What Size Tankless Water Heater Do I Need?

What most Orange County homes need. The bar is usually two showers running at once in winter — about 3.6 gallons a minute when the groundwater is cold. Plenty of units can't hold that. The tool above only shows you the ones that can, in your house, with your gas line.

Most Orange County homes need a tankless water heater delivering 3.6 to 5.6 GPM — enough for two or three fixtures at once in January. That's a unit rated 150,000 to 200,000 BTU. Size on winter water temperature, not summer; the same heater delivers about a third less when groundwater drops to 52–60°F.

A family of four in Orange County usually needs a tankless water heater rated around 150,000 BTU. At peak they run two showers at once — about 3.6 GPM at winter temperatures, or roughly 126,000 BTU of heat. Add a bathroom sink and you're at 4.2 GPM, which takes 180,000.

A 50-gallon tank never gave you 50 gallons of hot water. As you draw from it, cold water refills the tank and mixes, so roughly three quarters is usable before it runs cool — about 37 gallons. A tankless doesn't run out at all, which is why a smaller unit than you'd expect replaces a 50-gallon tank comfortably.

What actually replaces it. For most Orange County homes, a 150,000 to 199,000 BTU unit delivering 3.6 to 5.5 GPM. A 40-gallon tank replaces the same way. The tank's gallon rating tells you nothing about the flow rate you need — only how long the old one lasted before it ran out.

A two-bathroom Orange County home needs a tankless water heater delivering about 3.6 GPM — enough for two showers at once at a 70°F winter rise, or roughly 126,000 BTU of heat. One bathroom needs 1.8 GPM. Three bathrooms runs 5.3–6.7 GPM and may take two units.

Size a tankless water heater by adding up the gallons per minute of every fixture you'd run at the same time, then checking how far the heater must raise the incoming water temperature. In California a showerhead or kitchen faucet is capped at 1.8 GPM; a washing machine uses 2–3 GPM. Size for a 70°F rise.

A tankless water heater needs about 35,000 BTU for every gallon per minute, at the 70°F rise Orange County requires in winter. Two showers at 3.6 GPM works out to roughly 126,000 BTU of heat. Most homes here end up with a unit rated 150,000 to 200,000 BTU.

Size a tankless water heater in Orange County for a 70°F temperature rise. Groundwater here reaches about 70°F in summer but drops to 52–60°F in winter, and a 70°F rise delivers 122–130°F water in January. A heater sized on summer numbers loses roughly a third of its flow when the weather turns.

The same unit does less in winter. A Rinnai RX199iN is rated 8.6 GPM at a 45°F rise — a summer number. At the 70°F rise Orange County actually needs in January, the same unit delivers 5.5 GPM. That is why we size on winter, and why a spec sheet's headline number can mislead you.

A large residential tankless unit measures about 24" tall × 18" wide × 11" deep — roughly the size of a kitchen wall cabinet, and most models are smaller. It hangs on the wall, replacing a tank that sat on a 30" × 30" platform — 6.25 square feet of floor. That's the closet or garage corner you get back.

Usually no — a ½-inch gas line will not carry a full-size tankless water heater. It carries 189,200 BTU at 10 feet and drops to 129,800 at 20 feet, while a full-size unit fires at up to 199,000. Below about 16 feet a ½-inch line can feed a mid-size 150,000 BTU heater.

At standard residential pressure, a ¾" line carries 396,000 BTU at 10 feet, 271,700 at 20, 187,000 at 40, and 128,700 at 80. Capacity falls with distance, and every 90° elbow adds about 5 feet. A 199,000 BTU tankless needs ¾" out to roughly 36 feet.

A 200,000 BTU tankless water heater needs 182 CFH of gas — 200,000 divided by 1,100, the heating value used in SoCalGas territory. A ¾-inch line carries that to about 36 feet of developed length. Past 36 feet you need 1 inch. A ½-inch line won't carry it at any distance.

A tankless unit typically needs a ¾" gas line, versus the ½" line that fed your old tank. What matters is developed length, not distance — every 90° elbow adds about 5 feet. Run the code table and ½" stops carrying a mid-size unit at about 16 feet. Past that you go up a pipe size.

