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    • HERS RATER WRITTEN EXAM >
      • Section 1 Building Science Fundamentals >
        • 1a. Basic Terms & Definitions >
          • 1. Airflow in Buildings
          • 2. Equipment Efficiencies
          • 3. Power and Energy
          • 4. Effective Leakage Area
          • 5. Area Weighted R-Value
          • 6. Baseload / Seasonal Energy Use
          • 7. Driving Forces (Including Natural and Mechanical)
          • 8. Behavior of Radiation
          • 9. Thermal Resistance / Transmittance: R and U Values
          • 10. Latent / Sensible Heat
          • 11. Total Equivalent Length
          • 12. Dehumidification / Humidification
          • 13. Convert Pressure Units
          • 14. Thermal Bridges
          • 15. Pressure Boundary
          • 16. Stack Effect
          • 17. Exfiltration and Infiltration
          • 18. Natural / Mechanical Ventilation
          • 19. Net Free Area
          • 20. Input & Output Capacity
          • 21. Peak Electrical Demand
          • 22. Permeability and Perm Rating
          • 23. Standby Loss
          • 24. IAQ (indoor air quality): Moisture, CO, Dust
        • 1b. Principals of Energy, Air & Moisture Thermodynamics >
          • 1. Thermodynamics: Conduction, Convection, Radiation, ΔT
          • 2. Factors That Affect Insulation Performance
          • 3. House Pressurization/Depressurization by Various Forces
          • 4. Heat Gain / Loss
          • 5. Power and Energy
          • 6. Moisture Transport Mechanisms
          • 7. Identify Areas of Highest Relative Humidity
          • 8. Principles of Combustion
        • 1c. Combustion Safety >
          • 1. Combustion Analysis
          • 2. Carbon Monoxide (CO) Testing
          • 3. Combustion Appliance Venting, Draft, Combustion Air & Sizing
          • 4. Understand Combustion Safety Issues
      • Section 2 Buildings and Their Systems >
        • 2a. Building Components >
          • 1. Identify basic duct configurations and components
          • 2. Identify Basic Hydronic Distribution Configurations and Components
          • 3. Identify Basic Structural Components of Residential Construction
          • 4. Thermal Boundaries and Insulation Applications
          • 5. Basic Electrical Components and Safety Considerations
          • 6. Basic Fuel Delivery Systems and Safety Considerations
          • 7. Basic bulk water management components (drainage plumbing gutters sumps etc)
          • 8. Vapor barriers/retarders
          • 9. Radiant Barrier Principles and Installations
          • 10. Understand Fenestration Types and Efficiencies
          • 11. Understand Issues Involved With Basements, Crawlspaces, Slabs, Attics, Attached Garages, Interstitial Cavities, and Bypasses
          • 12. Understand Issues Involved With Ventilation Equipment
          • Understand Basic Heating / Cooling Equipment Components Controls and Operation
          • Understand Basic DHW Equipment Components Controls and Operation
          • Identify Common Mechanical Safety Controls
          • Identify Insulation Types and R-Values
          • Understand Various Mechanical Ventilation Equipment and Strategies: Spot, ERV, HRV
        • Conservation Strategies >
          • Appropriate Insulation Applications and Installation Based On Existing Conditions
          • Opportunity for ENERGY STAR Lighting and Appliances
          • Identify Duct Sealing Opportunities and Applications
          • Understand Importance of Air Leakage Control and Remediation Procedures
          • Blower Door-Guided Air Sealing Techniques
          • Water Conservation Devices and Strategies
          • Domestic Hot Water (DHW) Conservation Strategies
          • Heating & Cooling Efficiency Applications
          • Proper Use of Modeling to Determine Heating and Cooling Equipment Sizing and Appropriate Energy
          • Understand the Use of Utility History Analysis in Conservation Strategies
          • Appropriate Applications For Sealed Crawlspaces Basements and Attics
          • Identify / Understand High Density Cellulose
          • Appropriate Applications for Fenestration Upgrades Including Modification or Replacement
        • Comprehensive Building Assessment Process >
          • Determine Areas of Customer Complaints / Concerns in Interview
          • Understand / Recognize Need For Conducting Appropriate Diagnostic Procedures
          • Interaction Between Mechanical Systems, Envelope Systems and Occupant Behavior
        • Design Considerations >
          • Appropriate Insulation Applications Based On Existing Conditions
          • Understand Fire Codes as Necessary to Apply Home Performance in a Code-Approved Manner
          • Understand / Recognize Building Locations Where Opportunities for Retrofit Materials
          • Understand Climate Specific Concerns
          • Understand Indoor Environment Considerations for the Environmentally Sensitive
          • Understand Impact of Building Orientation, Landscape Drainage, and Grading
          • Opportunity Potential Renewable Energy Applications: Geothermal, Photovoltaic, Wind
          • Understand Impact of Shading on Heating / Cooling Loads
          • Awareness for Solar Gain Reduction / Solar Gain Opportunities
          • Understand Need for Modeling Various Options For Efficiency Upgrades
      • Measurement & Verification of Building Performance >
        • Measurement & Verification of Building Performance >
          • Air Leakage Test Results
          • Understand Building Shell / Envelope Leakage
          • Apply Fundamental Construction Mathematics and Unit Conversions
          • Calculate Building Tightness Levels (Minimum Ventilation Requirements)
          • Calculate Heating Degree Days and Cooling Degree Days
          • Identify Proper Appliance and Combustion Appliance Venting
          • Ventilation calculations and strategies
          • Proper methods for identifying / testing fuel leaks
          • Blower door setup, accurate measurement and interpretation of results
          • Combustion Appliance Zone (CAZ): depressurization, spillage, draft, Carbon Monoxide (ambient and flue)
          • Carbon Monoxide (CO) evaluation: ambient
          • Proper applications and use of temperature measuring devices
          • Pressure pan and room to room pressure diagnostics
          • Recognize contributing factors to comfort problems
          • Inspect for areas containing moisture or bulk water in undesirable locations
          • Understand and inspect for basic electric safety (e.g. frayed wires, open boxes, etc)
      • RESNET HERS RATER National Standards & Project Specifications >
        • Understand applicability content and intent of BPI National Standards – Do no harm, make buildings more healthy, comfortable, durable and energy efficient
        • Recognize need for a professional local/state/national codes evaluation
        • Be able to specify appropriate materials and processes needed for building performance projects
      • Analyzing Buildings Systems >
        • Recognize need for air sealing measures and their impact on other building systems
      • Conduct and Communications >
        • Conservation strategies
        • Conservation strategies
    • HERS RATER FIELD EXAM >
      • How To Put The House Under Worst Case & CAZ
      • What's What? Pa, CFM, CFM50, CAZ, Draft, Room Pressure
      • What To Know In The Attic
      • What To Know In The House
    • BLOWER DOOR TEST >
      • Manometer Setup
    • RESNET STANDARDS >
      • RESNET Standards Decoded
  • ESSENTIALS
    • HELP, I HATE MATH!
    • AUDITOR TO CREW COMMUNICATION
    • COMMON AUDITOR / CREW MISTAKES
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1. Understanding Airflow in Buildings

