Tankless Water Heater Sizing Calculator

Temperature Rise in this calculator is a pure subtraction -- Target Hot Water Temp minus Groundwater Temp -- so raising the target always raises it and raising the groundwater temperature always lowers it, both cleanly across their full declared ranges, and neither Simultaneous Showers, Sinks, Dishwasher, nor Washing Machine touch it at all: fixture count sets flow, temperature inputs set temperature rise, and the two never cross. Required BTU/hr is the product of that temperature rise and Total Flow Demand, and while Target Hot Water Temp and Groundwater Temp both move it near this calculator's defaults, neither one safely "dominates" it across the whole input range -- pushed to their extremes, the fixture-count inputs (especially Simultaneous Showers, at 2.0 GPM apiece) can swing Total Flow Demand, and therefore Required BTU/hr, more than either temperature input can.

That matters for reading Units Needed: a colder groundwater supply or a hotter target delivery temperature both push the required capacity up and can tip the recommendation from a single residential unit into "Multiple units needed" territory, but so can simply adding more simultaneous fixtures -- the BTU requirement this calculator sizes against is genuinely a joint function of both how much hot water is demanded at once and how far it has to be heated, not either one alone. When the condensing-unit BTU requirement exceeds a single unit's roughly 199,000 BTU/hr ceiling, the calculator reports the actual number of units needed (rounding up), rather than quietly recommending one undersized unit.