy does not reflect known dependencies of infiltration on outdoor weather and the complexities of HVAC system operation.

A Simple Equation Made Better
A new strategy has been developed that can more
accurately estimate infiltration in EnergyPlus and other
energy simulation tools. In this method, A, B, and D values in Equation (1) are determined based on key building characteristics. (Note that C is assumed to be equal to
zero since the infiltration rates were not highly impacted
by that term as demonstrated in Reference 3. The building characteristics considered are: building height (H in
m), exterior surface area to volume ratio (SV in m2/m3),
and net system flow (i.e., design supply air minus design
return air minus mechanical exhaust air) normalized by
exterior surface area (Fn in m3/s·m2).
Note that the exterior surface area was calculated
considering the building surfaces subject to infiltration, which are the above-grade walls and roof. This
is different than the approach taken in Appendix G of
ASHRAE/IES Standard 90.1-2013, but the two values
can easily be converted to one another.4 The following
Lisa C. Ng, Ph.D., and Steven J. Emmerich are mechanical engineers and Andrew K.
Persily, Ph.D., is the group leader of the Indoor Air Quality and Ventilation Group at the
National Institute of Standards and Technology (NIST), Gaithersburg, Md.

ASHRAE Journal - July 2014

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