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 raises incoming water to a 120°F setpoint. The incoming temperature comes from the average January low. It is taken as 42°F water below 25°F, 52°F water from 25°F to 39°F and 65°F water at 40°F or above. Rockwall’s 35°F January low falls 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.
Yes, if two showers run together. Two showers draw 5 gallons per minute, and lifting that by Rockwall’s 68°F takes about 170,000 BTU per hour, which the calculator puts in the large-unit class. Add a faucet or an appliance at the same moment and it suggests two linked units.
Yes, a large gas unit can. It delivers about 5.6 gallons per minute when the water gains 68°F, as it does in Rockwall, against the 5 that two showers draw. An electric unit manages roughly 3.6.
Yes. The calculator figures Rockwall’s incoming water at 52°F and the output at 120°F, a 68°F rise, and the colder the incoming water, the fewer gallons per minute a given unit delivers.
No, not for two showers at once. A 36 kW model, the largest the calculator uses, delivers about 3.6 gallons per minute at a 68°F rise, which is one shower, and an electrician has to confirm that the home’s electrical service can carry the 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