Tankless, tank or solar heater: which one wastes less water

When people change heaters they compare price, gas use and whether it handles two showers. What nobody compares is water, and there is a surprise: heater type barely changes the liters you waste waiting, because that water is not in the heater but trapped in the pipe.

Tank: the classic

Keeps 40-80 liters hot at all times. Spends more gas holding temperature, but hot water leaves the tank immediately when you open the tap... and you still wait, because the 10-20 meters of pipe between heater and shower are full of cold water. That wait is identical with any heater.

Tankless: gas-efficient with water fine print

• No storage: heats as water flows. More efficient on gas.
• On water it has TWO waits: the pipe's (same as everyone) plus its own startup, since it needs a minimum flow and a few seconds to ignite and stabilize.
• High-flow tankless units may need a booster pump to ignite with a hydro-return's flow; we explain it here.

Solar: free energy, same wait

The solar tank heats with the sun and gas backup covers cloudy days. On water it behaves like a tank heater: the pipe wait does not change, and since it usually sits on the roof, the distance to the bathroom sometimes GROWS, and wasted liters with it.

The honest table

• Gas: tankless wins, then solar (near zero), tank pays for the pilot and standby losses.
• Water wasted waiting: technical tie across all three, slight disadvantage for tankless due to startup. The real variable is heater-to-fixture DISTANCE.
• Fix for the waste: identical in all three cases, a hydro-return that sends the pipe's cold water back to the heater. Compatible with all three types.

If you are about to buy a heater

Choose by gas, capacity and budget, as always. But decide PLACEMENT thinking about water: every meter of pipe between heater and bathroom is ~0.15 to 0.5 liters wasted per use, every use, every day. A well-placed heater plus a hydro-return leaves the wait in seconds and the waste at zero; the calculator tells you what that is worth in your house per year.

A plumber's rule we like to repeat: the best heater badly placed wastes more water than the worst heater well placed.