GPM

GPM tells you flow: a modern showerhead runs about 1.8 to 2.5 GPM, and a tankless water heater is rated by how many GPM it can heat at once. Sizing a Navien tankless unit means adding up the fixtures you actually run at the same time. Cold Iowa groundwater in winter lowers the effective GPM of any tankless heater, so honest sizing accounts for that.

Why flow, not pressure, explains most 'weak water' complaints

Homeowners usually say 'low pressure' when the real problem is low flow — the two feel the same at the showerhead but have different causes and fixes. Pressure is the push; GPM is how much water actually arrives. A scaled-up showerhead, a half-closed valve, or corroded galvanized pipe can starve a fixture even when pressure at the meter is perfect. GPM is also the whole language of tankless sizing: a unit is bought by how many simultaneous gallons per minute it can raise from Iowa's winter groundwater temperature to shower temperature.

When GPM numbers should drive your decisions

Run the numbers any time you are considering a tankless water heater: add up the fixtures your household genuinely runs at once — a shower and the dishwasher, say — and size to that, remembering that winter inlet water in Eastern Iowa can cut a unit's rated output roughly in half versus the brochure's warm-climate numbers. Flow math also matters when one fixture is weak but the rest are fine (clean the aerator, check its stop valve) versus when the whole house is weak (think supply line, pressure, or old galvanized pipe).