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.
Every tankless unit is asked to deliver 120°F water, so the calculator needs to know how cold the supply is. It takes that from the town’s average January low, assuming 42°F water below 25°F, 52°F water from 25°F up to 40°F and 65°F water at 40°F or warmer. Nampa’s low of about 25°F lands in the 52°F band, a 68°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.
Yes, provided the unit is a large one. For Nampa’s 68°F rise, the roughly 170,000 BTU per hour needed to supply 5 gallons per minute sits in the calculator’s large-unit band, and 5 gallons per minute is about what two showers use together. Add a faucet or a dishwasher at the same time and the result becomes two linked units.
It can. With a 68°F rise a large gas unit puts out around 5.6 gallons per minute and two showers need about 5, which leaves a little to spare. If a third shower or an appliance joins in, two linked units are needed.
Yes, because the incoming water temperature follows the town’s January low. The rise is the gap between that water and the 120°F setpoint. The calculator assumes 52°F incoming water for Nampa, which gives a 68°F rise, while a place in the 65°F band needs a rise of only 55°F.
Not when two showers share it. Around 36 kW is the top of the standard electric range, and at a 68°F rise it gives about 3.6 gallons per minute, which is one shower. Because it needs a very large electrical service, an electrician has to confirm the house can supply it.
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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