A tankless water heater warms water as it passes through the appliance rather than storing a full tank of hot water. Opening a hot tap starts a sequence of flow sensing, heating and temperature control.
On demand does not mean unlimited flow
A correctly sized unit can keep supplying hot water while it operates, but there is a limit to how much water it can heat at once. Two showers and another fixture running together may ask more of the system than a single shower. Incoming water temperature also affects its output.
Gas and electric models have different installation requirements. Gas units may need suitable venting and fuel supply; electric units may need significant electrical capacity. The existing home’s setup matters as much as the advertised appliance features.
Decide with your usage in mind
List the fixtures that may run together during your busiest period. Ask how a proposed unit performs at that demand and what maintenance it requires. Our tankless water-heater service can help compare the options.
What happens when you open a tap
Water flow tells the heater to start. The appliance then adjusts its heat output to aim for a selected delivery temperature while water passes through its heat exchanger. When the tap closes, heating stops. The system still has water in the pipe between the heater and the fixture, so “on demand” does not mean hot water arrives instantly at every tap.
Each model has a minimum flow needed to start and a maximum amount it can heat at a given temperature rise. Colder incoming water requires a larger temperature rise, which can reduce the available hot-water flow. This is why the manufacturer’s capacity figures need to be read against the home’s real conditions rather than a headline number alone.
If an existing unit turns cold during use, note the pattern and any displayed code before requesting service. See why a tankless heater may turn cold for what to record.