Why Does a Tankless Water Heater Need a New Gas Line? Because Your Old Line Was Sized for 40,000 BTU, Not 199,000

That's the short version, and it's the part most homeowners already sense is coming. The longer version — the part that separates an honest quote from an $18,000 one — is how a plumber decides what your line actually needs.

Your old tank kept a reservoir of water hot around the clock and only fired its burner now and then to top up the temperature. Low demand, low BTU. A tankless keeps nothing in reserve — it lights the burner the moment water moves and heats it on the fly, which is why a whole-home unit is one of the largest single gas appliances in the house. Manufacturer literature puts most tankless units in the 140,000-to-200,000 BTU range. Rheem's own gas-piping guide calls a tankless "one of the largest single appliances on a typical gas system."

Natural gas carries roughly a thousand BTU in every cubic foot — the U.S. average runs about 1,037. So a 199,000-BTU unit needs close to 199 cubic feet of gas per hour to reach full output. Your old 40,000-BTU tank needed about 40. The pipe that comfortably delivered 40 was never asked to deliver 199, and a half-inch line doesn't have the cross-section to do it without choking. The 40,000-versus-199,000 gap is the whole story on our gas line page; this post is about the sizing work underneath it.

Here's where a real installer earns the job. The gas line doesn't just have to feed the tankless. It has to feed the tankless and everything else that can run at the same time — the furnace, the range, the dryer, the pool heater — with enough left over that none of them starve. Rinnai's install manual is blunt about it: the load calculation has to include "all gas products at the location and the maximum Btu usage at full load when all gas products are in use."

Four things set the required pipe diameter, per that same manual: the type of gas, the supply pressure at the meter, the pipe material, and the pressure drop across the run. A normal residential system runs at low pressure — around 7 inches of water column — and the piping has to be sized so the pressure drop stays at a half-inch of water column or less with every appliance firing at once. That half-inch is the entire budget. Spend past it and appliances at the end of the line don't get their gas.

Distance eats into that budget. The farther gas has to travel, the less a given pipe can carry, which is why an installer measures the run from the meter to the unit before picking a size — not after. In Rinnai's own worked example, a 199,000-BTU tankless sharing a line with one 65,000-BTU appliance adds up to 264 cubic feet per hour, and the required pipe comes out to at least 3/4-inch. Add length, or add more appliances, and the diameter only grows.

The pipe is half the picture. The gas meter and regulator on the side of your house have a flow rating, and it caps everything downstream. A typical household meter and regulator is rated around 250 cubic feet per hour. A house with a 40,000-BTU tank usually totals about 235 — comfortably under the cap. Swap that tank for a tankless and the whole-house total can jump past what a 250-CFH meter can physically pass.

The check is arithmetic: a meter's CFH rating times 1,024 gives its BTU capacity, and that number has to be larger than the sum of every appliance in the house. Rheem's guide is plain about the failure mode: "If the capacity of the system is too small, the gas appliances will not receive the volume of gas required for proper operation." A good installer runs that number before touching a pipe. If the meter is undersized, that's a call to the gas utility — not a bigger invoice for you.

This is the part you actually experience. A tankless only fires once water is moving — its minimum activation is 0.4 gallons per minute — and it modulates the burner up and down to match how much hot water you're pulling. The Rinnai RU199 needs its gas supplied at 3.5 to 10.5 inches of water column to do that. Run it on a pipe that's too small, and at peak demand the pressure at the unit sags below that floor. The burner can't reach the input it's calling for, and the water headed to your shower cools mid-stream. If your tankless runs hot and then goes cold partway through a shower, an undersized or shared gas line is one of the first things worth ruling out.

Underperformance is the mild outcome. Rheem's guide lists the rest of what an undersized gas system does over time: burners soot up, pilots and burners drop out, condensate forms and corrodes the heat exchanger, and — the one that matters most — sooting can clog a flue until an appliance can "produce harmful exhaust gases such as Carbon-Monoxide." Undersizing a gas line to save on pipe isn't a corner you cut. It's a safety problem with a delay on it.

When a tankless genuinely needs a bigger line, the work is defined, not open-ended. We run the load calculation for every gas appliance in the home at simultaneous full demand. We measure the run from the meter to where the unit will hang. We size the trunk to hold the pressure drop inside that half-inch-of-water-column budget — 3/4-inch is the usual answer for a near-200,000-BTU unit, larger if the run is long or the load is heavy. We confirm the meter and regulator can pass the new total, and we pressure-test the finished line before it carries gas. One appliance drove the change, so the work is scoped to serve that appliance safely — not to replace pipe that's already doing its job.

Andrew F. left us a note after one of these installs:

A correctly sized line is what makes a fast, clean install possible — the math happens before the wrench does.

If a quote to convert your water heater involves replacing all the gas piping you own, ask to see the load calculation that says so.

If you're weighing a tankless conversion — or a salesman has told you the whole house needs new gas lines to make one work — get the load calculation before you sign anything. We'll size the line to the unit, check the meter, and tell you straight whether a trunk line is all you need. In most homes, it is. Call or text us — 405-519-1868.