Residential Energy Efficiency & Load Calculations Flashcards

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?

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.

Which of the following is a key advantage of performing a blower door test before designing a geothermal system?

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.

Which climate zone classification system is referenced in the IECC and directly informs insulation R-value requirements for geothermal system load calculations?

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.

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?

Answer: 133 CFM

Volume = 2,000 × 8 = 16,000 ft³; ACH × Volume / 60 = 0.5 × 16,000 / 60 ≈ 133 CFM.

Which phenomenon causes basement walls below grade to have lower effective heat loss than above-grade walls with the same U-factor?

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.

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?

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.

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?

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.