Tankless Gas Water Heater Sizing: Flow, Temperature Rise, BTU/h, Venting & Gas Supply

A tankless gas water heater is sized by flow and temperature rise, not by gallons stored: the heat it must deliver is BTU/h = gpm × 60 × 8.33 × rise in °F, about gpm × 500 × rise. Two showers at the federal maximum of 2.5 gpm each, heating 50 °F water to 120 °F, need 174,930 BTU/h (51.3 kW) of heat, which is 184,137 BTU/h of gas input at 95% efficiency and 213,329 BTU/h at 82%. ENERGY STAR defines residential gas tankless heaters as having an input below 200,000 BTU/h, and the biggest Navien, Rinnai and Noritz models compared below are rated 199,000 to 199,900 BTU/h, so that pair of showers suits a condensing unit but is beyond a non-condensing one.

This article derives the formula, works the examples in US and metric units, and covers fixture flow rates, incoming water temperature, condensing versus non-condensing models, venting, gas supply, propane, recirculation and scale. Every figure comes from the documents listed under Sources. To run your own numbers, use the Water Heater Size Calculator in tankless mode.

The sizing formula, derived

Three facts give the formula:

• A US gallon of water weighs about 8.33 lb. A gallon is 3.785 412 L (NIST SP 811), and NIST’s data put water at 999.02 kg/m³ at 60 °F and 997.97 kg/m³ at 70 °F, which works out to 8.337 and 8.329 lb per gallon. The usual 8.33 is the 70 °F value; 8.34 is the 60 °F value.
• One BTU raises one pound of water by 1 °F. NIST lists 1 Btu_IT/(lb·°F) as exactly 4,186.8 J/(kg·K), and NIST’s measured specific heat of water between 40 and 140 °F is 4.179–4.206 J/(g·K), so water takes almost exactly 1 BTU per pound per °F.
• There are 60 minutes in an hour.

So each gpm of flow carries 60 × 8.33 = 499.8 lb of water an hour, and:

A tankless heater’s rated input is what it burns at full fire. Its output is the input times its efficiency at that firing rate, so the hot water it can supply at a given rise is fixed: 199,900 BTU/h × 0.95 ÷ (499.8 × 70) = 5.43 gpm at a 70 °F rise. The Uniform Energy Factor (UEF) on the label is a rating over a test draw pattern, not a full-fire efficiency, so treat it as an approximation. The makers’ own flow tables show what each unit really delivers: Navien lists the NPE-240A2 (199,900 BTU/h input) at 5.6 gpm with a 67 °F rise, which is 187,525 BTU/h of output, 94% of its input; Noritz lists the NR98 (199,900 BTU/h) at 5.6 gpm with a 60 °F rise, 167,933 BTU/h, 84% of input.

Worked examples

All examples use the formula above, with the hot setting at 120 °F (49 °C) unless stated. Inputs are output ÷ 0.95 (a condensing unit) and output ÷ 0.82 (a non-condensing unit).

Step by step for the first row: 5.0 gpm × 60 = 300 gal/h; × 8.33 = 2,499 lb/h; × 70 °F = 174,930 BTU/h; ÷ 0.95 = 184,137 BTU/h of gas input, which fits a 199,900 BTU/h condensing unit. Metric: 5.0 gpm = 18.93 L/min, 70 °F = 38.9 °C; 18.93 × 38.9 × 4.19 ÷ 60 = 51.3 kW. The same 51.3 kW from an electric tankless unit would draw 51,300 ÷ 240 = 214 A at 240 V.

The last two rows show why the temperature you actually use matters. Showers and basins are mixed with cold water, and the heat needed is the same whether you count the mixed flow at its use temperature or the smaller hot flow at the heater setting. The DOE Building America guideline sizes this way: with 65 °F inlet water and a 105 °F shower at 2.0 gpm, the unit must give a 40 °F rise at 2.0 gpm. Sizing every fixture at the full 120 °F setting is the cautious bound.

Fixture flow rates

Federal maximums apply to fixtures made after 1 January 1994 (10 CFR 430.32). WaterSense figures are EPA’s voluntary label limits. The ENERGY STAR figures are the averages its buying guide gives for estimating tankless demand. Real flow depends on pressure and on the fixture you actually have.

The Building America guideline’s table (taken from ASPE’s Domestic Water Heating Design Manual) gives higher figures for some uses: bathtub 2.4–4.0 gpm, laundry sink 4.0 gpm, kitchen sink 2.5 gpm, dishwasher 1.9 gpm. A tub filler is often the largest single draw in a house, so check its rating if a tub may be filled while a shower runs. EPA paused its draft revision of the bathroom-faucet specification (which proposed 1.2 gpm) in February 2025, so 1.5 gpm is still the WaterSense limit.

