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 aims for hot water at 120°F, the setting the US Department of Energy recommends. Colder January weather means colder water reaches the heater, so it sorts towns into three bands by average January low. At 40°F or above the water arrives at 65°F. From 25°F up to 40°F it arrives at 52°F. Under 25°F it arrives at 42°F, and the heater has to add 78°F. Troy and the six other towns in the picker all sit in that third 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 running together, about 5 gallons per minute. With Troy’s 78°F rise that takes roughly 195,000 BTU per hour, which is more than one large gas unit delivers. The result lands in the linked two-unit band, and you can test your own fixture mix in the calculator.
Not quite. Two showers draw about 5 gallons per minute, and a large gas unit gives about 4.9 at a 78°F rise. Two linked units cover both showers at full flow, while a single unit covers one shower at a time.
It is the number of degrees the heater has to add. Water arriving at 42°F and leaving at 120°F needs a 78°F rise. Flow ratings are stated at a particular rise, so the same unit gives fewer gallons per minute when the incoming water is colder.
Not on its own. A 36 kW electric model, near the top of the range, gives about 3.1 gallons per minute at a 78°F rise, enough for one shower. A house needs a very large electrical service for it, and an electrician has to confirm that.
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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