Understanding Tankless Water Heaters

How Tankless Water Heaters Work

Unlike traditional tank heaters that maintain a reservoir of hot water 24/7, tankless (on-demand) water heaters heat water only when you need it. When a hot water tap is opened, cold water flows through the unit and is heated instantly by either a gas burner or electric elements. This eliminates standby heat loss — the energy wasted keeping 40 to 80 gallons of water hot around the clock — resulting in energy savings of 24 to 34 percent for typical households.

Sizing a Tankless Water Heater

Proper sizing is critical for tankless heaters. Unlike tank heaters measured in gallons, tankless units are rated by flow rate (gallons per minute) and temperature rise. A shower requires about 2.0 GPM, a kitchen faucet about 1.5 GPM, and a dishwasher about 1.0 GPM. Add up the flow rates of fixtures you expect to use simultaneously and choose a unit with at least that capacity. In cold climates, the incoming water temperature is lower, requiring a larger temperature rise and reducing the unit's effective flow rate.

Installation Considerations

Gas tankless units require larger gas lines (typically 3/4-inch) and proper venting — most use sealed direct-vent systems. Electric units require dedicated high-amperage circuits (often 40 to 60 amps per unit). Both types need adequate water flow to activate, so low-flow fixtures may not trigger the unit. Installation costs are typically higher than tank heaters, but federal tax credits and energy savings offset the difference over the unit's 20-year lifespan.

Maintenance Requirements

Tankless water heaters require annual maintenance to prevent scale buildup in the heat exchanger, especially in hard water areas. Flushing involves circulating white vinegar through the unit for 45 minutes using a small pump and two hoses connected to the service valves. Most manufacturers include isolation valves for this purpose. The inlet water filter screen should be cleaned every few months. Neglecting maintenance can reduce efficiency, lower flow rates, and void the manufacturer's warranty. Despite the maintenance, tankless units last nearly twice as long as tank heaters.