How They Work
How Tankless Water Heaters Work
No tank, no stored hot water — heat happens the moment you turn on the tap.
A traditional water heater keeps dozens of gallons hot around the clock, reheating as the stored water cools. A tankless unit works the opposite way: when you open a hot-water tap, cold water flows through the unit, powerful burners or electric elements heat it as it passes, and hot water comes out the other side. Close the tap, and the heating stops.
This on-demand design is why tankless units are compact enough to mount on a wall — there is simply no tank to house. It is also why they do not “run out” the way a tank does: there is no stored supply to deplete.
But here is the honest part most marketing skips: a tankless unit can only heat so much water at once. Every unit has a rated flow — how many gallons per minute it can heat by the needed temperature rise. Stay within that rating and the hot water keeps coming. Exceed it — say, two showers plus the dishwasher all at once on an undersized unit — and the temperature drops. That is not a malfunction; it is physics. Which is exactly why sizing matters more for tankless than for any other water heater decision.