HERS Energy Modeling and Analysis Questions and Answers 1 — Questions and Answers
Question 1: A HERS Rater completes an energy model for a new home, resulting in a HERS Index of 55 and an estimated annual energy cost of $1,500. The homeowner, who works from home and keeps the thermostat at 75°F in the summer, receives their first-year utility bills totaling $2,200. What is the most likely explanation for the discrepancy between the modeled cost and the actual cost?
- The energy modeling software has a mandatory +/- 50% margin of error.
- The actual operating conditions and occupant behavior differed from the standardized assumptions used in the model. (Correct answer)
- The rater must have entered the wrong utility rates into the software.
- The blower door test result was inaccurate, causing the model to be invalid.
Correct answer: The actual operating conditions and occupant behavior differed from the standardized assumptions used in the model.
HERS ratings are based on RESNET/ANSI 301 standard operating conditions (e.g., thermostat settings, internal gains, hot water usage) to create a standardized 'asset rating' for comparing homes. This is different from an 'operational rating' that predicts actual use. Occupant behavior is the primary variable that causes actual energy use to differ significantly from the model's standardized prediction.
Question 2: According to the RESNET/ANSI 301 Standard, which of the following attributes of the HERS Reference Home is set to be identical to the Rated Home to create a basis for comparison?
- Window efficiency (U-factor and SHGC)
- Air leakage rate (ACH50)
- Building geometry, size, and orientation (Correct answer)
- HVAC system efficiency (AFUE/SEER)
Correct answer: Building geometry, size, and orientation
The HERS Reference Home is a computer-generated model with the same geometry, size, and orientation as the Rated Home. Its energy efficiency features (like insulation, window performance, air tightness, and system efficiencies) are set to levels consistent with a baseline energy code (historically the 2006 IECC), creating the benchmark against which the Rated Home is compared to generate the HERS Index of 100.
Question 3: An energy model for an existing home in a cold climate results in a HERS Index of 120. To determine the most cost-effective upgrade recommendation, what is the best first step for the rater to take using the modeling software?
- Create separate model files for each potential upgrade to isolate its individual impact on the HERS Index and energy cost. (Correct answer)
- Combine all potential upgrades into a single improved model to show the maximum possible savings.
- Use the software's default IECC Code compliance improvement package without modification.
- Adjust the utility rates in the software to see how cost savings change with different energy prices.
Correct answer: Create separate model files for each potential upgrade to isolate its individual impact on the HERS Index and energy cost.
This process is known as sensitivity analysis. To determine the cost-effectiveness of individual measures, a rater must first determine the energy savings associated with each one. The standard process is to model each upgrade individually against the base case to see the resulting change in the HERS Index and estimated energy costs. This isolates the impact of each upgrade, which can then be compared against its installation cost.
Question 4: When creating a 'Confirmed Rating' for a new home, which of the following inputs MUST be determined by on-site performance testing rather than being taken from construction plans or default values?
- The R-value of the wall insulation.
- The Solar Heat Gain Coefficient (SHGC) of the windows.
- The envelope leakage rate (ACH50). (Correct answer)
- The foundation type (slab, crawlspace, or basement).
Correct answer: The envelope leakage rate (ACH50).
A confirmed HERS rating requires on-site inspection and performance testing. While insulation levels and window specs are visually verified from labels and installation, envelope leakage (measured in Air Changes per Hour at 50 Pascals, or ACH50) must be measured with a blower door test. This tested value is a critical, non-negotiable input for an accurate energy model.
Question 5: Which of the following best describes the primary purpose of using standardized operating conditions (e.g., thermostat settings, number of occupants, internal gains) in a HERS rating energy model?
- To accurately predict the future energy bills for the specific family that will occupy the home.
- To provide a consistent, repeatable basis for comparing the energy efficiency of different homes, regardless of occupant behavior. (Correct answer)
- To meet the minimum data input requirements for federal tax credit applications.
- To simplify the data entry process and reduce modeling time for the HERS Rater.
Correct answer: To provide a consistent, repeatable basis for comparing the energy efficiency of different homes, regardless of occupant behavior.
The HERS Index is an 'asset rating,' not an 'operational rating.' By using standardized assumptions for how the home is operated, the model can compare the intrinsic energy efficiency of one home's construction and systems to another's. This allows for a fair, apples-to-apples comparison, similar to how the EPA uses a standard driving cycle to determine a car's MPG rating.
Question 6: A HERS Rater is modeling a two-story home with a vented, unconditioned attic. The attic floor is insulated, and the air handler and all ductwork are located in this attic. How must the location of the duct system be specified in the energy modeling software for an accurate rating?
- Inside conditioned space, because it serves the house.
- In a buffer space, such as an unconditioned basement.
- The location does not need to be specified if duct leakage is tested.
- Outside conditioned space. (Correct answer)
Correct answer: Outside conditioned space.
The thermal boundary of the home is at the insulated attic floor. A vented attic is considered 'outside conditioned space' because it is exposed to outdoor temperature fluctuations. Any mechanical systems or ductwork located in the attic must be modeled as being outside, which correctly accounts for the significant thermal losses and gains they will experience, as well as the impact of duct leakage to the outdoors.
HERS Energy Modeling and Analysis Questions and Answers 1
A HERS Rater completes an energy model for a new home, resulting in a HERS Index of 55 and an estimated annual energy cost of $1,500.
The homeowner, who works from home and keeps the thermostat at 75°F in the summer, receives their first-year utility bills totaling $2,200.
What is the most likely explanation for the discrepancy between the modeled cost and the actual cost?