About Tanklessly

A tankless heater is sold on its maximum flow, and that figure is measured at a temperature rise most houses only see in summer. In January, with 40-degree water coming in, the same unit may run half as many showers.

Tanklessly starts with the house: how many showers and appliances run at once, how cold the water is when it arrives, and the set point you want. From those it works out the flow the heater has to hold at your rise, and checks every gas and electric unit in the catalog against it.

The second question is whether the house can run the unit at all. An electric whole-house model can draw well over 100 amps; a 199,000 BTU gas model needs a gas line that delivers close to 200 cubic feet an hour and its own venting. Those requirements sit next to every unit, so an electrician or gas fitter can confirm them before you buy.

Heat-pump water heaters, point-of-use mini tanks, flush kits, isolation valves, recirculation pumps and venting parts are covered the same way: the maker's figures, one grid, nothing filled in.

Who runs it

Tanklessly is an independent publication. It doesn't sell or install water heaters and doesn't take payment to list one. Some links earn a commission, explained in the affiliate disclosure.

Write to us

Found a flow figure that doesn't match the maker's chart, or a kit that doesn't fit what we say it fits? Email contact@tanklessly.com.

Updated October 2026.