How They Work
How a Tankless Heater Works
No tank and no stored hot water — the heat happens when you open the tap.
A standard heater keeps tens of gallons hot all day, reheating as the stored water cools. A tankless unit flips that: open a hot tap and cold water runs through the unit, where strong burners or electric elements heat it on the way past. Close the tap and the heating stops.
That on-demand design is why the units are small enough to hang on a wall — there is no tank to house. It is also why they never “run out” the way a tank does.
The honest catch that marketing skips: a tankless unit can only heat so much water at one time. Each unit has a rated flow — the gallons per minute it can heat by the needed temperature rise. Stay inside that rating and hot water keeps flowing. Push past it — two showers plus the dishwasher on a small unit — and the temperature falls. That is physics, not a defect, and it is why sizing is the single most important tankless decision.