Why "it's only 25 feet" is usually wrong. A run with 25 feet of pipe and six elbows has a developed length of 55 feet, not 25. At 55 feet ¾" carries about 144 CFH — roughly 158,000 BTU. A 199,000 BTU tankless needs 181 CFH, so that job takes 1-inch pipe.

Why the line has to grow. A 40-gallon tank heater draws a steady 34,000 BTU. A tankless unit modulates between 15,000 and 199,000 BTU — nearly six times the tank's draw — because it heats on demand instead of over time. The line and the meter both have to carry that maximum.

A starved tankless burns dirty. It can't reach full fire, so combustion goes incomplete. On a condensing unit that makes the condensate more acidic than the heat exchanger is built to handle, and it corrodes the unit from the inside. You lose years off its life, and the manufacturer voids the warranty for improper gas supply.

The manufacturers put this in writing. Navien's limited warranty for its NPN-E tankless series excludes "performance problems caused by improper sizing of the boiler, the gas supply line, the venting connection, combustion air openings" — and voids coverage for "subjecting the heat exchanger to pressures or firing rates greater or lesser than those shown on the rating plate."

Gas pipe capacity tables are published in cubic feet per hour, but a tankless water heater is rated in BTU. Divide BTU by the heating value of the gas to convert. Most online charts use 1,000 BTU per cubic foot; in SoCalGas territory it's 1,100, so a 199,000 BTU heater needs 181 CFH.

Size the gas line for a tankless water heater in four steps. Multiply your GPM by 35,000 for the BTU output you need at a 70°F rise. Divide by the heater's efficiency for its input rating. Divide that by 1,100 for cubic feet per hour. Look it up at your developed length — pipe run plus 5 feet for every 90.

Where the 16 feet comes from. Navien doesn't publish a maximum length — they tell you to use the gas code sizing table. Run a 150,000 BTU unit at 1,100 BTU per cubic foot and you need 136 CFH. The code table puts ½" pipe at that capacity right around 16 feet of developed length.

Sometimes a tankless water heater does push a gas meter past its rating. The meter has to carry the whole house at once — furnace, range, dryer and tankless together. Add them up, and if the total exceeds what the meter is rated for, it needs upsizing. That's a call to SoCalGas, not a plumbing job.

A tankless water heater goes hot, then cold, then hot again when you turn the tap off and back on within a minute or two. The water in the pipe is still hot, but the slug behind it cooled while the burner was off. The cool burst usually lasts a few seconds.

The cold water sandwich is the brief burst of cool water you get from a tankless water heater when you turn a tap off and back on quickly. The heater only fires when water flows, so a short pause leaves un-heated water sitting in the line. A recirculation pump largely eliminates it.

Tankless water heaters have three real downsides. The gas line often has to be upsized, which is the cost that surprises people. Descaling is needed roughly annually in Orange County's hard water, or the warranty can lapse. And there's the cold water sandwich — a brief cool burst when you cycle a tap.

Most plumbers aren't against tankless water heaters, but the ones who are have a reason: a tankless punishes a bad install in ways a tank doesn't. Undersize the gas line and it burns dirty and voids the warranty. Skip the descaling and it fails early. Done properly it outlasts a tank.

Condensing units in our lineup run 0.85 to 0.98 UEF and vent in PVC, which is cheaper to install. Non-condensing run 0.79 to 0.82 and need Category III stainless venting, which costs more but may reuse an existing flue. In Orange County we install condensing on 90–95% of jobs.

A whole-house electric tankless water heater delivers about 3.5 GPM at the 70°F rise Orange County needs in winter, and draws 50 to 150 amps — requiring a 100 to 300 amp service. The gas units we install do 3.3 to 5.6 GPM without touching your electrical panel.

Electric tankless units are advertised at a low temperature rise. A unit sold as "3.5 GPM" often means 3.5 GPM at a 35°F rise — summer conditions in a warm climate. At the 70°F rise Orange County needs in January, the same unit delivers closer to half that.

The largest single tankless water heaters we install all land in the same place at a 70°F rise: the Noritz EZ111DV and Navien NPE-240S2 at 5.6 GPM, the Rinnai RX199iN at 5.5. Smaller heaters track BTU input — a 180,000 BTU model does roughly 4.8 to 5.0 GPM, a 150,000 BTU model 4.2.