Will a standard unit cover two showers at a 55°F rise?
Yes. Two showers use about 5 gallons per minute, and at a 55°F rise that works out to about 137,500 BTU per hour. The standard whole-house band runs up to 150,000, so it fits.
Tankless Flow Calculator
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.
What runs at the same time
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.
How cold the water comes in
A tankless unit has to raise incoming water to a 120°F setpoint. National City’s average January low of 48°F falls in the warm band, so the calculator assumes 65°F water arriving and a 55°F rise.
How the size is worked out
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 and electric limits
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.
Common questions
Does adding a faucet to two showers change the size?
Yes. A faucet adds about 1.8 gallons per minute, for a total of 6.8. At a 55°F rise that is about 187,000 BTU per hour, which lands in the large unit band.
Is the rise the gap between incoming and outgoing water?
Yes. For National City the calculator starts with water at 65°F and heats it to the 120°F setpoint, leaving a 55°F rise for the unit to supply. Tankless flow ratings are always quoted at a particular rise.
Can one electric unit run two showers here?
Not quite. The biggest electric units, around 36 kW, deliver about 4.5 gallons per minute at a 55°F rise, and two showers need about 5. They also call for a very large electrical service, which an electrician has to confirm first.
Does a tankless water heater need a bigger gas line?
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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