A tankless water heater is an on-demand appliance that heats water only as it flows, firing a gas burner or electric elements when a flow sensor detects an open tap. With no stored water there is no standby heat loss, so its DOE UEF rating usually beats a comparable tank, but output is limited to a set gallons-per-minute flow.
Why it matters to you
Because nothing is kept hot between uses, a 0.93 UEF gas tankless unit delivers the same hot water as a 0.62 UEF gas tank with about 33% less energy input (1 − 0.62/0.93). The trade-off is a flow limit: an undersized unit cannot keep up with two showers at once.
Practical guidance
Size a tankless unit on how many fixtures run at the same time and how cold your incoming water gets, not on the brochure maximum. Descale the heat exchanger and clean the inlet screen on the manufacturer's schedule, especially with hard water.
Tank vs. tankless water heaters at a glance
| Storage tank | Tankless | |
|---|---|---|
| How it heats | Keeps a stored volume hot at all times | Heats water only as it flows |
| Hot water limit | Tank volume, can run out | Flow rate, can be outrun but never runs empty |
| Standby heat loss | Yes, stored water cools and is reheated | None, nothing is stored |
| Typical gas UEF | About 0.58–0.70 (standard models) | About 0.81–0.97 |
| Typical lifespan | About 8–12 years | Often 15–20 years with regular descaling |
| Routine maintenance | Annual flush, anode rod checks | Periodic descaling, inlet screen cleaning |
| Space | Floor-standing tank | Compact, wall-mounted |
How is a tankless water heater different from a tank model?
A tank model keeps a fixed volume of water hot at all times and can run out once that volume is used. A tankless model heats water as it passes through, so it never runs empty, but it can be outrun when demand exceeds its flow rate at your incoming water temperature.
Can a tankless water heater run out of hot water?
It cannot run out the way a tank does, but it can be overwhelmed. Each unit delivers a limited number of gallons per minute at a given temperature rise, and the rise it must achieve is larger where incoming water is colder. Sizing on real simultaneous use prevents lukewarm showers.
Does switching from a tank to tankless need other work?
Often, yes. Gas tankless units can need a larger gas line and different venting than the tank they replace, and electric whole-house units can draw enough current to require new circuits or more service capacity. Have gas, venting, and electrical needs assessed before choosing a unit.
What maintenance does a tankless water heater need?
Periodic descaling of the heat exchanger and cleaning of the inlet water filter screen. Mineral scale restricts flow and lowers efficiency over time, and many manufacturers list routine maintenance as a condition of their warranty, so keep the service records.