A tankless heater makes hot water as you use it, so its size comes down to flow and temperature rise. Choose the showers and fixtures that run at the same time to see what a unit has to deliver.
Pick your place and we set the incoming water temperature from its winter weather.
A tank stores hot water. A tankless unit makes it as you go, so it is sized in gallons per minute, not gallons. Add up what realistically runs together. We use 2.5 gallons per minute for a showerhead, which is the federal limit, about 1.8 for a faucet and about 2 for a dishwasher or washer. Two showers at once already means 5.
The unit has to lift incoming water up to the setpoint, and we use 120°F, the setting the US Department of Energy recommends. Hialeah’s average January low is 60°F, above the 40°F line, so the calculator treats incoming water as warm, at 65°F, for a rise of 55°F. Colder places start from 52°F or 42°F, which means a bigger rise and a bigger unit for the same house.
The heat a unit has to deliver each hour is the flow in gallons per minute, times the rise in degrees, times 500. That figure comes from the weight of a gallon of water. We round it and place it in one of our own planning bands, based on that delivered heat. Up to 90,000 BTU per hour is a small unit. Up to 150,000 is a standard whole-house unit. Up to 190,000 is a large unit. Above that, two units are linked together.
A tankless unit trades flow for rise. The same unit gives more gallons per minute at a small rise than at a big one, which is why every tankless rating is stated at a rise.
Gas units go up to about 199,000 BTU per hour of input, and two can be linked. Electric units top out near 36 kW, about 123,000 BTU per hour, and they need a very large electrical service. The result shows how many gallons per minute each one delivers at your rise. A plumber or an electrician checks the gas line, the electrical service and the venting before any unit is chosen.
For two showers running at once, a standard whole-house unit. Two showers draw 5 gallons per minute, and at Hialeah’s 55°F rise that is about 137,500 BTU per hour of heat into the water. Set your own mix of showers and fixtures in the calculator to see your class.
Yes. Two showers draw 5 gallons per minute, and at a 55°F rise one standard unit covers that. A faucet added on top moves the load into the large class, and a dishwasher or washer added instead takes it past one unit, so the calculator suggests two linked together.
It is the gap between the water coming in and the setpoint. For Hialeah the calculator uses 65°F in and 120°F out, a rise of 55°F. Every tankless flow rating is given at a stated rise, and a smaller rise lets the same unit deliver more gallons per minute.
Not quite. The largest electric units in the calculator, about 36 kW, deliver around 4.5 gallons per minute at a 55°F rise, a little under two showers, and they need a very large electrical service that an electrician has to confirm.
Often. A 199,000 BTU per hour unit draws several times what a tank heater does, and the existing gas line and venting may not be sized for it. A plumber checks both first.
Data: fixture flow rates from the federal showerhead limit and typical faucet and appliance flows, the 120°F setpoint recommended by the US Department of Energy, and incoming water bands set from Daymet winter data 2015 to 2024 for each place.
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