The main line is input rating. Federal rules treat a gas storage water heater as residential at 75,000 Btu/h or less and an electric one at 12 kW or less; above that it is commercial equipment. Commercial units recover faster and carry commercial warranties, while some residential warranties drop to one year in a business.
Where the federal line falls
The federal line falls at 75,000 Btu/h for gas tanks and 12 kW for electric tanks. The U.S. Department of Energy writes these definitions in the Code of Federal Regulations. The consumer definition in 10 CFR 430.2 covers storage units "including gas storage water heaters with an input of 75,000 Btu per hour or less, oil storage water heaters with an input of 105,000 Btu per hour or less, and electric storage water heaters with an input of 12 kilowatts or less."
The commercial side, 10 CFR 431.102, picks up right where that stops. It counts as commercial "Gas-fired storage water heaters with a rated input both greater than 75,000 Btu/h and less than 4,000 Btu/h per gallon of stored water" and "Electric storage water heaters with a rated input both greater than 12 kW and less than 4,000 Btu/h per gallon of stored water."
Tankless units have their own split. A gas tankless heater stays in the consumer class up to 200,000 Btu per hour, and an electric one up to 12 kilowatts, per 10 CFR 430.2.
| Heater type | Residential (consumer) limit | Above this, it's commercial |
|---|---|---|
| Gas storage (tank) | 75,000 Btu/h or less | Over 75,000 Btu/h |
| Electric storage (tank) | 12 kW or less | Over 12 kW |
| Oil storage (tank) | 105,000 Btu/h or less | Over 105,000 Btu/h |
| Gas tankless | 200,000 Btu/h or less | Over 200,000 Btu/h |
| Electric tankless | 12 kW or less | Over 12 kW |
For tanks, the commercial definitions carry one more condition: less than 4,000 Btu/h of input per gallon of stored water. A unit with more input per gallon than that falls under the instantaneous (tankless) definitions instead.
### The in-between class: residential-duty commercial
There's a gray zone. DOE defines a "residential-duty commercial water heater" as a gas-fired storage, oil-fired storage or electric instantaneous commercial heater that meets all of these conditions: it uses single-phase power if it needs electricity, it "Is not designed to provide outlet hot water at temperatures greater than 180 °F," and it doesn't cross the size limits DOE lists for that type. For a gas storage heater, those limits are a rated input over 105,000 Btu/h or rated storage over 120 gallons. A gas tank rated between 75,000 and 105,000 Btu/h, with 120 gallons or less, that meets the other conditions, lands in this class. It's commercial on paper, but light-duty in size.
A.O. Smith's Cyclone HE is a good example of how thin the line can be. The company says its "BTX-80 uses 76,000 BTU/h of gas to deliver 86 GPH at 100°F rise." That's only 1,000 Btu/h over the residential cap, and it's sold as a commercial heater.
What actually changes between the two
### Recovery
Recovery is how many gallons the heater can bring up to temperature in an hour. More input means faster recovery. A common residential 50-gallon gas tank, A.O. Smith's ProLine GCR-50, pairs "a 50-gallon (nominal) tank and a 40,000 BTU gas burner" with "a first hour rating of 84 gallons and a recovery rate of 43 gallons per hour." The light commercial BTX-80 above recovers 86 gallons per hour at a 100 degree rise, about double the residential figure.
Big commercial units go much further. A.O. Smith says its Cyclone Mxi models run "from 120,000 to 499,900 BTU/h." That's the kind of input a restaurant, hotel laundry or large salon may need. For how the county sizes a restaurant heater by gallons per hour, see how to size a restaurant water heater in Maricopa County.
### Tank build and ratings
Commercial tanks can be built to a different standard. A.O. Smith says its Cyclone LV puts the heat exchanger "into a large ASME-certified storage tank with a capacity up to 250-gallons." ASME refers to the American Society of Mechanical Engineers, whose pressure vessel code covers how a tank like that is built and stamped.
Both kinds have to meet the plumbing code's basics. The Phoenix version of the 2024 International Plumbing Code (IPC) says "Water heaters shall be third-party certified." It also says water heaters on the drinking water system "shall comply with NSF 372," the lead content standard. Those apply to home and commercial units alike.
### Temperature
The residential-duty definition caps design outlet temperature at 180 F. A commercial heater designed for hotter water falls outside that class. For the 180 F rinse some dish machines need, see whether a restaurant needs a booster heater.
### Warranty
This is the difference owners feel most. A.O. Smith's residential warranty is written for a house: "The warranty is restricted to the water heater used in a single-family residence in the United States of America. This warranty is reduced to one year if the water heater is used in a commercial, or industrial application, or if the water heater is used to supply more than one dwelling unit."
So a residential tank in a coffee shop or a fourplex may keep only a one-year warranty on that brand. Other brands write their own terms, so read the warranty before you buy. Commercial models carry commercial warranties written for that use.
When a business can use a residential water heater
A residential heater in a business isn't against federal rules. The DOE classes are about the product, not the building. The questions that decide it are practical.
Is demand low and spread out? A small office with a break room sink and two restrooms may never stress a residential tank. That's a fine fit.
Is there a peak rush? A salon rinsing color at several bowls, or a cafe washing dishes at close, can drain a 40,000 Btu tank faster than it can recover. That's where a commercial unit or a larger system pays off. If yours already runs out at peak, see why a commercial water heater runs out of hot water.
Does the health code set a size? Food businesses in Maricopa County go through the county's sizing method, covered on the restaurant sizing page linked above.
Can you live with the warranty? If the brand drops to one year in commercial use, factor in a shorter coverage period.
Does it need a permit? Permit rules differ for businesses and homes. See whether you need a permit to replace a water heater in Phoenix for when a swap needs one.
| Business | Typical fit | Why |
|---|---|---|
| Small office, restrooms and break room | Residential tank or point-of-use | Low, steady use |
| Retail store with one restroom | Residential tank or point-of-use | Very light use |
| Hair salon with several shampoo bowls | Light commercial or high-recovery unit | Peak rinsing at many bowls at once |
| Coffee shop or cafe | Commercial, sized by the county method | Dishwashing and sanitizing loads |
| Full-service restaurant | Commercial tank, tankless rack or boiler system | High, spiky demand and high temperatures |
| Fourplex on one heater | Check the warranty terms | A.O. Smith's residential warranty drops to one year if the heater supplies more than one dwelling unit |
The table is our general guide, not a code rule. Your peak-hour demand decides the size, and a licensed plumber can work that out from your fixtures.
For the types of commercial systems, from tanks to tankless racks to heat pumps, see the types of commercial water heaters.
HQ Plumbing & Air installs and repairs commercial and residential water heaters across the Valley, with free estimates and upfront pricing.