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 commonly recommended setting. Commerce City has an average January low of 18°F, so the calculator figures incoming water at 42°F. That is a rise of 78°F, the biggest the calculator uses, and it makes the same house a bigger job than it would be in a warm place.
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.
Count the showers that run together. Two showers need about 5 gallons per minute, and at a 78°F rise that lands in the two-units class. One shower on its own fits a standard whole-house unit. The calculator shows the class for your own mix.
Not quite. Two showers need about 5 gallons per minute, and at a 78°F rise one large gas unit delivers about 4.9. The calculator shows that load as two linked units, or as a plan to stagger the second shower.
It is the gap between the incoming water and the 120°F setpoint. The calculator uses 42°F incoming water for Commerce City, so the rise is 78°F. A unit delivers fewer gallons per minute as the rise grows.
No. The largest common electric units, about 36 kW, deliver around 3.1 gallons per minute at a 78°F rise. That is about one shower, and the unit needs 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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