Quick Terms: 

  1. CFM - cubic feet per minute
  2. CFM50 - cubic feet per minute at 50 Pascals
  3. ACHn - natural air changes per hour
  4. ACH50 - the number of air changes per hour at 50 Pascals
  5. FPM - feet per minute
Terms in depth:

1. CFM - cubic feet per minute is how airflow is measured.  It is the combination of a home's holes (leakage) and the pressure difference between the inside and outside.  Energy auditors and HERS Raters use a blower door to take the pressure difference in a home between the inside and outside at a standard 50 Pascals.  
2. CFM50 - cubic feet per minute at 50 Pascals (Pa for short).  This is the airflow (in Cubic Feet per Minute) needed to create a change in building pressure of 50 Pascals. Energy auditors take blower door readings to measure a home's air and duct leakage.  To standardize air leakage measurements, each home is pressurized or depressurized (it doesn't technically matter, what is important is the pressure difference, the rest depends on your showmanship, troubleshooting skills and the house itself) to 50 Pascals.  When you run a blower door, you will have two readings. One will tell you the pressure the house is at (you dial this in by adjusting the fan speed up or down to 50 Pa, this will never change), the other reading will show you the house leakage at 50 Pa, in CFM (this number will vary and will need to be compared to the minimum ventilation requirements for the house to tell you if the house is too tight, too leaky and how much you can seal the home before it becomes a potential H&S hazard).  A duct blaster is another piece of equipment to measure only duct leakage.  It is the most accurate way to measure duct leakage, but only required in HERS Ratings for Energy Star Homes.  A duct blaster is like a mini blower door and the home is taken to 25 Pa of pressure difference, not 50 Pa like a blower door.
3. ACHn - natural air changes per hour. This is the number of times the air is changed in a home (or space) per hour under natural conditions.  This term is not unique to us energy auditors and is used by engineers to design make-up air for buildings.  Each building has a target ACH it has to reach per code.  For homes, ACH is a nice term for homeowners to hear and relate to, rather than CFM50.  One ACH means that all the air in the house is replace one time every hour.

