Domestic Hot Water Systems
What this domain tests
Domestic hot water questions test load calculation, efficiency ratings, distribution losses, and system selection.
Core rules
- Heat rate: Btu/h = 500 × gpm × ΔT; daily energy = 8.33 × gallons × ΔT.
- Recovery efficiency and standby loss both matter; EF and UEF are tested ratings.
- Distribution losses and recirculation can exceed tank losses in large buildings.
- Lower the storage setpoint and insulate pipes and tanks before adding capacity.
- Solar thermal and heat-pump water heaters reduce purchased energy but have their own COP and backup requirements.
Worked example
A building uses 500 gallons/day with an 80°F temperature rise. Daily heat = 8.33 × 500 × 80 = 333,200 Btu. At 80% efficiency, input = 333,200 ÷ 0.80 = 416,500 Btu/day, or about 152 MMBtu/yr. If a low-flow retrofit cuts usage to 400 gallons/day, input falls to about 122 MMBtu/yr - a 20% saving without changing the heater.
Traps
Sizing on tank volume rather than peak draw; ignoring recirculation and pipe losses; using EF as if it were a steady-state combustion efficiency; and forgetting that tempering valves and mixing can mask savings.