Glossary · Hot water
Tankless water heater
Also called: tankless · tankless water heaters · on-demand water heater · instantaneous water heater · demand water heater
A water heater with no storage tank that heats water only as it flows, avoiding the heat a tank loses while it sits waiting.
What it means
A tankless unit fires up when a hot tap opens, so it avoids the standby losses of keeping a tank hot. Its flow capacity is limited, so households that run several hot taps at once may need a larger unit. Gas models need proper venting and gas supply.
Why it matters in an audit
Savings are largest for households using moderate amounts of hot water; for all-electric homes a heat pump water heater usually saves more.
Typical numbers
Reference values with their sources. Your auditor’s report gives the figure for your own house.
| What | Value | Source |
|---|---|---|
| Savings vs. a storage tank (DOE) | 24%–34% more efficient for homes using 41 gallons (about 155 L) of hot water a day or less | energy.gov ↗ |
| ENERGY STAR gas tankless | UEF ≥ 0.95 and at least 2.8 gallons per minute at a 67°F temperature rise | energystar.gov ↗ |