What size gas line does a tankless water heater need?

Why a tankless unit changes the gas math

A tank heater warms water slowly and stores it, so its burner is small. Rheem's Performance power vent tanks, for example, are rated up to 42,000 Btu per hour^[5]. A tankless heater has no stored water, so it has to heat water as fast as it flows. Rinnai's RU199 models draw up to 199,000 Btu per hour at full fire^[1].

That is roughly five times the gas flow of the old tank, and the pipe that fed the tank was sized for the tank. If the line is too small, pressure drops when the heater fires at full rate, especially when the furnace or range runs at the same time. Rinnai's manual says the supply line must deliver enough gas for the heater and every other gas appliance at the location "without loss of pressure," and that the supplied pressure must stay within limits "with all gas appliances operating"^[1]. For natural gas on those models, that window is 3.5 to 10.5 inches of water column (in. w.c.) at the heater^[1].

What the code says

The model fuel gas code sets three rules that drive the sizing^[2]:

• Pipe must deliver enough gas to meet the maximum demand, at no less than each appliance's minimum required pressure (section 402.1).
• Size for the full connected load. The flow to plan for is the sum of the maximum input of the appliances served, assuming all of them could run at full capacity at once. A smaller load is allowed only where a diversity of load can be established (section 402.2).
• Use an approved method. That means the code's sizing tables or equations, the sizing tables of a listed piping system (such as flexible corrugated tubing), or engineering methods (section 402.3).

Under the common "longest length" method, each section of pipe is sized using the longest run from the meter to the most remote appliance, plus the load that section carries (section 402.4.1)^[2]. Your city adopts a specific code edition, sometimes with local changes, and the inspector enforces that version.

How the sizing tables work

Plumbers convert the load to cubic feet of gas per hour (CFH). Natural gas carries about 1,000 Btu per cubic foot; EIA's conversion factor is 1,036^[7], and the gas utility can give the local value. Then they read a table for the pipe material, supply pressure and allowed pressure drop.

Here is part of the natural gas table Rinnai reproduces from NFPA 54 (Schedule 40 steel pipe, supply under 2 psi, 0.5 in. w.c. pressure drop)^[1]:

Length of run	1/2 inch	3/4 inch	1 inch	1-1/4 inch
10 feet	172 CFH	360 CFH	678 CFH	1,390 CFH
20 feet	118	247	466	957
40 feet	81	170	320	657
60 feet	65	137	257	528
100 feet	50	104	195	400

Capacity falls fast with length. That is why the same heater may work on 3/4 inch pipe in one house and need 1 inch in the next.

A worked example (our arithmetic, for illustration only)

Say a home has a 199,000 Btu tankless unit, an 80,000 Btu furnace and a 65,000 Btu range, and the farthest appliance is 40 feet of pipe from the meter.

1. Total load: 344,000 Btu per hour, or about 344 CFH at 1,000 Btu per cubic foot.
2. Main line from the meter (344 CFH at 40 feet): 1 inch carries 320, which is not enough, so the table points to 1-1/4 inch (657)^[1].
3. Branch to the water heater (199 CFH at 40 feet): 3/4 inch carries 170, not enough; 1 inch carries 320^[1].
4. For comparison, the old 40,000 Btu tank (40 CFH at 40 feet): 1/2 inch carries 81, so the original 1/2 inch branch was fine for the tank^[1].

Real sizing also counts fittings, the actual pipe material, and your local code's tables. Treat this as a picture of why upgrades happen, not as a design.

Exceptions that change the answer

• Higher supply pressure. Rinnai also prints a table for a 3.0 in. w.c. drop, intended for homes with a supply of 8.0 in. w.c. or more. In it, 1/2 inch pipe carries 454 CFH at 10 feet^[1]. Some homes have elevated-pressure systems with regulators near the appliances, which the code sizes in two parts (section 402.4.3)^[2]. Only the utility and a plumber can confirm what you have.
• Flexible corrugated tubing (CSST). It is sized from the tubing maker's own tables, not the steel pipe table^[2].
• Propane. It uses separate tables, listed in thousands of Btu per hour at an 11 in. w.c. inlet. In Rinnai's example, 1/2 inch pipe carries 291,000 Btu per hour at 10 feet^[1]. Propane appliances also need a higher supply pressure: 8.0 to 13.5 in. w.c. on the RU199^[1].
• Smaller tankless units. Navien lists its NPE-180A2, a 150,000 Btu unit, as "1/2 inch gas pipe capable up to 24 feet," subject to proper sizing under the National Fuel Gas Code^[3]. Navien's FAQ adds that pipe size, length and pressure drop all affect delivery^[4].

Signs the gas line may be too small

• The tankless unit shows ignition or flame-loss codes, especially when the furnace, dryer or range is also running. See the Rinnai and Navien code lists.
• Water temperature swings or drops during heavy use while other gas appliances run.
• Other gas appliances flicker or struggle when the water heater fires.

These symptoms have other causes too, including venting and scale. A plumber confirms a supply problem with a pressure gauge (manometer) reading taken with all appliances firing.

What it costs

HomeGuide's January 2026 guide puts upgrading to a larger gas supply line at $350 to $2,000, and permit fees for gas or electrical work at $50 to $200^[6]. Price depends on the run length, the pipe material, how open the framing is, and whether the meter also needs an upgrade. These are national ranges, not quotes. See tankless water heater cost for the full install picture.

What a homeowner can safely do

• Read the rating plate on each gas appliance and write down its Btu input.
• Note roughly where the meter is and how far the water heater sits from it.
• Ask each contractor to write the gas line size, material and route into the quote, and to confirm they sized it from the code tables.

Do not loosen, cut or extend gas piping yourself. If you smell gas at any point, leave the house without using switches or phones, and call the gas utility or 911 from outside^[8].

When to call a pro

• Any change from a tank to a gas tankless unit, or any new gas appliance added to the house.
• Ignition or flame-loss codes that appear when other gas appliances run.
• Any gas pipe sizing, extension, pressure test or meter question: this is licensed, permitted work.
• A gas smell near the heater or the piping: leave first, then call the utility.