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RED Calc Free Help Design Infiltration
Design Infiltration with AIM-2 Tool User Guide

Design Infiltration with AIM-2
RED Calc Tool User Guide

What this tool can do for you

This tool calculates the stack-induced, the wind-induced, and combined infiltration rates, as well as the resulting sensible heat loss or gain, for given indoor/outdoor temperatures and wind speed. The calculations use the same, powerful infiltration model, AIM-2, that was used to calculate the weather and shielding factors for the ASHRAE 62.2-2013 ventilation standard. The primary use cases include:

  • Determining the design infiltration rate that can be used in a heating or cooling load calculation for system sizing.
  • Investigating how infiltration rate depends on building leakage, building height, flues, leakage distribution, sheltering, terrain, and weather conditions.
  • See how the stack-induced and wind-induced infiltration rates combine in a sub-additive way because of their mutual interference. By sub-additive we mean that the combination is less than the sum of the two.

EPA pictorial of infiltration/exfiltration sites
Infiltration/exfiltration sites, EPA

Calculated values

  • Stack-induced infiltration for given indoor and outdoor temperatures.
  • Wind-induced infiltration for given wind speed.
  • Design (combined) infiltration for given indoor/outdoor temperatures and wind speed.
  • Design infiltration sensible heat loss (heating) or heat gain (cooling) energy.

Tips

  • Clicking the label for any input or result will cause a popup help box to appear. This help box includes the allowed and normal values (for inputs). Read more.
  • This tool, based on the AIM-2 infiltration method, is not intended for buildings of more than three stories above grade, or for multi-unit buildings.
  • An important use for this tool is the determination of the design infiltration heating and cooling load for the purposes of sizing heating and cooling systems. To use it for this purpose, you must enter the design indoor and outdoor temperatures and wind speed at the location of the building. Please see the References section below for help finding the appropriate weather data.
  • Use the default percentages for leakage distribution unless you have reason to believe you have more accurate vales. The default percentages are dependent on the building height and whether or not the building has a vented crawlspace.
  • Make sure you read the pop-up help for "Terrain category" and "Building shelter class" before making your selection from the drop-downs. Click/touch on the input label to read the pop-up help.
  • Regarding the "Building has open flue/chimney" section inputs:
    • If the building has flues and/or chimneys that are closed, such as fireplaces with tight-fitting dampers, do not include them in this section.
    • If there are multiple open flues, add their areas and average their heights.
  • If the "Design indoor temperature" is higher than the "Design outdoor temperature", "Design infiltration heat loss" appears as the last result of the tool. On the other hand, if the "Design indoor temperature" is lower than the "Design outdoor temperature", "Design infiltration heat gain" appears as the last result of the tool.
  • The stack-induced and wind-induced infiltration rates combine in a sub-additive way because of their mutual interference. By sub-additive we mean that the combination is less than the sum of the two.
  • When either the stack-induced or wind-induced infiltration rate is more than four times the other, you will find that the combined infiltration rate is actually less than the larger of the two. This is a consequence of the way the two modes of infiltration interfere with each other.

Inputs and field measurements

  • Building leakage @ 50Pa - value from standard blower door test at 50 pascals. Ideally, this should be corrected to standard conditions at seal level. This can be done using either the ASHRAE 6.2-2010 or ASHRAE 62.2-2013 ventilation tools.
  • Pressure exponent - use default of 0.65 unless you know the value for the building in question.
  • Building height - the average distance between (a) the bottom pressure boundard, or grade, whichever is higher, and (b) the top pressure boundary. Generally one story is 8.2 feet (2.5 meters), two stories is 16.4 feet (5 meters), etc. The enclosed volume should correspond to the door positions used during the blower door test as much as possible.
  • Building has a vented crawlspace - checkbox for indicating that the building sits on top of a vented (or open) crawlspace. This affects the default leakage distribution and the wind infiltration calculation.
  • Building shelter class - five possible selections from a drop down.
  • Terrain category - four possible selections from a drop down.
  • Design indoor temperature - the design indoor temperature for heating is usually 70°FF (21°FC) and for cooling is 75°F (24°FC).
  • Design outdoor temperature - use design weather data for building location. You can also use any outdoor temperature of interest.
  • Design wind speed - use design weather data for building location. You can also use any wind speed of interest.
  • Optional inputs:
    • Ceiling leakage % - estimated air leakage through upper ceiling.
    • Floor leakage % - estimated air leakage through lower floor area.
    • Height flue extends above grade - if the grade is uneven, use an average.
    • Flue cross-section type - for convenience in calculating the flue cross-setional area.
    • Flue dimensions - inputs for diameter and rectangular shapes.