Incoming water temperature

The rise is the hot setting minus the coldest water the heater will see, which is usually in late winter. The Building America guideline says to get that minimum from the local water utility and to use 120 °F as the output temperature for most uses. If you are on a well, a US Geological Survey study gives a rule: water from 30–60 ft deep is generally 2–3 °F warmer than the mean annual air temperature (5–6 °F in exceptional places), and water from 20 to 200 ft runs about 3–6 °F above it. That paper dates from 1925 and its map is generalized, so a thermometer in a glass of cold tap water after a long run in winter is a better check. Water from rivers and reservoirs swings much more with the seasons than groundwater does.

The table shows how far one 199,900 BTU/h unit goes at each inlet temperature, using the formula and the two efficiencies above. The inlet temperatures are examples, not data for any region.

Compare the makers’ own tables: Navien rates the NPE-240A2 at 11.2 gpm (42 L/min) with a 35 °F rise, 8.7 gpm (33 L/min) at 45 °F and 5.6 gpm (21 L/min) at 67 °F; Noritz rates the NR98 at 9.8 gpm at a 30 °F rise, 6.7 at 50 °F, 5.6 at 60 °F and 4.8 at 70 °F. The headline gpm on a box is usually the figure at a small rise.

Condensing vs non-condensing

A condensing tankless heater has a stainless steel secondary heat exchanger that recovers the latent heat in the water vapour of its exhaust, and the condensate it makes has to be drained (DOE Building America guideline). Ratings from current spec sheets:

For comparison, the federal minimum for gas instantaneous water heaters over 50,000 BTU/h is UEF 0.81 (0.80 for the very-small draw pattern) in 10 CFR 430.32(d), and ENERGY STAR certifies gas tankless heaters at UEF ≥ 0.95 with at least 2.8 gpm over a 67 °F rise. A condensing unit uses less gas for the same hot water and, as the examples show, delivers more flow from the same 199,000–199,900 BTU/h burner. A non-condensing unit costs less to buy but needs metal venting and reaches its flow limit sooner.

Venting

• Condensing: Navien’s NPE-2 manual approves 2 or 3 in. Schedule 40 or 80 solid-core PVC or CPVC (UL 1738 listed in the US), approved polypropylene or approved stainless steel. Maximum length is 75 ft (23 m) in 2 in. pipe and 150 ft (45 m) in 3 in., less 8 ft (2.4 m) per 90° elbow in 2 in. and 5 ft (1.5 m) in 3 in.; if the exhaust temperature limit is disabled, the exhaust must be CPVC, polypropylene or stainless. Rinnai lists the same 75 ft and 150 ft maximum equivalent lengths for 2 and 3 in. PVC/CPVC or polypropylene twin pipe on the RX series.
• Non-condensing: the Noritz NR98 requires 4 in. Category III or 5 in./3 in. Noritz concentric stainless steel pipe. Rinnai’s RE199i uses a 3 in./5 in. concentric vent connection, and Rheem’s RTG series a 3 in./5 in. concentric inlet and exhaust with up to 43 ft of vent run, listed as compatible with Metal-Fab, PROTECH and Ubbink concentric venting.

Use only the vent materials and terminations the installation manual lists for your model; Navien’s manual says unapproved vent materials void the certification and the warranty.

Gas supply pressure and pipe size

A tankless heater needs its full gas flow at a minimum inlet pressure while it fires at maximum, with every other gas appliance running. If the pressure drops below the maker’s minimum (3.5 in. w.c. on natural gas for the Navien NPE-2 and Rinnai RX, 4.0 in. w.c. for the Rinnai RE and V-series), the unit will not reach its rated output. 1 in. w.c. is 249 Pa, so 3.5 in. w.c. is 0.87 kPa (8.7 mbar).

Gas pipe is sized from the total connected load, the longest length including fittings, the supply pressure, the allowed pressure drop and the pipe material, using the fuel gas code your jurisdiction has adopted. That is why no article can give one pipe size for a tankless heater. As an illustration only, the natural-gas table Navien reprints from the National Fuel Gas Code (0.5 in. w.c. drop, 0.60 specific gravity) lets 3/4 in. pipe carry 360 ft³/h at 10 ft, 199 ft³/h at 30 ft, 151 ft³/h at 50 ft and 104 ft³/h at 100 ft. Using Navien’s simplification of 1,000 BTU per ft³, a 199,900 BTU/h heater needs about 200 ft³/h on its own, so in that table a 3/4 in. line would run out at about 30 ft before any other appliance is added.

Most units have a 3/4 in. gas inlet. Some condensing models are certified for smaller pipe: Navien says the NPE-2 runs on 1/2 in. gas pipe up to 24 ft equivalent length (see its manual’s sizing tables for the limits), and Rinnai lists the RX series as 1/2 in. gas line compatible. Navien adds that a flexible connector must have a large enough inside diameter for the full input, and that you should check the local jurisdiction approves the smaller pipe.

