Often, yes. A water heater or boiler with a higher Btu input burns more gas every hour, and the pipe and meter must carry that on top of every other appliance running at once. A same-size swap usually needs no gas work. An upgrade that raises the total load needs a pipe sizing check and, in Phoenix, a permit.
How do you know if the gas line is big enough?
Add up the load and check it against the pipe. Section 402.2 of the 2024 International Fuel Gas Code, which Phoenix enforces, sizes gas pipe on "the sum of the maximum input of the appliances served," assuming everything could run at full capacity at the same time. So the question isn't whether the pipe can feed the new heater alone. It's whether it can feed the heater plus the kitchen, the other heaters and anything else on that line.
The math works in cubic feet per hour. Natural gas carries roughly 1,000 Btu per cubic foot, so a heater's rating plate converts easily. The code's sizing table for standard pressure, Table 402.4(1), then shows what each pipe size carries over a given length. For Schedule 40 steel pipe over 100 feet:
| Pipe size | Carries about (100 ft, standard pressure) | Roughly |
|---|---|---|
| 1 inch | 148 cubic feet per hour | 148,000 Btu/h |
| 1-1/4 inch | 304 cubic feet per hour | 304,000 Btu/h |
| 1-1/2 inch | 455 cubic feet per hour | 455,000 Btu/h |
| 2 inch | 877 cubic feet per hour | 877,000 Btu/h |
Take a simple case. A 1-1/4 inch branch runs 100 feet to a water heater room. Swap a 200,000 Btu/h heater for a 400,000 Btu/h model, and the demand on that branch goes from about 200 to about 400 cubic feet per hour. That's past what 1-1/4 inch carries at that length, so the branch moves up to 1-1/2 inch. The main feeding it has to carry the new total too. Longer runs carry less, so a heater far from the meter needs bigger pipe than the same heater close to it.
What happens if the gas line is too small?
The heater gets less pressure than it needs, right when demand peaks. Section 402.1 requires the piping to deliver gas at each appliance inlet at not less than the appliance's minimum supply pressure. Manufacturers spell out what that means for their units.
A.O. Smith's service handbook for its BTH commercial high-efficiency water heaters is a good example. It lists a nominal natural gas supply pressure of 7 inches of water column and a low gas pressure switch at 4.8 or 5.2 inches, depending on the model. It also says the supply pressure "must be stable within 1.5" W.C." from static to full fire. Bigger drops "may cause rough starting, noisy combustion or nuisance outages." And its diagnosis is direct: "If the supply pressure drops more than 1.5" W.C. ... then the supply gas system including the gas line and/or the gas regulator may be restricted or undersized."
The handbook also says to measure supply pressure "with all gas fired appliances connected to the common main firing at full capacity." That's the real test. A heater can light fine at 6 a.m. and lock out at noon when the kitchen, the dish machine booster and a second heater all fire together. Our page on why a commercial water heater isn't making enough hot water covers the other causes of that complaint.
### Does a higher-pressure system change the answer?
It can make the pipe smaller, but it adds regulators. A.O. Smith notes that "High pressure building supply systems use pressures that exceed 14" W.C. (1/2 PSI Gauge). These systems must use field supplied regulators," and that the gas valve on its BTH models has "a maximum gas supply pressure limit of 14" W.C." The regulator must be a positive lock-up type rated at or above the heater's input, and the manual says to place it "no closer than 3 equivalent feet" from the heater. Section 410.1 of the fuel gas code requires a listed line pressure regulator wherever an appliance runs at lower pressure than the supply. Our page on 2 psi gas systems explains when that design makes sense.
Which upgrades usually need gas work?
Anything that raises the building's total gas load. Common ones:
- A bigger storage heater in place of a smaller one, often after a kitchen or laundry expands.
- A second heater added to keep up with demand.
- Multiple tankless units replacing one tank. Each unit's rating plate lists its input, and together they can add up fast.
- A boiler with an indirect tank replacing a water heater.
The same load math applies to other big gas equipment added at the same time, like a standby generator.
Section 624.1 of the code requires water heaters to be listed and installed per the manufacturer's instructions, and Section 631.2 requires the installer to set and test a boiler's controls. The type of heater you choose decides the load. Our guide to types of commercial water heaters compares them.
### Does it need a permit?
It depends on whether the gas load goes up. The City of Phoenix gas clearance table lists "Replacement in kind of any gas appliance with no gas piping alterations" as needing no city permit. It lists "Appliance upgrade with an increase in BTU consumption requiring piping capacity calculations" as needing a permit, a city inspection and a city clearance. New or altered piping needs the same. Other Valley cities set their own rules, so check with yours.
What about the gas meter?
It has to keep up too. The meter belongs to Southwest Gas, and its Arizona tariff says "It is the customer's responsibility to notify the Utility of changes in their load requirements." A big heater upgrade can push a building past its meter's capacity, especially in a restaurant already running a full cooking line. Let Southwest Gas know about the new load before installation day. Our guide to getting a bigger Southwest Gas meter covers the load list, lead times and meter set.
What does the gas side of the install include?
Beyond the pipe size, the connection itself has rules:
- 1A shutoff valve in the same room and within 6 feet of the heater, upstream of the union or connector, per Section 409.5.1.
- 2A sediment trap downstream of that shutoff, as close to the heater's inlet as practical, unless the heater has one built in (Section 408.4).
- 3The right connector. Rigid pipe, or a listed appliance connector kept in the same room.
- 4A pressure test on the new piping before it's covered or put in service. Our guide to how a gas line pressure test works walks through it.
- 5A running pressure check with every appliance firing, so you know the heater gets what its manual asks for.
HQ Plumbing & Air installs commercial water heaters and the gas piping behind them across the Phoenix area, from load calculations and pipe upsizing to permits and pressure checks at full fire. We're licensed and insured, and you get a free estimate with the price up front.
Size the gas for the heater you're buying, with everything else running.
To see how the heater itself is sized for a Maricopa County restaurant, read how the county sizes a restaurant water heater.