Tank vs Tankless Water Heater

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Sizing Units Are Not Interchangeable

DOE sizes storage water heaters with the peak-hour worksheet and first-hour rating. You add gallons used in the busiest hour. You buy a tank whose EnergyGuide FHR meets that total. Sequential showers in the same hour all count. They do not have to overlap.

Tankless (demand) water heaters are sized in gallons per minute at a stated temperature rise. You add the fixtures that will run at the same time, using their actual GPM. A 2.0 GPM shower plus a 1.3 GPM dishwasher is 3.3 GPM. The unit must hold outlet temperature at that flow when groundwater is whatever it is in January. Manufacturer charts are temperature-rise curves, not first-hour ratings. There is no honest way to say “this tankless is a 50-gallon heater.”

The calculator still prints an FHR next to the tankless discussion because that is the first-hour equivalent you are giving up when you abandon storage. If your DOE peak hour is 66 gallons, a tankless system has to be able to produce that much hot water across the hour and survive the peak simultaneous GPM. Those can both be true, or only one of them. Two bathrooms overlapping at 2.0 GPM each is 4.0 GPM. Plenty of “whole-house” tankless units miss 4.0 GPM at a 70°F rise. They will feel like unlimited hot water for one shower and like a 0.9 GPM dribble when the second shower opens.

When a Storage Tank Is the Right Call

Tanks are boring in the best way. They forgive sequential use. They do not care that three people showered between 6:30 and 7:20 as long as the FHR held. They work on a standard 240-volt / 30-amp circuit (electric) or a standard flue (atmospheric gas). They cost less to hang. They are serviceable by every plumber in town.

Choose a tank when:

• The household staggers showers more than it overlaps them.
• Electrical service cannot swallow a 150–200 amp tankless (electric tankless is a panel problem disguised as a plumbing problem).
• You need a simple swap in the same footprint, same fuel, same vent.
• Winter inlet temperatures would force a huge gas tankless or a cascade of two units to hold GPM.
• A soaking tub is filled once in a while — a tank can dump a large volume quickly; many single tankless units cannot fill a tub at a usable temperature without waiting.

Tanks lose on standby heat, floor space, and the hard stop when the first hour is gone. If the honest peak hour is already past an 80-gallon electric FHR, you are looking at two tanks, a commercial-ish gas tank, or tankless anyway. The calculator will flag that instead of inventing a 120-gallon residential carton.

When Tankless Is the Right Call

Tankless earns its keep when the constraint is duration, not overlap. One person who takes long showers, a household that rarely runs two baths at once, a small mechanical room, or a desire to stop paying to keep 50 gallons hot all afternoon. Gas tankless, correctly sized on the winter temperature-rise curve, can run a shower indefinitely. That is the feature. It is not the same as running two showers and a washing machine on a unit that was specced from a summer GPM number on a hang-tag.

Choose tankless when:

• Simultaneous GPM is modest (one shower, maybe a faucet) and you want unlimited duration.
• You have the gas load and venting (or a dedicated high-amp electric service) to support the unit at winter rise, not catalog rise.
• Floor space is gone and a wall-hung unit is the only thing that fits.
• You are willing to maintain descaling on hard water. Skip this and the GPM you bought will not be the GPM you have in year five.

Install Constraints That Decide the Job Before Sizing Does

Gas tankless needs more than a flue. It wants a large gas line, combustion air, and usually a condensate drain on condensing units. Swapping a 40,000 Btu/h atmospheric tank for a 160,000–199,000 Btu/h tankless on a 1/2-inch gas pipe is how you get error codes instead of hot water. Electric tankless needs copper and breakers the existing water-heater circuit does not have. Heat-pump tanks need a space that can donate heat and a place for condensate; they also publish an FHR like any other tank — size them with the DOE worksheet, not with a tankless GPM chart.

Recirculation is the other silent decider. A tank with a recirc loop needs extra recovery or a timer. A tankless with recirc needs a model that supports it, or a small buffer tank, or you will get cold-water sandwiches and a unit that never sleeps. None of that shows up in the carton gallon number or the GPM hang-tag.

Start with how the house draws: sequential volume vs simultaneous flow. Run the calculator so those two numbers are on the same page. Then let fuel, venting, and the panel break the tie. That is a real decision. It is not a brand preference.

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