CRD Residential Energy Efficiency & Load Calculations 5 — Questions and Answers
Question 1: A designer is sizing a geothermal system for a retrofit home that was originally designed for a gas furnace. What is the primary concern with using the old equipment nameplate capacity as the new geothermal sizing target?
- Old gas furnace systems are frequently oversized, which would lead to an oversized geothermal system (Correct answer)
- Gas furnaces operate at lower efficiency than geothermal, so the same size is always too small
- Geothermal heat pumps cannot replace gas furnaces in existing duct systems
- Nameplate capacity of gas furnaces is measured in watts, not BTU/hr
Correct answer: Old gas furnace systems are frequently oversized, which would lead to an oversized geothermal system
Gas furnaces are historically oversized by 50–100%, so reusing that capacity as a geothermal sizing target results in significant oversizing and poor part-load efficiency.
Question 2: Which of the following is a key advantage of performing a blower door test before designing a geothermal system?
- It quantifies actual air leakage to improve infiltration inputs in Manual J calculations (Correct answer)
- It measures soil thermal conductivity for ground loop design
- It determines the correct refrigerant charge for the heat pump
- It verifies that duct static pressure meets Manual D requirements
Correct answer: It quantifies actual air leakage to improve infiltration inputs in Manual J calculations
A blower door test measures actual envelope air tightness in ACH50, allowing the designer to calculate more accurate infiltration rates for Manual J.
Question 3: Which climate zone classification system is referenced in the IECC and directly informs insulation R-value requirements for geothermal system load calculations?
- ASHRAE/IECC Climate Zones 1–8 (Correct answer)
- Köppen Climate Classification
- USDA Hardiness Zones
- NOAA Climate Regions
Correct answer: ASHRAE/IECC Climate Zones 1–8
IECC uses climate zones 1–8 (based on ASHRAE 169) to define prescriptive insulation, window, and air sealing requirements that inform design loads.
Question 4: In a 2,000 sq ft home with an 8-foot ceiling, an infiltration rate of 0.5 ACH represents approximately how many CFM of outdoor air infiltrating?
- 133 CFM (Correct answer)
- 1,000 CFM
- 16,000 CFM
- 267 CFM
Correct answer: 133 CFM
Volume = 2,000 × 8 = 16,000 ft³; ACH × Volume / 60 = 0.5 × 16,000 / 60 ≈ 133 CFM.
Question 5: Which phenomenon causes basement walls below grade to have lower effective heat loss than above-grade walls with the same U-factor?
- The soil temperature surrounding the basement remains warmer than outdoor air in winter, reducing the effective DTD (Correct answer)
- Below-grade walls are always better insulated by code
- Moisture in soil acts as an insulator around basement walls
- Underground walls are exempt from heat loss calculations in Manual J
Correct answer: The soil temperature surrounding the basement remains warmer than outdoor air in winter, reducing the effective DTD
Below-grade soil temperatures (typically 50–60°F) are much warmer than winter outdoor air temperatures, so the effective temperature difference driving heat loss is smaller.
Question 6: When calculating cooling loads for a home with a dark-colored roof in a hot climate, which correction factor must be applied in Manual J?
- Equivalent Temperature Difference (ETD) or Cooling Load Temperature Difference (CLTD) adjusted for roof color and mass (Correct answer)
- A SHGC value for the roof surface
- An infiltration correction for roof-level stack effect
- A latency factor for roofing moisture absorption
Correct answer: Equivalent Temperature Difference (ETD) or Cooling Load Temperature Difference (CLTD) adjusted for roof color and mass
Manual J uses CLTD (or ETD) values that account for roof color, mass, and exposure to properly calculate solar-driven heat gain through the roof assembly.
Question 7: A home energy auditor finds that a home's actual annual heating energy use is 30% higher than the Manual J-based estimate. Which factor is MOST likely responsible?
- Higher-than-assumed infiltration due to air leakage not accounted for in the original calculation (Correct answer)
- The geothermal system's COP is higher than the manufacturer's rating
- The ground loop was oversized, reducing system efficiency
- Occupants maintain a lower thermostat setpoint than design assumptions
Correct answer: Higher-than-assumed infiltration due to air leakage not accounted for in the original calculation
Underestimating infiltration in Manual J is the most common cause of actual heating energy exceeding predicted values, especially in older or poorly sealed homes.
CRD Residential Energy Efficiency & Load Calculations 5
A designer is sizing a geothermal system for a retrofit home that was originally designed for a gas furnace.
What is the primary concern with using the old equipment nameplate capacity as the new geothermal sizing target?