Terminology

• Storage tank: glass-lined steel or stainless tank heated by gas burner or electric elements
• Tankless (instantaneous): heats water on demand
• Heat pump water heater (HPWH): moves heat from surrounding air into the tank
• Condensing water heater: high-efficiency gas unit with plastic or stainless venting
• T&P relief valve: temperature and pressure relief valve
• Recirculation / heat trace: keeps hot water available near fixtures
• Mixing valve: blends hot and cold water to a set temperature
• Thermal expansion tank: absorbs expansion in closed systems
• First hour rating / recovery: capacity measures

Standards

Ratings

• Temperature relief valves set at 210°F maximum; pressure relief at 150 psi or the rated working pressure, whichever is less
• Relief valve discharge piping full size, by gravity, ending 6 in. maximum above the floor or receptor, with no valves or traps
• Drain pans at least 1-1/2 in. deep with a 3/4 in. drain where leakage could cause damage
• Hot water piping from source to fixtures 50 ft maximum developed length, unless recirculated or heat-traced (commercial energy code limits may govern)
• Tankless outlet temperature 140°F maximum for domestic use
• Attic installations need a passage at least 30 in. high by 22 in. wide, 20 ft long maximum, and a 30 by 30 in. working space

Test standards

• ANSI Z21.22: relief valves and vacuum relief valves
• ASSE 1017: thermostatic mixing valves
• NSF 372: lead content in drinking water components
• ASSE 1070: water temperature limiting devices

Components

• Heater: tank, tankless module, or heat pump unit
• Relief valves and discharge piping
• Venting and combustion air: for fuel-fired units
• Mixing valve and recirculation pump
• Expansion tank, drain pan, and condensate drain

Detailing

• Locate heaters centrally to keep hot water runs short
• Give heat pump water heaters enough room air volume and condensate drainage
• Route vents to terminations away from openings and intakes
• Provide access for replacement without removing permanent construction
• Plan electrical capacity for heat pump and electric units in electrified buildings

Uses

Specific uses

• Homes and dwelling units
• Restaurants and commercial kitchens
• Hotels, dormitories, and healthcare
• Gyms and locker rooms
• Multifamily central plants

Pros and cons

Pros

• Heat pump units use far less energy than electric resistance
• Tankless units save space and standby losses
• Central plants with recirculation give quick hot water

Cons

• Gas units need venting and gas service
• Heat pump units need space, airflow, and condensate drains
• Legionella and scald risks require temperature management

Manufacturing and cost

Construction

• Factory-built; set, piped, vented, and wired on site; relief valves and pans installed with the heater

Main manufacturers and suppliers

• A. O. Smith
• Rheem
• Bradford White
• Rinnai
• Navien
• Lochinvar

Trade associations: AHRI and ASPE.

Typical costs

• Tank water heater, installed: about $900 to $1,800
• Tankless water heater, installed: about $1,400 to $3,900
• Heat pump water heater, installed: about $1,200 to $4,600

Sources

• UpCodes: 2024 IPC Chapter 5 water heaters (GSA adoption without amendments), https://up.codes/viewer/general-services-administration/ipc-2024/chapter/5/water-heaters
• UpCodes: 2024 IPC Chapter 6 water supply and distribution (hot water supply), https://up.codes/viewer/general-services-administration/ipc-2024/chapter/6/water-supply-and-distribution
• HomeAdvisor: water heater installation cost, https://www.homeadvisor.com/cost/plumbing/install-a-water-heater
• NerdWallet: water heater cost, https://www.nerdwallet.com/article/mortgages/water-heater-cost
• Fixr: cost to install a water heater, https://www.fixr.com/costs/install-water-heater
• Wikimedia Commons: Cutaway diagram of an electric water heater.jpg, by Apfel's Systems and Electric Heating (Public domain), https://commons.wikimedia.org/wiki/File:Cutaway_diagram_of_an_electric_water_heater.jpg