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 heat incoming water up to a 120°F setpoint, the commonly recommended setting. We set the incoming water by the average January low: 65°F where that low is 40°F or warmer, 52°F where it falls between 25°F and 40°F, and 42°F below 25°F. Tacoma’s January low of 36°F puts it in the middle group, which means 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, when only that shower is running. At 2.5 gallons per minute and a 68°F rise, one shower takes about 85,000 BTU per hour, which the calculator counts as a small unit. Run a faucet alongside it and the result steps up to a standard whole-house unit.
Yes, a large gas unit handles it. Two showers draw 5 gallons per minute, and with a 68°F rise the load is about 170,000 BTU per hour, the calculator’s large unit class. Adding a faucet, a dishwasher or a washer on top of that moves the answer to two linked units.
Yes. The rise is the gap between the water arriving at the unit and the setpoint, here 52°F in and 120°F out for a 68°F rise. A tankless rating is always given at a stated rise, and the larger the rise, the fewer gallons per minute the same unit can deliver.
No, not for more than one shower at a time. The biggest common electric units, around 36 kW, give about 3.6 gallons per minute at a 68°F rise, and they 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.
Call us to discuss your current issue and how we can assist you. The call is free and you get a free, personalized estimate with no obligation. We're here to help.
Call (855) 321-3426