The main difference is the pipe. A plumber mainly installs and repairs water, drain and gas piping and fixtures in homes and businesses. A pipefitter installs pipe that may carry chemicals, acids and gases, mostly in industrial and commercial settings. In Arizona, hire by contractor license class: C-37 for plumbing, C-4 for boilers, steamfitting and process piping.
What each trade actually works on
Plumbers work on building water, drain and gas systems, and fitters work on industrial, process and high-pressure pipe. The U.S. Bureau of Labor Statistics (BLS) treats them as one job group, then splits out what each does. According to the BLS, plumbers "install and repair water, gas, and other piping systems in homes, businesses, and factories." They also set fixtures like bathtubs and toilets. They hook up appliances like dishwashers and water heaters. They clear drains and fix broken pipes.
Pipefitters and steamfitters, the BLS says, are "sometimes simply called fitters." They "install and maintain pipes that may carry chemicals, acids, and gases," and "these pipes are mostly in manufacturing, commercial, and industrial settings." The BLS adds that fitters work on pipe in power plants. They also work on heating and cooling systems in large office buildings. Steamfitters are a narrower group: in the BLS's words, they "specialize in systems that are designed for the flow of liquids or gases at high pressure." Other fitters specialize as gasfitters or sprinklerfitters.
The BLS also gives a scale example that matches what you see on job sites. Residential water systems use copper, steel and plastic pipe that one or two plumbers install. Industrial plant water systems, by contrast, are made of large steel pipe that usually takes a crew of pipefitters to install.
| Plumber | Pipefitter / steamfitter | |
|---|---|---|
| Typical pipe contents | Drinking water, hot water, waste, vent, natural gas | Chemicals, acids, gases, high-pressure steam and process fluids |
| Typical setting | Homes, businesses, factories | Manufacturing, commercial and industrial sites, power plants |
| Typical work | Fixtures, appliances, drains, water heaters, gas lines | Process piping, boiler and steam systems, large building heating and cooling piping |
| Arizona commercial license class that fits | C-37 Plumbing | C-4 Boilers, Steamfitting and Process Piping |
The table is a guide to the usual split, not a hard wall. The BLS is clear that the trades overlap: although they "have distinct responsibilities, they often have similar duties." All three install pipe and fittings that carry water, gas and other fluids. They pick materials, connect pipe, and pressure test the system so it's airtight and watertight.
Same training path, different specialties
The two trades start from the same place, which is why people mix them up. The federal ONET database files all of them under one occupation code, 47-2152.00 Plumbers, Pipefitters, and Steamfitters. Its sample job titles for that single code include Plumber, Service Plumber, Pipefitter, Steamfitter, Pipe Welder and Sprinkler Fitter. ONET's older specialty codes for the separate trades, 47-2152.01 and 47-2152.02, now redirect to that combined profile.
O*NET describes the whole group as workers who "assemble, install, alter, and repair pipelines or pipe systems that carry water, steam, air, or other liquids or gases." Its task list covers both ends of the trade. At one end: installing sinks, toilets, dishwashers and water heaters. At the other: welding small pipes or special piping, and installing automatic controls that regulate pipe systems.
Training looks alike too. The BLS says most people in these trades "learn their trade through a 4- or 5-year apprenticeship." Apprentices typically get 2,000 hours of paid on-the-job training each year. They also get technical instruction in safety, local plumbing codes and blueprint reading. Apprentices also study math, applied physics and chemistry. The paths split on the work an apprentice is placed on. One spends years on service calls and water heaters. Another spends them welding steel pipe in a plant.
Licensing is where the two trades differ. The BLS says "most states and some localities require plumbers to be licensed," while "some states require pipefitters and steamfitters to be licensed; they may also require a special license to work on gas lines." So a license is the norm for plumbers, while for fitters it depends on the state.
How Arizona sorts the work: C-37 vs C-4
In Arizona, plumbing falls under the C-37 license class and boiler, steam and process pipe falls under C-4. What matters for hiring is the company's license class. Under A.R.S. 32-1151, it's unlawful to act as a contractor without a license in good standing "in the name of the person, firm, partnership, corporation, association or other organization" doing the work, unless an exemption applies. The Registrar of Contractors issues those licenses by class. The scopes are written into the Arizona Administrative Code.
