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 Michigan list is below 25°F in January, so the checker works with a 78°F rise.
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.
Usually the flow for two showers at once, about 5 gallons per minute, at your local temperature rise. With the 78°F rise we use across our Michigan places, that comes to about 195,000 BTU per hour, which is past one large gas unit and into our two-unit band. The calculator shows the class for your own mix of showers and fixtures.
Sometimes. Two showers is about 5 gallons per minute, and at a 78°F rise one large gas unit covers about 4.9 gallons per minute. That is just short, so two showers at once can call for two linked units, or for staggering a second appliance. One shower alone fits a standard unit.
It is the difference between the incoming water temperature and the setpoint. With 120°F out and 42°F in, the rise is 78°F. Every tankless flow rating is stated at a rise, so a unit rated for a small rise delivers less when the water comes in colder.
Rarely. The largest common electric units, about 36 kW, deliver around 3.1 gallons per minute at a 78°F rise. That covers one shower and not much more, and the unit needs a very 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.
Your free, personalized quote begins with one phone call. Describe the problem, name the room it is in and give the age of the house. The rest we work out from there.
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