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 to a 120°F setpoint, the commonly recommended setting. We set the incoming temperature from the average January low: 42°F water below 25°F, 52°F water from 25°F to 39°F, and 65°F water at 40°F or above. Fort Worth’s 36°F January low falls in the middle band, so the unit has to 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.
Plan around how many showers run together. Two at once draw 5 gallons per minute, and lifting that by the 68°F used for Fort Worth takes about 170,000 BTU per hour, which the calculator places in the large-unit class. A dishwasher or another faucet running at the same time calls for two units linked together.
Yes, with a large gas unit. At the 68°F rise figured for Fort Worth, one delivers about 5.6 gallons per minute, and two showers need 5. An electric unit falls short at roughly 3.6 gallons per minute.
Yes. The temperature rise is how many degrees the unit adds between the water coming in and the water going out. For Fort Worth the calculator uses 52°F in and 120°F out, a 68°F rise, and a unit delivers fewer gallons per minute as the rise grows.
No, not when two showers run together. The largest electric models, about 36 kW, manage roughly 3.6 gallons per minute at a 68°F rise, which is one shower. An electrician also has to confirm that 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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