Water Heater Types

Storage Tank Water Heaters

The most common type in residential and light commercial applications. A tank (20–120 gallons for residential; 75–500+ gallons for commercial) maintains water at setpoint temperature continuously. Heat sources include natural gas, propane, fuel oil, and electric resistance elements. The key sizing parameter is First-Hour Rating (FHR) — the volume of hot water deliverable in the first hour of operation starting with a full, heated tank — which accounts for both stored capacity and recovery rate.

Tankless (Instantaneous) Water Heaters

Tankless units heat water on demand with no storage. Key sizing parameter is flow capacity (GPM) at a specified temperature rise. A unit rated for 5.0 GPM at 70°F rise (incoming 50°F groundwater to delivery 120°F) cannot deliver 5.0 GPM at a 90°F rise. Gas-fired tankless units range from 140,000 to 400,000 BTU/hr input; electric units are typically limited by available electrical service (240V, 80–200A). Advantages: no standby loss, unlimited hot water capacity (within GPM rating), small footprint. Disadvantages: high flow demand can exceed single-unit capacity, minimum flow rate required to activate burner (typically 0.5 GPM), sensitive to inlet water temperature in cold climates.

Heat Pump Water Heaters (HPWH)

HPWHs extract heat from ambient air and transfer it to the water via a refrigeration cycle, achieving energy factor values of 3.0–4.0 (delivering 3–4 BTU of heat per BTU of electrical energy consumed). They require a sufficiently large unconditioned or semi-conditioned space (minimum 700–1,000 cubic feet of air volume), and they cool and dehumidify the surrounding space as a byproduct — beneficial in warm climates, potentially problematic in cold climates where the heat is extracted from conditioned air. HPWHs qualify for federal tax credits and many utility rebates. They operate more slowly than electric resistance units and often include a backup electric resistance element for high-demand periods.

Sizing Methodology

Residential Storage Sizing

The demand method from the ASHRAE HVAC Applications handbook calculates peak hourly demand based on occupancy:

  • Shower: 15–20 gallons at 105°F (10–15 minutes at 1.5–2.0 GPM)
  • Bathtub fill: 20–40 gallons
  • Clothes washer (hot cycle): 15–30 gallons
  • Dishwasher: 5–10 gallons
  • Kitchen sink: 2–5 gallons per use

Sum the expected simultaneous uses during the morning peak hour. Select a water heater whose FHR meets or exceeds this value. The FHR is listed on the EnergyGuide label for all residential water heaters and must be tested per DOE procedures.

Commercial Sizing: Design Hour Demand Method

For commercial applications, ASHRAE provides design hour demand (DHD) data by building type. Hotels: 15–20 gallons per guest room per hour during peak. Hospitals: 15–25 gallons per bed per hour. Food service (full restaurant): 1.5–2.0 gallons per seat per meal period. Office buildings: 1–2 gallons per worker per day (low demand, can often be served by point-of-use units). The storage volume should be sized to supply 1.0–1.5 hours of peak demand, with the recovery rate supplying ongoing demand beyond the storage period.

Recovery Rate

Recovery rate is expressed in GPH (gallons per hour) at a specified temperature rise (typically 100°F rise from 40°F inlet to 140°F). For gas-fired units: Recovery GPH = (Burner BTU/hr × thermal efficiency) / (8.33 lb/gal × 1 BTU/lb·°F × 100°F). A 199,000 BTU/hr gas water heater at 80% efficiency recovers approximately (199,000 × 0.80) / (8.33 × 100) = 191 GPH.

Legionella Control: Temperature Requirements

Legionella pneumophila colonizes domestic hot water systems at temperatures between 77°F and 113°F, with optimal growth at 95–115°F. ASHRAE Standard 188 (Legionellosis: Risk Management for Building Water Systems) and ASSE 1082 / HTM 04-01 establish the following thermal control strategy:

  • Storage temperature: ≥ 140°F (60°C) — kills Legionella within minutes
  • Distribution circulation return temperature: ≥ 124°F (51°C)
  • Point-of-use delivery temperature: ≤ 120°F (49°C) — scald prevention per ASSE 1016 / ASSE 1070

The conflict between storage temperature (140°F) and delivery temperature (120°F) is resolved with a thermostatic mixing valve (TMV) or point-of-use mixing valve installed on the system outlet. The TMV (ASSE 1017 for whole-system, ASSE 1016 for individual shower, ASSE 1070 for lavatory) blends hot water from the heater with cold water to achieve the desired delivery temperature while the storage system maintains Legionella-inhibiting temperatures.

ASHRAE 188 requires a Water Management Plan (WMP) for buildings with centralized domestic hot water systems, cooling towers, or decorative fountains. The WMP must include schematic diagrams, control points, monitoring frequencies, and corrective action thresholds.

Energy Performance Metrics

Energy Factor (EF) was the legacy DOE metric, defined as the ratio of useful energy output to total energy input over a 24-hour simulated use cycle (with 64 gallons of hot water drawn in six equal draws). It has been replaced by the Uniform Energy Factor (UEF) per DOE test procedure 10 CFR Part 431 / 430. UEF test protocols vary by usage draw pattern (Very Small, Low, Medium, High) selected based on tank volume and first-hour rating. UEF values by technology type (approximate):

  • Electric resistance storage: UEF 0.90–0.95
  • Natural gas storage: UEF 0.60–0.70
  • Natural gas tankless: UEF 0.82–0.96
  • Heat pump water heater: UEF 2.8–4.0

Standby Loss and Insulation

Standby loss is the rate of heat loss from a storage tank to surrounding ambient air when no hot water is being drawn. It represents wasted energy paid to maintain storage temperature 24/7. Standby loss is reduced by increasing tank insulation (R-16 or higher foam insulation on modern tanks vs R-7 on older units), locating the water heater in conditioned space, and using pipe insulation on distribution piping (minimum R-3 for the first 8 feet of hot and cold supply to the water heater per IECC §R403.5.1). Tank wraps (additional fiberglass blankets) on older tanks provide marginal improvement. The most effective strategy is replacing old tanks with high-UEF units or converting to tankless or HPWH systems.