Tankless Water Heater Gas Line Sizing & Upgrades in Dallas, TX

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Trading a gas tank for a whole-house tankless water heater almost always means new gas pipe too. The tankless burner fires at two and a half to five times a tank's rate, usually more than the old half-inch branch can carry. Dallas 24 PLUMBER, Texas plumbing license # M-27587, sizes the pipe, vent and condensate drain before anything is ordered, and the plumber doing the work pulls the City of Dallas permit.

Gas smell in the house? Leave the building first and bring everyone with you. Don't touch light switches or light anything. Once outside, call 911 or Atmos Energy, then call us at (844) 313-1397. See what to do if you smell gas.

Tank burner vs tankless burner: the numbers

A tank reheats stored water slowly, so its burner is small: the national fuel gas code's planning annex puts a 30- to 40-gallon tank near 35,000 BTU per hour and a 50-gallon tank near 50,000. A tankless unit heats water on the fly, and the annex estimates about 142,800 BTU per hour just to deliver two gallons a minute.

Under federal product definitions, residential gas tankless units stop just short of 200,000 BTU per hour, and many whole-house models are rated 140,000 or more. Each cubic foot of natural gas carries roughly 1,000 BTU, so a heater rated at 199,000 draws close to 199 cubic feet an hour flat out, against about 40 for a 40,000 BTU tank.

Sizing the pipe by length and total load

Gas pipe loses capacity with distance. The 2021 fuel gas code table for Schedule 40 steel at normal house pressure (inlet under 2 psi, allowing a pressure drop of 0.5 inch of water column) shows it:

40-foot run: half-inch about 81 cubic feet per hour, three-quarter-inch 170, one-inch 320.
50-foot run: half-inch about 72, three-quarter-inch 151, one-inch 284, inch-and-a-quarter 583.

The code's longest-length method measures from the meter to the farthest outlet, assumes every appliance runs at full input at once, and sizes each stretch for the load it carries, all from that one row. Say the longest run is 50 feet, with a furnace, range and dryer (annex estimates: 100,000, 65,000 and 35,000 BTU) plus a new 199,000 BTU heater. Its branch needs one-inch pipe, and the shared stretch by the meter, now near 399 cubic feet an hour, needs inch-and-a-quarter. Real sizing uses your rating plates.

Slab homes usually get the new line through the attic; older pier-and-beam houses may allow a crawlspace route. A starved heater throws error codes or shuts down when a second shower opens. The meter itself can be outgrown too, and only Atmos can change it.

New venting, and a drain for condensing models

Unlike a tank's natural-draft metal flue, an indoor tankless heater's fan-driven vent must be the type, size and length its manufacturer lists, so it gets its own new vent; outdoor models exhaust through their own cabinet and need no vent pipe. If the old heater shared a flue with the furnace, that flue must be checked and resized if needed for the furnace alone: a vent sized for two appliances can draft poorly with one and let exhaust spill back.

Non-condensing units need heat-rated vent pipe, often stainless steel. Condensing units use a second heat exchanger to cool the exhaust enough for plastic vent, typically PVC, and their mildly acidic condensate drains through corrosion-resistant pipe no smaller than the heater's drain connection.

In a city historic district such as Winnetka Heights or Junius Heights, a new wall or roof vent or an outdoor unit alters the exterior, which the city's permit guide says typically needs a Certificate of Appropriateness first.

What the city checks before the heater fires

Water heater work needs a City of Dallas plumbing permit, filed in DallasNow before work starts. The city's November 2025 gas test guidelines (2021 IRC, IPC and IFGC) require an inspection and release for any change or addition to a gas system.

Pick the model: its input rating drives the sizing.
Site check: we look at the meter, trace the existing pipe and measure the longest run.
Rough-in: the new line goes in with a shut-off valve near the heater and a sediment trap ahead of its gas inlet, plus the vent and condensate drain.
City test: the capped piping is brought to 3 psi on a diaphragm gauge while a city plumbing inspector watches it hold for at least 15 minutes. See gas pressure test.
Release and first fire: after a passed test and the city's gas release (Atmos reconnects the meter if it was shut off), air is bled from the line, joints are leak-checked and the heater runs with several taps open.

Gas already off? See getting gas back on in Dallas.

Part of your water heater installation

Our water heater installation and replacement page covers the rest of the job, including the relief valve discharge pipe and removing the old heater, and the water heaters hub compares tankless with tank models. More gas appliances coming? See gas line installation.

Cost turns on pipe size, length and route, any upsizing near the meter, the vent and drain work, and the permit. Coupons (terms on the coupons page): $50 off for new customers; $150 off a new water heater. We plan tankless upgrades all over Dallas, from Kessler Park and Lakewood bungalows to newer infill near Fair Park.

Tankless gas line FAQ

Can my half-inch gas line feed a tankless water heater?

Rarely, for a whole-house model. Half-inch steel pipe moves about 81 cubic feet of gas an hour over a 40-foot run, roughly 81,000 BTU, while many whole-house units are rated 140,000 BTU or more, up to just under 200,000. Some manufacturers allow half-inch pipe only under set conditions, such as higher supply pressure and a limited run length.

What permit does a tankless gas line need in Dallas?

A City of Dallas plumbing permit, pulled by the plumber doing the work. Because gas piping is added or changed, the city also inspects it, and an inspector watches the pressure test.

Will Atmos Energy have to replace my gas meter?

Not always. The meter is Atmos equipment; the piping after it is your responsibility. If the tankless burner pushes your total load well past what the meter was set up for, Atmos decides whether a larger one is needed, so we add up the load early.

What is the condensate drain on a condensing model for?

Its second heat exchanger cools the exhaust until water condenses out. That water can be mildly acidic, so it runs in corrosion-resistant pipe, sloped at least 1/8 inch per foot, to an approved drain, and some models call for a neutralizer.

Plan your tankless gas line

For a planned upgrade, phone (844) 313-1397 with the model you're weighing or the input rating from its spec sheet. We'll check the meter, pipe and vent path and book the work at a time that suits you. Smell gas? Get outside first and call 911 or Atmos Energy, then call us at any hour and we send our on-call plumber right away.