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 120°F, the temperature the US Department of Energy recommends. Colder January weather means colder water arriving, so the calculator uses three bands for incoming water. It is 65°F where the January low is 40°F or more, 52°F from 25°F up to 40°F, and 42°F below 25°F. St. Clair Shores falls in the coldest band, a rise of 78°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 on two showers at once, about 5 gallons per minute. At the 78°F rise in St. Clair Shores that takes roughly 195,000 BTU per hour of heat, a little above one large gas unit, so the result falls in the linked two-unit band. Enter your own fixtures in the calculator to check.
Not quite. Two showers draw about 5 gallons per minute, and a large gas unit delivers about 4.9 at a 78°F rise. Two linked units cover both at full flow, while a single unit covers one shower at a time.
It is the number of degrees the heater adds. Water arriving at 42°F and leaving at 120°F needs a 78°F rise. Flow ratings are quoted at a stated rise, so a unit gives fewer gallons per minute when the incoming water is colder.
Not on its own. A 36 kW electric unit, near the top of the range, delivers about 3.1 gallons per minute at a 78°F rise. That covers one shower, and the house would need a very large electrical service, which 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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