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 is set to deliver 120°F, the Department of Energy’s recommended setting. The incoming water temperature comes from the town’s average January low, in three bands: 65°F for a low of 40°F or more, 52°F from 25°F up to 40°F, and 42°F below 25°F. At about 26°F, Mount Vernon 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.
Yes, when two showers run together. Two showers draw 5 gallons per minute, which at Mount Vernon’s 68°F rise means about 170,000 BTU per hour and a large high-capacity unit. A faucet or dishwasher running as well moves the answer to two linked units.
Yes. One large gas unit delivers about 5.6 gallons per minute at a 68°F rise, and two showers draw 5. Add a faucet or an appliance at the same time and the load is more than one unit can supply.
Yes. A flow rating always comes with a temperature rise, which is how many degrees the heater adds. Heating 52°F water to 120°F is a 68°F rise, and the colder the water arrives, the fewer gallons per minute one unit can heat.
Not quite. The largest electric units, about 36 kW, give about 3.6 gallons per minute at a 68°F rise. That covers a single shower but not two together, and the unit needs a 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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