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.
Incoming water has to be lifted to the setpoint, and we use 120°F. We assume incoming water at 65°F where the January low averages 40°F or more, 52°F between 25°F and 40°F, and 42°F below that. Chandler’s January low of 42°F falls in the warmest band, so the rise is 55°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.
That depends on how many showers run together. Two at once come to 5 gallons per minute, and at Chandler’s 55°F rise that works out to about 137,500 BTU per hour, which the calculator places in its standard whole-house class.
Yes. Two showers need 5 gallons per minute, and one standard gas unit covers that at a 55°F rise. Adding a faucet pushes the load into the large class, and a dishwasher or washer on top calls for two linked units.
Rise is the number of degrees the unit adds to the incoming water. Water arriving at 65°F and leaving at 120°F is a 55°F rise, and because flow ratings are quoted at a stated rise, colder incoming water lowers the gallons per minute a unit delivers.
No. At a 55°F rise the largest electric units, around 36 kW, give about 4.5 gallons per minute, a little less than two 2.5 gallon showers. They also need a very large electrical service that an electrician has to confirm.
Yes, it can. A unit rated near 199,000 BTU per hour draws several times what a tank heater does, so the gas line and venting that served a tank may be too small, and a plumber checks both before a unit is chosen.
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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