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. We use 120°F, the setting the US Department of Energy recommends. Centennial’s average January low is 19°F, which the calculator treats as cold groundwater, so it figures incoming water at 42°F and a rise of 78°F.
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.
The calculator works from the showers running at once, not from household size. Count the showers that overlap in your busiest hour and add any faucet or appliance. One shower on its own lands in the standard whole-house class.
Not quite. Two showers need about 5 gallons per minute, a little more than the 4.9 that one large gas unit delivers at Centennial’s 78°F rise. The calculator lands on two linked units for that load.
Rise is the gap between the incoming water and the 120°F setpoint. Centennial’s incoming water is figured at 42°F, so the rise is 78°F. The same unit delivers fewer gallons per minute as the rise grows.
At a 78°F rise the largest electric units, about 36 kW, deliver around 3.1 gallons per minute. That is more than one shower and less than two, and the unit needs a very large electrical service, which an electrician can check.
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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