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.
A tankless unit has to lift the incoming water to a 120°F setpoint. Encinitas has an average January low of 48°F, so the calculator figures incoming water at 65°F and a rise of 55°F. Where winters are colder it uses 52°F or 42°F incoming water and a bigger rise, which means the same household needs more heat from the unit.
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.
Start with two showers running together, about 5 gallons per minute. At the 55°F rise used for Encinitas that is about 137,500 BTU per hour, which falls in the standard whole-house class. The calculator works out your own mix of showers and fixtures.
Yes. Two showers draw about 5 gallons per minute, and at a 55°F rise one standard gas unit covers that. A third fixture, such as a faucet, moves the load into the large class, and a dishwasher or washer on top can call for two linked units.
It is the gap between the water coming in and the 120°F setpoint. The calculator takes Encinitas water as arriving at 65°F, so the rise is 55°F. Tankless ratings are always stated at a rise, and a unit delivers less when the water arrives colder.
Not quite. The largest electric units, near 36 kW, give about 4.5 gallons per minute at a 55°F rise, a little under two showers. They also need 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.
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