Water Heating Efficiency Flashcards

What is the purpose of a tempering (mixing) valve installed at the water heater outlet?

Answer: To mix hot and cold water to deliver a safe, lower temperature at fixtures while allowing the tank to be stored hotter for Legionella control

A tempering valve allows the tank to be set at 140°F to kill Legionella while delivering blended water at a safer 120°F or lower to fixtures, reducing scalding risk without compromising sanitation.

How does water hardness affect water heater efficiency over time?

Answer: Mineral scale deposits on heating elements and tank walls reduce heat transfer efficiency and increase energy use

Calcium and magnesium deposits from hard water form scale on heating elements and tank interiors, acting as insulation that forces the heater to work harder and consume more energy to maintain setpoint temperature.

Under IECC 2021 requirements, what is the minimum efficiency required for gas storage water heaters with inputs ≤75,000 BTU/hr?

Answer: EF 0.67 or UEF 0.69

IECC 2021 requires gas storage water heaters ≤75,000 BTU/hr to meet a minimum UEF of 0.69 (or equivalent EF 0.67), pushing manufacturers toward improved insulation and heat exchanger designs.

What is 'thermal stratification' in a storage water heater, and why is it beneficial?

Answer: The natural layering of hot water at the top and cooler water at the bottom, which preserves usable hot water volume

Thermal stratification allows hot water to accumulate at the top of the tank where the outlet is located, maximizing the volume of usable hot water and reducing mixing losses that would otherwise force more frequent reheating.

A HERS rater is verifying a heat pump water heater installation. Which of the following would be a deficiency requiring correction?

Answer: The unit's exhaust air is directed back into the same small (300 cubic foot) sealed closet as the inlet

Recirculating exhaust air back into the same small sealed space causes the HPWH to rapidly cool its own air source, dramatically reducing efficiency and potentially triggering constant resistance backup operation.

Which metric is used by the HERS Index to compare the rated home's water heating energy use against the reference home?

Answer: Annual water heating energy consumption in kBtu/yr calculated by the rating software

HERS software calculates annual water heating energy in kBtu/yr for both the rated home and the reference home, and the ratio contributes proportionally to the overall HERS Index score.

What is the role of an expansion tank on a closed water heater system?

Answer: To absorb the increased volume of water that results from thermal expansion, preventing pressure buildup that could damage the tank or relief valve

When a check valve or pressure-reducing valve creates a closed system, heated water expands with nowhere to go; an expansion tank provides a compressible air cushion that absorbs this volume increase and protects system components.