Size from the house, not the box. Flow you need = the fixtures running at once (about 2 GPM a shower, 1 GPM a bathroom sink, 1.5 GPM a kitchen sink, 2 GPM a dishwasher or washer, per Noritz's sizing sheet). Temperature rise = your set point (usually 120°F) minus your winter inlet water. Then choose a unit whose flow at that rise covers it. Our size planner does the arithmetic and lists the units that keep up.
Step 1: count the fixtures that run together
Two showers and a dishwasher at the same moment is 6 GPM; four showers and a dishwasher, 10 GPM (Noritz's worked examples). A WaterSense showerhead flows 2.0 GPM or less; a standard head, 2.5 GPM, so check yours.
Step 2: find your winter rise
Inlet water is coldest in January. Rinnai's ground-water map runs from about 35°F in the far north to 77°F in the far south. At a 120°F set point that is a rise of 43°F to 85°F. Noritz's advice is to size on the winter rise: an undersized unit gives the right temperature but fewer fixtures at once, and the shortfall shows in winter.
Step 3: match flow at that rise
| Inlet | Rise | 199,000 BTU condensing | 150,000 BTU non-condensing | 27 kW electric | 18 kW electric |
|---|---|---|---|---|---|
| 37°F | 83°F | 4.6 | 2.8 | 2.2 | 1.5 |
| 47°F | 73°F | 5.2 | 3.2 | 2.5 | 1.7 |
| 57°F | 63°F | 6.0 | 3.7 | 2.9 | 1.9 |
| 67°F | 53°F | 7.1 | 4.4 | 3.5 | 2.3 |
| 72°F | 48°F | 7.9 | 4.8 | 3.8 | 2.6 |
Gas figures use input × efficiency ÷ (500 × rise) with 0.95 for condensing and 0.77 for non-condensing units; electric figures use kW × 3,412 ÷ (500 × rise). The 199,000 BTU column reproduces Rinnai's published figures.
Examples in the catalog
Large gas units: Rheem RTGH-95DVLN-3 199,900 BTU Natural Gas Condensing Tankless Water Heater, Rinnai CX199iN 199,000 BTU Condensing Tankless Water Heater and Rinnai RX199iN Condensing Tankless Water Heater. Whole-house electric: Atmor AT-36N-XTP 36 kW Electric Tankless Water Heater, Eemax EEM24036 36 kW Electric Tankless Water Heater and Eemax HA036240 36 kW Electric Tankless Water Heater.
Products mentioned
Rheem RTGH-95DVLN-3 199,900 BTU Natural Gas Condensing Tankless Water Heater
199,900 BTU/h · 9.5 GPM max
Rinnai CX199iN 199,000 BTU Condensing Tankless Water Heater
199,000 BTU/h · 11.1 GPM max · Propane
Atmor AT-36N-XTP 36 kW Electric Tankless Water Heater
36 kW