A water heater heats and supplies hot water for a building. Storage (tank) water heaters are the most common type but not the most efficient, because they keep a large tank of water hot even when none is being used.[1] Tankless models heat water only on demand, and heat pump models move heat from the surrounding air into the water.[2][3] Most water heaters run on electricity or natural gas.[1]
Storage water heaters
In a storage water heater, hot water is drawn from the top of the tank while cold water enters at the bottom and triggers heating. Gas models have a burner and a central flue; electric models have upper and lower heating elements.[6] Insulating the tank with a fiberglass blanket can cut standby heat loss by 25–45%, saving about 4–9% on water-heating costs.[6] Gas, oil and propane units must be vented outdoors.[1]
Storage heaters are sized by their First Hour Rating, the amount of hot water they can deliver in an hour, rather than by tank size. The University of Georgia suggests:[1]
| Household | Suggested First Hour Rating |
|---|---|
| 2 people | 45–55 gallons |
| 4 people | 65–75 gallons |
| 7 or more | 100+ gallons |
Georgia's amendments to the plumbing code add their own table of minimum residential water heater sizes.[7]
Tankless water heaters
In a tankless or demand water heater, a flow sensor detects an open hot-water tap and fires the burner or element, so there is no stored hot water and no standby loss. A 2009 U.S. Department of Energy Building America fact sheet put the typical efficiency of gas tankless units at about 0.82, compared with about 0.60 for conventional gas tanks, and estimated savings of 10–20% on water-heating bills.[2]
Gas tankless heaters typically supply about 5 gallons per minute, enough for two showers at once, while electric models supply about 2–3.[1][2] Units usually need a minimum flow of about 0.5 gallons per minute to fire, may take a few extra seconds to deliver hot water, and gas models generally need electricity, so they do not work in a power outage. In hard water, the heat exchanger can build up scale and should be descaled.[2] Models with a constantly burning pilot light can waste enough energy to offset their advantage.[8]
Heat pump water heaters
A heat pump water heater works like a refrigerator in reverse, moving heat from the surrounding air into the water. ENERGY STAR-certified models use less than half the energy of a standard electric water heater and, under its assumptions, can save a four-person household about $490 a year.[3] They need to be installed in a space that stays between 40 and 90 °F with about 1,000 cubic feet of air around them, and they cool and dehumidify that space.[9][1]
Efficiency standards
Federal efficiency standards that took effect on April 16, 2015, raised requirements for residential tank water heaters of about 20 to 80 gallons, and required tanks larger than 55 gallons to use heat pump (electric) or condensing (gas) technology.[10]
Temperature and safety
The U.S. Consumer Product Safety Commission urges setting water heaters to 120 °F to prevent scalds, which mostly injure older adults and children under five.[4] Many heaters come set at 140 °F, and lowering the setting to 120 °F can cut water-heating costs by 6–10%.[6] Electric heaters often have two thermostats, which must both be set to the same temperature.[4]
| Water temperature | Time for an adult to suffer a third-degree burn |
|---|---|
| 150 °F | 2 seconds |
| 140 °F | 6 seconds |
| 130 °F | 30 seconds |
| 120 °F | 5 minutes |
Source: CPSC.[4] For large buildings, CDC water-management guidance notes that Legionella bacteria grow best at 77–113 °F and recommends storing hot water above 140 °F, with mixing near the point of use to limit scald risk.[11]
Tank water heaters have a temperature and pressure (T&P) relief valve near the top that lets hot water or steam escape safely if a thermostat fails; it should be checked once a year.[6] Residential T&P valves typically open at 150 psi or 210 °F.[5]
Maintenance and lifespan
Sediment and scale reduce a tank's efficiency and capacity; flushing a gallon or two from the drain valve until the water runs clear, anywhere from monthly to yearly depending on water hardness, helps remove it.[6] The sacrificial anode rod, which protects the tank from corrosion, should be inspected every two years and replaced about every five, and popping or banging noises suggest sediment buildup.[5] Tank water heaters usually last 10–15 years and tankless units more than 20.[1][2]
References
- ^ ^ ^ ^ ^ ^ ^ ^ ^ Atiles, Jorge H.; Laing, Jonathan (July 2006). "Choosing a New Water Heater" (HACE-E-60-06). University of Georgia Cooperative Extension. fcs.uga.edu
- ^ ^ ^ ^ ^ "Tankless Water Heaters". U.S. Department of Energy Building America (Consortium for Advanced Residential Buildings), 2009. www1.eere.energy.gov
- ^ ^ "Soak in the Savings: Choose an ENERGY STAR Certified Electric Storage Water Heater". ENERGY STAR. energystar.gov
- ^ ^ ^ ^ "Avoiding Tap Water Scalds" (Publication 5098). U.S. Consumer Product Safety Commission, March 2012. cpsc.gov
- ^ ^ ^ "Water Heating". Building Maintenance & Construction, University of Hawaiʻi Maui College. pressbooks.oer.hawaii.edu
- ^ ^ ^ ^ ^ "Water Heater Maintenance". Montana State University Extension. montana.edu
- ^ "Georgia State Amendments to the International Plumbing Code (2024 Edition)". Georgia Department of Community Affairs, revised January 1, 2026. dca.georgia.gov
- ^ "Tankless". Vermont Energy Saver, Vermont Department of Public Service. energysaver.vermont.gov
- ^ "Heat Pump Water Heaters". ENERGY STAR. energystar.gov
- ^ "Water heaters get an efficiency makeover courtesy of the Department of Energy". American Council for an Energy-Efficient Economy, February 25, 2015. aceee.org
- ^ "Monitoring Building Water". Centers for Disease Control and Prevention. cdc.gov