CRD Residential Energy Efficiency & Load Calculations 3 — Questions and Answers
Question 1: A building has 2,400 sq ft of wall area with an overall U-factor of 0.065 BTU/hr·ft²·°F and a DTD of 65°F. What is the wall heat loss?
- 10,140 BTU/hr (Correct answer)
- 15,600 BTU/hr
- 23,920 BTU/hr
- 6,760 BTU/hr
Correct answer: 10,140 BTU/hr
Heat loss = U × Area × DTD = 0.065 × 2,400 × 65 = 10,140 BTU/hr.
Question 2: In Manual J calculations, the 'latent load' refers to:
- Heat associated with moisture removal from the air (Correct answer)
- Heat conducted through the building envelope
- Heat introduced by occupants' body heat
- Heat stored in thermal mass materials
Correct answer: Heat associated with moisture removal from the air
Latent load is the cooling energy required to condense and remove moisture from indoor air, distinct from sensible (temperature-related) heat.
Question 3: Which window property most directly affects the cooling load from solar radiation?
- Solar Heat Gain Coefficient (SHGC) (Correct answer)
- Visible Light Transmittance (VLT)
- U-factor
- Center-of-glass R-value
Correct answer: Solar Heat Gain Coefficient (SHGC)
SHGC measures the fraction of incident solar radiation admitted through a window, making it the primary factor for solar-driven cooling loads.
Question 4: For a geothermal heat pump with an EWT of 50°F in heating mode, which effect does a lower entering water temperature have on capacity?
- Capacity decreases and power consumption increases (Correct answer)
- Capacity increases and COP improves
- Capacity stays constant regardless of EWT
- Capacity increases but COP drops to below 1
Correct answer: Capacity decreases and power consumption increases
As EWT drops, the heat pump must work harder, reducing heating capacity and COP while increasing electrical consumption.
Question 5: Which of the following best describes 'degree-days' as used in energy consumption estimation?
- Cumulative sum of daily mean temperature deviations from a base temperature (65°F) (Correct answer)
- Number of days per year when heating is required
- Total hours per year below freezing
- Monthly average outdoor temperature minus indoor setpoint
Correct answer: Cumulative sum of daily mean temperature deviations from a base temperature (65°F)
Heating degree-days (HDD) sum the daily differences between 65°F and the average outdoor temperature, providing a seasonal measure of heating energy demand.
Question 6: Air sealing a home before sizing a geothermal system is important because:
- Reduced infiltration lowers the design load, potentially allowing a smaller, more efficient system (Correct answer)
- It increases static pressure, improving ground loop flow rates
- Air sealing increases latent loads that the heat pump must handle
- It is required by the geothermal equipment warranty
Correct answer: Reduced infiltration lowers the design load, potentially allowing a smaller, more efficient system
Reducing infiltration directly decreases both sensible and latent loads, so the designer can right-size the geothermal system rather than oversize it.
Question 7: Which ACCA manual covers the design of duct systems to properly distribute conditioned air from a geothermal air handler?
- Manual D (Correct answer)
- Manual J
- Manual S
- Manual N
Correct answer: Manual D
ACCA Manual D provides the procedure for sizing residential duct systems to distribute airflow from any HVAC system, including geothermal air handlers.
CRD Residential Energy Efficiency & Load Calculations 3
A building has 2,400 sq ft of wall area with an overall U-factor of 0.065 BTU/hr·ft²·°F and a DTD of 65°F.
What is the wall heat loss?