HERS Water Heating Efficiency 3 — Questions and Answers
Question 1: What is the primary advantage of a demand (tankless) water heater over a storage water heater?
- It delivers water at higher pressure
- It eliminates standby heat losses because it heats water only when needed (Correct answer)
- It can supply unlimited hot water at any flow rate
- It requires no venting when installed indoors
Correct answer: It eliminates standby heat losses because it heats water only when needed
Tankless water heaters eliminate standby losses entirely because they heat water on demand and do not maintain a stored volume at setpoint temperature between uses.
Question 2: A solar water heating system's 'solar fraction' refers to:
- The percentage of the collector area exposed to direct sunlight
- The proportion of total annual water heating energy supplied by solar energy (Correct answer)
- The ratio of collector tilt angle to local latitude
- The fraction of the day during which solar radiation exceeds 1,000 W/m²
Correct answer: The proportion of total annual water heating energy supplied by solar energy
Solar fraction is the percentage of a home's annual domestic hot water energy demand met by the solar thermal system, with the remainder supplied by a backup conventional heater.
Question 3: For HERS credit purposes, what information is required to model a solar water heating system?
- Brand name, purchase price, and installer certification number
- Collector area, collector orientation/tilt, and solar fraction or system details (Correct answer)
- Tank volume, number of collectors, and local utility rate
- Solar irradiance data only, obtained from a satellite measurement service
Correct answer: Collector area, collector orientation/tilt, and solar fraction or system details
HERS modeling of solar water heating systems requires collector area, orientation, tilt angle, and either a rated solar fraction or sufficient system details to calculate one using software like REM/Rate.
Question 4: What does 'first-hour rating' (FHR) indicate about a storage water heater?
- How many BTUs the burner produces in one hour
- The gallons of hot water the unit can deliver in the first hour of use starting with a full tank (Correct answer)
- The time required to heat a full tank from 50°F to 120°F
- The efficiency decline experienced during the first hour of operation
Correct answer: The gallons of hot water the unit can deliver in the first hour of use starting with a full tank
First-hour rating indicates how many gallons of hot water the heater can supply per hour starting with a full tank, reflecting both tank capacity and recovery rate.
Question 5: Which pipe configuration minimizes hot water wait time and distribution energy losses?
- Trunk-and-branch system with a single large main line
- Home-run manifold with dedicated lines to each fixture
- Structured/compact plumbing layout that minimizes pipe length between heater and fixtures (Correct answer)
- Recirculation loop with a continuous pump running 24/7
Correct answer: Structured/compact plumbing layout that minimizes pipe length between heater and fixtures
A structured plumbing layout that minimizes total pipe length between the water heater and fixtures reduces both the wait time for hot water and the heat lost to the surrounding structure.
Question 6: What temperature setting is generally recommended for residential water heater storage tanks to balance scalding prevention and Legionella control?
- 100°F (38°C)
- 110°F (43°C)
- 120°F (49°C) (Correct answer)
- 140°F (60°C)
Correct answer: 120°F (49°C)
120°F is the standard recommended storage temperature—hot enough to inhibit Legionella growth while low enough to reduce scalding risk and standby losses compared to higher settings.
Question 7: How does a desuperheater connected to a heat pump HVAC system contribute to water heating efficiency?
- It pre-heats domestic hot water using waste heat recovered from the refrigerant during the cooling cycle (Correct answer)
- It increases the HVAC system's SEER rating by adding an extra heat exchange stage
- It reduces water heater standby losses by circulating cooled water through the tank
- It provides a backup heating source when outdoor temperatures drop below 20°F
Correct answer: It pre-heats domestic hot water using waste heat recovered from the refrigerant during the cooling cycle
A desuperheater captures superheat from the heat pump compressor discharge—heat that would otherwise be rejected—and uses it to pre-heat domestic hot water, providing essentially free hot water during cooling operation.
HERS Water Heating Efficiency 3
What is the primary advantage of a demand (tankless) water heater over a storage water heater?