Does a standard unit cover two showers at a 55°F rise?
Yes. A pair of showers uses about 5 gallons per minute. Heating that by 55°F takes about 137,500 BTU per hour, under the 150,000 top of the standard whole-house band.
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
Every tankless unit has to heat incoming water up to a 120°F setpoint. With an average January low of 45°F, Poway sits in the warm band, so the calculator starts from 65°F water 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 a third faucet running change the size?
Yes. One more faucet adds about 1.8 gallons per minute, bringing the total to 6.8. Heated 55°F, that is about 187,000 BTU per hour, so the answer moves up to the large unit band.
Is the rise the difference between incoming and outgoing water?
Yes. Poway’s incoming water is taken as 65°F, and the heater has to bring it to 120°F. The 55°F it adds is the rise, and tankless flow ratings are stated at a rise.
Can one electric unit run two showers in Poway?
Not quite. A 36 kW electric unit gives roughly 4.5 gallons per minute at a 55°F rise, and two showers use about 5. It also needs 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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