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A tankless water heater can deliver hot water for as long as fuel, water and electrical power remain available. That does not mean it can supply any number of fixtures at any flow.
The accurate promise is endless duration at a limited flow rate and temperature rise.
Flow rate is the capacity limit
A tankless unit heats water as it passes through a heat exchanger. The colder the incoming water and the hotter the requested outlet, the more energy each gallon requires. As temperature rise increases, available gallons per minute decrease.
That is especially important in Central Ohio winter, when entering water can be much colder than summer water. A model advertised at a high flow under mild test conditions may deliver less at the temperature rise the home actually needs.
Add the fixtures that may run together
Sizing begins with simultaneous demand—not the total number of fixtures in the house. Two showers, a kitchen faucet and a washing machine running together create a different load from one shower at a time.
DOE's guidance recommends adding the flow rates of hot-water uses that may operate simultaneously and selecting capacity at the relevant inlet temperature. Manufacturer charts should show the model's flow at different temperature rises.
Why a tank can feel more forgiving
A storage heater builds a reserve of hot water. For a short period, that tank may serve a flow larger than the burner or elements could heat continuously. Once the stored volume is depleted, recovery takes time.
A tankless unit has no large reserve. Stay within its continuous capacity and duration is effectively unlimited. Exceed the capacity and outlet temperature may fall, the unit may restrict flow or fixtures may compete.
Tankless does not guarantee instant hot water
Wait time is mainly a distribution issue. Water already sitting in the pipe between the heater and faucet must move out. A tankless unit at the same location as the old tank may take a similar time—or slightly longer because of activation and ignition.
Recirculation, pipe layout and insulation can improve delivery, but they must be designed to avoid unnecessary energy use and to remain compatible with the heater.
Tankless is not automatically the lowest-cost choice
It can reduce standby losses and save space, but installation may require larger gas piping, new venting, condensate drainage, water treatment or electrical upgrades. Maintenance is important, particularly where scale can form in the heat exchanger.
The decision should compare household demand, fuel, installation scope and expected use—not the word “endless.”
Frequently asked questions
Can a tankless heater run two showers?
Many can, but verify the combined flow and required temperature rise against the exact model's performance table.
Will tankless work during a power outage?
Most gas tankless units need electricity for controls, ignition and venting. They generally do not operate without appropriate backup power.
Why does very low flow stay cold?
Tankless units have a minimum activation flow. A trickle or very low-flow fixture may not start the heater. DOE guidance notes typical minimum-flow requirements vary by model.
Size the hot-water experience
West Jefferson Plumbing & Heating installs and services tank and tankless water heaters across Central Ohio. We evaluate winter inlet temperature, simultaneous fixtures, gas or electrical capacity and delivery layout for homes in Circleville and the surrounding counties.
Suggested internal links: Water Heater Services; Water Heater Installation; Article 21 in this package Authoritative sources: DOE Tankless Water Heaters guide; DOE tankless transition guide