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.
The unit lifts incoming water to a 120°F setpoint, the commonly recommended setting. We estimate the incoming temperature from the average January low. Below 25°F we use 42°F water, from 25°F to 39°F we use 52°F, and at 40°F or above we use 65°F. Garland’s 36°F January low puts it in the middle band, so the unit has to add 68°F.
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.
It comes down to how many showers run together. Two at once draw 5 gallons per minute, and lifting that by Garland’s 68°F takes about 170,000 BTU per hour, which the calculator puts in the large-unit class. Running a dishwasher or another faucet at the same moment calls for two units linked together.
Yes, with a large gas unit. One of those puts out about 5.6 gallons per minute when the water climbs 68°F, as it does in Garland, and two showers need 5. An electric unit falls short at roughly 3.6 gallons per minute.
Yes. Temperature rise is the number of degrees the unit adds between the water coming in and the water going out. Garland’s water is figured at 52°F in and 120°F out, a 68°F rise, and a given unit delivers fewer gallons per minute as the rise grows.
No, not when two showers run together. The biggest common electric models, about 36 kW, manage roughly 3.6 gallons per minute when the water has to climb 68°F, which is one shower. An electrician also has to confirm the home’s electrical service can carry that load.
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