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 calculator assumes a 120°F setpoint, the commonly recommended setting. Incoming water is estimated from the average January low, at 42°F when that is below 25°F, 52°F between 25°F and 39°F, and 65°F at 40°F or above. Rowlett’s January low of 36°F lands in the middle band, so the unit must add 68°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 draw 5 gallons per minute. Heating that flow through Rowlett’s 68°F rise takes about 170,000 BTU per hour, which the calculator places in the large-unit class. Adding a dishwasher or a second faucet at the same moment calls for two linked units.
A large gas unit will. Through Rowlett’s 68°F rise it delivers about 5.6 gallons per minute against the 5 that two showers need, while an electric unit falls short at roughly 3.6.
The rise is the number of degrees added between the water coming in and the water going out. Rowlett’s figure is 52°F in and 120°F out, a 68°F rise, and each unit delivers fewer gallons per minute as the rise grows.
Not with two showers running together. About 36 kW is the largest electric size, and it delivers roughly 3.6 gallons per minute at a 68°F rise, enough for one shower. An electrician also has to confirm the home’s electrical service can carry the load.
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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