C-37 Plumbing. Rule R4-9-102 says this class "allows the licensee to install, alter, and repair all plumbing when performed solely within property lines and not on public easements or right-of-ways, except as hereinafter provided." The scope then lists water supply piping, fixtures and appliances, with tanks and pressure vessels. Municipal water systems are excluded. It also covers drains and vents, septic tanks, roof leaders, lawn sprinklers, water conditioning, and pool piping. It goes further than many people expect. It includes gas piping, compressed air and vacuum systems, fuel oil and nonpotable liquids. It also includes hot water heating and hot water supply systems "operating at pressures not exceeding 30 PSIG, or temperatures not exceeding 220° F," and "steam heating and steam supply systems not exceeding 15 PSIG operating pressure."
C-4 Boilers, Steamfitting and Process Piping. This is the fitter's class. The same rule says it "allows the licensee to install, alter, and repair steam and hot water systems and boilers," including chimney connections, flues, refractories, burners, piping, fittings, valves, thermal insulation and accessories. It also covers fuel and water lines to boilers, and "process and specialty piping and related equipment." Unlike the C-37 steam and hot water items, the C-4 text sets no pressure or temperature ceiling.
Read side by side, the two scopes draw the line for you. A C-37 contractor's steam work stops at 15 PSIG. Its hot water heating and supply work stops at the 30 PSIG or 220° F limits. Boilers, plus process and specialty piping, are written into C-4.
There are related classes you may see on a bid:
- C-74 is the C-4 scope including solar.
- CR-4 is a dual license. Rule R4-9-104 says it "allows the scopes of work permitted by the commercial C-4 Boilers, Steamfitting and Process Piping and the residential R-4 Boilers, Steamfitting and Process Piping licenses."
- CR-37 is the dual plumbing license. Rule R4-9-104 says it "allows the scopes of work permitted by the commercial C-37 Plumbing and the residential R-37R Plumbing licenses." Our guide to C-37, R-37 and CR-37 walks through the plumbing classes in detail.
- C-16 Fire Protection Systems is its own class for sprinkler and other fire protection piping. On a commercial job, a sprinkler fitter's work falls here.
So who should you hire?
Match the job to the license class, not to what someone calls themselves. A worker with the title "pipefitter" can be on a plumbing contractor's crew, and a plumber can work for a boiler contractor. The company's classification is what tells you whether the job is inside its legal scope.
For a house or condo, you want a plumbing contractor for water lines, drains, water heaters, gas piping and fixtures. Rule R4-9-103 says the residential R-37 class "allows the licensee to install and repair water and gas piping systems, fire protection as it relates to water sprinkler systems, and sewage treatment systems," along with "all fixtures, vents, and devices common to the industry." CR-37 pairs the commercial C-37 scope with the residential R-37R Plumbing scope. If your home has a hot water boiler, ask whether the job is plumbing work or falls under the residential R-4 Boilers, Steamfitting and Process Piping class.
For a commercial building, sort the job by what the pipe does:
- 1Domestic water, drains, restrooms, kitchens, water heaters, grease and floor drains, building gas piping: a C-37 or CR-37 plumbing contractor.
- 2Compressed air or vacuum lines, and low-pressure hydronic or steam systems within the limits above: C-37 covers these too, so the plumbing contractor can often take them.
- 3Boiler rooms, steam systems above 15 PSIG, and process or specialty piping for manufacturing equipment: a C-4, C-74 or CR-4 contractor.
- 4Fire sprinklers and fire protection piping: a C-16 fire protection contractor.
On a big project, you may have both trades on site at once. A boiler contractor sets and pipes the boiler, and the plumbing contractor runs the domestic water and drains that connect to the rest of the building. Write down who owns each connection point before work starts, so nothing falls between two scopes.
Whichever trade you need, check the license before you sign. Our page on how to verify a plumber's ROC license in Arizona shows where to look up the class and status. If you're weighing a cheaper unlicensed option, read licensed contractor vs handyman in Arizona first. For gas work specifically, see who can install a gas line in Arizona.
HQ Plumbing & Air is a licensed plumbing contractor serving the Phoenix area. We're licensed and insured, we give free estimates, and you get upfront pricing before any work starts. If your job is plumbing, we can look at it and give you a free estimate. If it turns out to be boiler or process piping, we'll tell you that straight so you can find the right license class.