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 heater has to lift incoming water to a 120°F setpoint, the temperature the US Department of Energy recommends. Colder winters mean colder water arriving, so the calculator uses three bands. A January low of 40°F or higher means 65°F water. From 25°F up to 40°F it means 52°F, and below 25°F it means 42°F, a 78°F rise. Westminster’s 20°F January low puts it in that last band.
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.
Plan on two showers at once, about 5 gallons per minute. At a 78°F rise that is roughly 195,000 BTU per hour of heat, just above one large gas unit, so the calculator lands on the linked two-unit band. Add your own mix of fixtures to see your result.
Not quite. Two showers pull about 5 gallons per minute, and a large gas unit delivers about 4.9 at a 78°F rise. Two showers at full flow call for a pair of linked units, and a single unit suits one shower at a time.
It is the difference between the incoming water temperature and the setpoint. With 120°F out and 65°F in, the rise is 55°F. Every tankless flow rating is stated at a rise, so a unit rated for a small rise delivers less when the water comes in colder.
Not at a 78°F rise. A 36 kW electric unit gives about 3.1 gallons per minute there, which is less than two showers at once, and it 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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