Natural gas vs propane

The sizing formula is the same on propane: the heater needs the same BTU/h either way. What changes is the supply side. Propane carries more energy per cubic foot (DOE Building America guideline), so the same pipe carries more heat: in the propane table Navien reprints (0.5 in. w.c. drop), 3/4 in. pipe carries 608,000 BTU/h at 10 ft, 336,000 at 30 ft and 255,000 at 50 ft, against about 151,000 BTU/h for natural gas at 50 ft. Propane also runs at a higher inlet pressure, typically 8 to 13 or 13.5 in. w.c. on these models.

Conversion differs by maker. Navien’s NPE-2 cannot be converted without its propane and high-altitude conversion kit, which ships inside the cabinet; Rinnai’s RX series has an adaptive gas valve and needs only a label kit. Propane systems also need regulators sized for the full load; Navien’s manual shows a two-stage regulator system as its recommended arrangement.

Recirculation and minimum flow

A tankless heater fires only when the flow passes its activation rate: 0.5 gpm (1.9 L/min) for the Navien NPE-2, 0.4 gpm (1.5 L/min) for Rinnai’s RE and V-series. The Building America guideline describes the “cold water sandwich”: a slug of cold water between hot when a tap is turned off and on again. Recirculation keeps hot water near the fixtures. Navien’s A2 models have a built-in buffer tank and pump, can run internal or external recirculation, and Navien says this lowers the minimum flow needed, resolves the hot/cold/hot effect and delivers hot water sooner, with higher energy use while the loop is kept hot (a schedule or push-button demand control cuts that). Rheem’s RTG-R models also have built-in recirculation.

Scale and descaling

Hard water leaves scale in a tankless heat exchanger, which restricts flow and shortens its life (Building America guideline). Navien’s NPE-2 manual sets these rules:

Navien says to flush with white vinegar or a descaler made for tankless heaters, through the service valves. The Building America guideline reports that makers’ intervals range from twice a year to once every three years, and that some require a softener once hardness exceeds 8 to 14 grains per gallon. Check your model’s manual and warranty terms.

Sources

All documents were downloaded and read on 14 October 2026. Inputs, outputs and metric conversions in the tables are calculated from the formula above.

• U.S. Department of Energy, 10 CFR 430.32, Energy and water conservation standards, paragraphs (d) water heaters, (o) faucets, (p) showerheads, eCFR current text, 2026. ecfr.gov
• U.S. EPA WaterSense, Showerheads and Bathroom Sink Faucets and Accessories pages, 2026. epa.gov/watersense/showerheads, epa.gov/watersense/bathroom-faucets
• ENERGY STAR, Whole Home Tankless Gas Water Heaters (average fixture flows) and Water Heater Key Product Criteria (gas instantaneous UEF ≥ 0.95, max gpm ≥ 2.8 over a 67° rise), 2026. energystar.gov, energystar.gov criteria
• K. Brozyna and A. Rapport (IBACOS), Measure Guideline: Transitioning to a Tankless Water Heater, U.S. DOE Building America, September 2012. eere.energy.gov (PDF)
• NIST Special Publication 811, Guide for the Use of the International System of Units, 2008 edition, Appendix B (gallon, pound, Btu_IT/h, Btu_IT/(lb·°F), inch of water). nist.gov (PDF)
• NIST Chemistry WebBook, SRD 69, Thermophysical Properties of Fluid Systems: water at 1 atm, 40–140 °F (density and Cp). webbook.nist.gov
• W. D. Collins, Temperature of Water Available for Industrial Use in the United States, USGS Water-Supply Paper 520-F, 1925. pubs.usgs.gov (PDF)
• Navien, NPE Condensing Water Heaters Installation Manual (NPE-180/210/240A2 and S2, NPE-150S2), rev. 9.11.2023: §2.3 specifications, §3.1 water quality, §3.3 gas supply and sizing tables, §3.6.4 vent materials. Navien, NPE-2 Series Specification Sheet. navieninc.com
• Rinnai, SENSEI RX Series Residential Condensing Spec Sheet (100000883) and RE Series Non-Condensing Residential Indoor Spec Sheet; V75i (REU-VC2528FFU-US) Spec Sheet. RX (PDF), RE (PDF)
• Noritz America, Model NR98 (GQ-2857WX US) Series Specification Sheet, rev. 01/2018 (UEF, venting, flow-rate table).
• Rheem, RTG/RTG-R Series High Efficiency Non-Condensing Tankless feature sheet, form RH-RTG-R-FS, 02/26. rheem.com (PDF)