Domestic Hot Water Systems
Load, losses, and temperature
DHW energy has three components: the useful load (gallons × 8.34 lb/gal × temperature rise), standby/storage loss from the tank, and distribution loss from piping and recirculation. In recirculating commercial systems, distribution and standby can easily be 20–40% of total water-heating energy - which is why pipe insulation, recirculation control (timed or demand-based), and tank insulation often beat equipment replacement on payback.
Know the equipment set: storage, instantaneous, and semi-instantaneous heaters; heat-pump water heaters (COP typically 2–3.5, with space-cooling benefit and space-heating penalty); condensing gas; solar thermal with SRCC-rated collectors; and drain-water heat recovery. Rating metrics are EF/UEF under standardized test conditions - actual inlet water temperature and draw pattern differ from the rating.
Hygiene matters: store at 140°F or above with a tempering valve to deliver 120°F at fixtures; lowering storage temperature too far creates Legionella risk.
Traps. Ignoring distribution losses so savings are overstated; using rated EF at an actual inlet temperature far from the test condition; mixing gpm with total gallons; forgetting the water and sewer savings from low-flow fixtures; and recommending a setpoint reduction that conflicts with Legionella guidance.
Worked example
A 200-room hotel at 60% occupancy, 2 guests/room, 30 gal/guest-day = 7,200 gal/day. With 8.34 lb/gal and a 65°F rise (55°F to 120°F): 7,200 × 8.34 × 65 = 3.90 MBtu/day useful. At 80% EF the input is 4.88 MBtu/day; over 365 days that is 1,782 MBtu ≈ 17,820 therms/yr. Add roughly 25% for distribution and standby, and pipe insulation plus recirculation controls become the first measures to cost.