To calculate ACHn you can use a very general rule of thumb and divide the ACH50 by 20, or you can use a do a multi-point blower door test. To do a multi-point blower door set the CFM to 15, 20, 25, 35, etc up CFM60 and plot on a graph the leakage (what you measured) on the y-axis and the pressure (CFM15, 25, etc) on the x-axis.  Then you can extrapolate to find the actual leakage at any pressure.  Generally, the rule of divide by 20 is good enough for a quick calculation, if you use software with your blower door, the extrapolation is what the computer uses to give ACHn.

Calculate ACH50

4. ACH50 - the number of air changes per hour at 50 Pascals.  This is second way a blower door results are reported (the other being CFM50).  To calculate the ACH50 first find the building volume by taking the square footage of the home and multiplying it by the average ceiling height.  Then take the CFM50 x 60 / volume to get ACH50.

An ACH50 of 19 is very high and you know the house has a lot of room for improvements.  An ACH50 of 1 or less is a very tight, possible Energy Star Certified home.
5. FPM - feet per minute. Blowometers and anemeters measure the velocity of air in FPM.  To get the flow rate, what advanced energy auditors and HVAC specialist are really concerned about in order to diagnose poor airflow. Given a room size, ceiling height and orientation we can know the exact flow rate of supply air coming out the ductwork.  If we can measure it (using the tools above) and adjust it (using baffles, dampeners and some duct re-design) we can solve a homeowner's comfort issues.  To convert FPM to CFM, multiply the FPM by the area of the duct... If you have a 10" flex duct, the area would be ((10" x 1/12) / 2))^2 x 3.14.  If you have a 10 x 6 rectangular sheet duct, the area would be 10 x 6.  FPM is irrelevant in airflow without the area of the ductwork so don't just go off the FPM if you start diagnosing poor airflow.
External Link:http://www.buildingenergysolutions.com/pdfs/bdtest.pdf

Next Section

  1. 1a. Basic terms and definitions
    1. Understand airflow in buildings / ducts: CFM, CFM50, CFM25, ACHn, ACH50, FPM
    2. Understand equipment efficiencies: AFUE, SSE, SEER, EER, HSPF
    3. Understand power and energy: watts, BTU/hr, ton of refrigeration  watt-hours, BTU, therm, decatherm
    4. Understand effective leakage area
    5. Understand area weighted R-Value
    6. Understand baseload / seasonal energy use
    7. Understand driving forces (including natural and mechanical: Pressure, temperature, moisture differential
    8. Understand behavior of radiation: emissivity, reflectivity, absorbtivity
    9. Understand thermal resistance / transmittance: R and U Values; including conversions
    10. Understand latent / Sensible heat: evaporation, condensation / specific heat, heat capacity
    11. Understand total equivalent length
    12. Understand basics of dehumidification / Humidification as well as measurement equipment
    13. Understand and convert Pressure units: Inches of Water Column (iwc), Pascal (Pa)
    14. Understand, identify thermal bridges
    15. Understand pressure boundary 
    16. Understand/define stack effect 
    17. Understand and define exfiltration and infiltration 
    18. Natural / mechanical ventilation 
    19. Understand net free area 
    20. Understand input / output capacity 
    21. Understand peak electrical demand 
    22. Understand permeability and perm rating 
    23. Understand standby loss 
    24. IAQ (indoor air quality): moisture, CO, dust
1b. Principals of energy, air & moisture
  1. Thermodynamics: conduction, convection, radiation, ΔT including air movement due to temperature gradients
  2. Factors that affect insulation performance: density, installation, moisture
  3. House pressurization/depressurization by various forces
  4. Heat gain / loss: internal, solar, heat transmission, air leakage 
  5. Power and energy: BTU content of fuels, capacity of combustion appliances and electrical appliances 
  6. Moisture transport mechanisms: bulk water, air leakage, diffusion, capillary action 
  7. Identify areas of highest relative humidity 
  8. Principles of combustion: combustion analysis, CO 
1c. Combustion science
  1. Combustion analysis: oxygen, flue-gas temperature, carbon monoxide 
  2. Carbon Monoxide (CO) testing of combustion appliances 
  3. Basics of: Combustion appliance venting, draft, and combustion air including identification of proper sizing/vent tables 
  4. Understand combustion safety issues: Combustion air, draft, worst case / baseline depressurization, spillage, backdrafting, unvented combustion appliances 
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