Why Getting Navien Water Heater Sizing Right Matters Navien water heater sizing comes down to two numbers: how many gallons per minute (GPM) your home needs, and how much the unit must heat that water. Get both right, and you'll have endless hot water. Get either one wrong, and you'll either run cold or overpay for capacity you never use. Here's a quick-reference sizing guide for most Utah homes: Simultaneous Fixtures Peak GPM Needed Typical Household 1-2 fixtures 3.5 GPM Sma