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 up to the setpoint. We use 120°F, the commonly recommended setting. Where the average January low is 40°F or higher, we figure incoming water at 65°F, a rise of 55°F. Between 25°F and 40°F we use 52°F, a rise of 68°F, and below 25°F we use 42°F, a rise of 78°F. Every place on our central Arkansas list has a January low between 29°F and 31°F, so the checker works with the 68°F rise throughout.
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, which is about 5 gallons per minute. At the 68°F rise used for central Arkansas, that works out to about 170,000 BTU per hour, a large unit. The calculator gives the class for your own mix of showers and fixtures.
One large gas unit can. At a 68°F rise it delivers about 5.6 gallons per minute, a little more than two showers need. Run a faucet or a dishwasher at the same moment and the load can pass what a single unit puts out.
Rise is the gap between the incoming water and the setpoint. Water arriving at 52°F and leaving at 120°F has been lifted 68°F. A flow rating is always quoted at some rise, and a unit rated at a smaller one gives less hot water when the supply is colder.
Seldom. The biggest common electric models, about 36 kW, deliver roughly 3.6 gallons per minute at a 68°F rise. Two showers need more than that, and the unit calls for heavy electrical service that an electrician has to confirm.
It can. A unit rated at 199,000 BTU per hour pulls several times the gas a tank heater does, so the gas line and the venting both need a look before a model is picked.
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.
Describe the trouble, name the room and give us a rough age for the house. The quote is free, it is built around your situation, and saying no is fine.
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