Background

The Alberta Infiltration Model (AIM-2) applies to low-rise (up to three stories) detached, single-family, residential structures. The development of the empirical model began in the 1980s and was more finely developed over the next decade by David Wilson, Iain Walker, Larry Palmiter, Tami Bond, Max Sherman, and others.

For the mathematical model used for this RED Calc Free tool, "over 3400 hours of measured ventilation rates from the test houses at the Alberta Home Heating Research Facility were used to validate the predictions of infiltration rates and to compare the AIM-2 predictions to those of other ventilation models. The AIM-2 model had bias and scatter errors of less than 15% for wind-dominated ventilation, and less than 7% for buoyancy ("stack-effect") dominated cases." [Please see Walker and Wilson in "Related external resources" below.]

References

  • ASHRAE. 2013 ASHRAE Handbook: Fundamentals. Chapter 14, Climate Design Information, table at end of Chapter "Design Conditions for Selected Locations". Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. This document may be purchased at www.ashrae.org.
    Comment: This chapter includes weather data for selected sites including heating dry bulb (99.6% and 99%) outdoor design temperatures; cooling dry bulb (0.4%, 1%, and 2%) outdoor design temperatures; and annual (1%, 2.5% and 5%) wind speed data. We suggest using the 99% dry bulb temperature for the heating "Design outdoor temperature" input; 1% dry bulb temperature for the cooling "Design outdoor temperature" input; and 2.5% annual wind speed data for the "Design wind speed" inputs of this tool.
  • ACCA. Residential Load Calculation - Manual J, 8th edition. Table 1A: Outdoor Design Conditions for the United States and Canada. Arlington, VA: Air Conditioning Contractors of America. 2011. This document may be purchased at www.acca.org.
    Comment: This table includes weather data for selected sites including heating dry bulb 99% outdoor design temperatures; cooling dry bulb 1% outdoor design temperatures; but does not include wind speed data. We suggest you refer to the ASHRAE reference above for wind data.

Related tools

  • Advanced Infiltration with AIM-2: This tool calculates the hourly (low, average, and high) stack, wind, ventilation and combined infiltration for a selected date range (one day to one year); and the infiltration load on a heating and/or cooling system for the same date range. Additionally, daily average infiltration rates (stack, wind, and combined) and daily infiltration load on heating and cooling systems are displayed on separate charts for a selected TMY3 weather station location and the selected date range.
  • ASHRAE 62.2-2013 Ventilation: Determine whole-building ventilation requirements for new and existing dwellings, with the choice of using advanced blower door options and the alternative compliance path.
    Comment: Use the "Use Advanced Blower Door Inputs" of this related tool for a more accurate "Building leakage @ 50Pa" input.
  • ASHRAE 62.2-2010 Ventilation: Determine whole-building ventilation requirements for new and existing dwellings, with the choice of using advanced blower door options and the alternative compliance path.
    Comment: Use the "Use Advanced Blower Door Inputs" of this related tool for a more accurate "Building leakage @ 50Pa" input.

Related external resources

  • Walker, Iain and Wilson, David. Field Validation of Algebraic Equations for Stack and Wind Driven Air Infiltration Calculations LBNL 42361. 1998. Lawrence Berkeley National Laboratory. http://epb.lbl.gov/publications/pdf/lbnl-42361.pdf
    Comment: This is the paper that explains the AIM-2 infiltration model. From the abstract: "Over 3400 hours of measured ventilation rates from the test houses at the Alberta Home Heating Research Facility were used to validate the predictions of ventilation rates and to compare the AIM-2 predictions to those of other ventilation models. The AIM-2 model had bias and scatter errors of less than 15% for wind-dominated ventilation, and less than 7% for buoyancy ("stack-effect") dominated cases."

Version 2016-07-06_01:30
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