FC Load Calculations & Equipment Sizing 2 — Questions and Answers
Question 1: What is a common negative consequence of installing an oversized furnace in a residence?
- Higher annual fuel efficiency
- Short cycling, leading to reduced comfort and increased wear (Correct answer)
- Lower installation cost due to simpler controls
- Improved indoor air quality from longer run times
Correct answer: Short cycling, leading to reduced comfort and increased wear
An oversized furnace heats the space too quickly and short cycles, which reduces comfort through uneven temperatures, increases mechanical wear, and lowers overall efficiency.
Question 2: Which ACCA manual provides guidance specifically for residential duct system design?
- Manual J
- Manual S
- Manual D (Correct answer)
- Manual T
Correct answer: Manual D
ACCA Manual D is the standard for designing residential duct systems, ensuring proper airflow to match the calculated loads from Manual J.
Question 3: What is the maximum recommended oversizing percentage for a gas furnace according to ACCA Manual S guidelines?
- 10%
- 40% (Correct answer)
- 25%
- 100%
Correct answer: 40%
ACCA Manual S generally allows gas furnaces to be oversized by up to 40% of the calculated heating load to account for setback recovery and future needs.
Question 4: A home has a calculated design heating load of 60,000 BTU/h. Which furnace output capacity is the most appropriate selection?
- 40,000 BTU/h output
- 80,000 BTU/h output (Correct answer)
- 120,000 BTU/h output
- 20,000 BTU/h output
Correct answer: 80,000 BTU/h output
An 80,000 BTU/h output furnace represents a 33% overage over the 60,000 BTU/h load, which falls within the ACCA Manual S acceptable range for gas furnaces.
Question 5: How does a home's infiltration rate affect the heating load calculation?
- Higher infiltration decreases heating load by bringing in fresh air
- Higher infiltration increases heating load by introducing cold outside air (Correct answer)
- Infiltration only affects cooling loads, not heating loads
- Infiltration rate cancels out with exhaust fan calculations
Correct answer: Higher infiltration increases heating load by introducing cold outside air
Higher infiltration brings more unconditioned cold outside air into the building, increasing the heating energy required to bring that air up to the design temperature.
Question 6: What is the purpose of calculating 'sensible heat ratio' (SHR) in HVAC equipment sizing?
- To determine the correct furnace input BTU rating
- To balance the proportion of sensible versus latent cooling capacity (Correct answer)
- To calculate the flue gas temperature rise
- To size the gas supply line diameter
Correct answer: To balance the proportion of sensible versus latent cooling capacity
Sensible Heat Ratio (SHR) describes what fraction of a unit's total cooling capacity handles sensible (temperature) versus latent (moisture) loads, ensuring proper dehumidification.
Question 7: When performing a load calculation for a home addition, what must the technician verify about the existing furnace?
- Whether the furnace has a variable-speed blower
- Whether the existing furnace has sufficient capacity to handle the additional load (Correct answer)
- Whether the furnace was installed by a licensed contractor
- Whether the furnace uses a two-stage gas valve
Correct answer: Whether the existing furnace has sufficient capacity to handle the additional load
Before extending an existing heating system to a new addition, the technician must verify that the existing furnace has enough remaining capacity to handle the additional heating load.
FC Load Calculations & Equipment Sizing 2
What is a common negative consequence of installing an oversized furnace in a residence?