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 must raise incoming water to the setpoint, and we use 120°F, the setting most often recommended. Where the average January low is 40°F or more we assume incoming water at 65°F, a 55°F rise. From 25°F to 40°F we assume 52°F, a 68°F rise, and under 25°F we assume 42°F, a 78°F rise. January lows in all seven Colorado places are under 25°F, so the checker uses a 78°F rise everywhere.
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.
Size it for two showers running at once, about 5 gallons per minute. At the 78°F rise we use for every Colorado place, that works out to about 195,000 BTU per hour, which is more than one large gas unit supplies and lands in our two-unit band. Enter your own showers and fixtures in the calculator for your class.
It is close but usually not quite. Two showers draw about 5 gallons per minute, while one large gas unit delivers about 4.9 at a 78°F rise. Two showers together can need a pair of linked units, or a second appliance has to wait. A single shower suits a standard unit.
Rise is the gap between the water coming in and the temperature you set. With 42°F coming in and 120°F going out, the rise is 78°F. Makers quote flow at a stated rise, and the same unit delivers less when the incoming water is colder.
Not often. The biggest common electric models, around 36 kW, deliver about 3.1 gallons per minute at a 78°F rise. That serves one shower and little else, and the home needs a very large electrical service, which an electrician must 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.
Ring for a free, personalized quote. Mention the problem, the room it is in and the year the house was built. If it is a cabin or second home, tell us that too, since a house that sits cold for weeks raises its own questions.
Call (855) 321-3426