When you're thinking about switching from a traditional hot water tank to a tankless model, it helps to understand exactly how these units work. Unlike a storage-tank heater that keeps 40–80 gallons hot around the clock, a tankless water heater — also called an on-demand heater — heats water only when you actually need it. This article explains the mechanism, the trade-offs, and who benefits most.
How a Tankless Heater Works
A tankless unit has no storage tank. Instead, when you open a hot water tap, cold water flows into the unit and a flow sensor activates the heating system. In a gas model, a burner fires; in an electric model, electric elements switch on. As the water passes through a heat exchanger, it is rapidly brought up to your set temperature and delivered continuously to the faucet. When you close the tap, the flow stops and the heater shuts off — so it only uses energy while hot water is actually running.
The Key Difference: No Standby Loss
A storage tank constantly reheats the same water to keep it hot, even overnight and while you're at work. That wasted energy is called "standby loss." Because a tankless heater never stores hot water, it eliminates standby loss almost entirely, which is why on-demand systems are generally more efficient than tank models. They're also very quiet — there's no large tank cycling on and off, just a slight sound when the unit starts and stops.
Our Top Picks
Rinnai RUCS65iP Tankless Gas Water Heater
A popular gas tankless unit that demonstrates how on-demand heating delivers endless hot water at up to 6.5 GPM.
Rheem RTEX-24 Electric Tankless Water Heater
A compact electric tankless heater — a simple way to try on-demand hot water without a gas line.
Final Editorial Verdict
Flow Rate: The Thing to Get Right
The main limitation of a tankless heater is flow rate, measured in gallons per minute (GPM). A tank can dump its whole volume at once; a tankless unit can only heat so much water per minute. If too many fixtures run at the same time — say a shower, the dishwasher, and the washing machine — a single small unit can struggle to keep up. Sizing the unit to your household's peak simultaneous demand (and your incoming water temperature) is essential. Colder incoming water reduces the effective flow rate, so the same unit delivers less hot water in winter than in summer.
Pros and Cons
Advantages: Endless hot water on demand, higher energy efficiency, a much smaller footprint that mounts on a wall, and a longer lifespan — because they don't hold water, tankless units resist the sediment buildup that shortens a tank's life, often lasting nearly twice as long.
Disadvantages: Higher up-front purchase and installation cost, limited simultaneous flow unless properly sized (or you install multiple units), and a possible short lag before hot water reaches the tap. Hard water can still scale the heat exchanger, so periodic descaling is recommended.
Is a Tankless Heater Right for You?
A tankless heater is a strong choice if you want long-term energy savings, value the endless hot water, or need to reclaim the floor space a bulky tank consumes. If your household frequently runs several hot water fixtures at once, plan to size the unit generously or use more than one. Weigh the higher up-front cost against years of lower energy bills and a longer service life.