g/plumbing • Posted by u/water_heater_wally • 1 days ago

Tankless water heater sizing for a family of 6 — 199k BTU enough?

Customer wants to replace a 75-gallon tank water heater with a tankless unit. Family of 6, 4 bathrooms, laundry on hot. Located in Minnesota so incoming groundwater is cold — around 38°F in winter.

I priced out a Rinnai RU199i (199,000 BTU). Is that enough for 2 simultaneous showers plus laundry? I'm trying to understand the temperature rise calculation so I can be confident in my sizing.

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u/pipefitter_pat • 1 days ago

The calculation: flow rate (GPM) × temperature rise (°F) × 0.454 = BTU required. For 2 showers at 2.0 GPM each + laundry at 1.5 GPM = 5.5 GPM. Temperature rise from 38°F to 120°F = 82°F. 5.5 × 82 × 0.454 = 204,919 BTU. You're right at the edge of the 199k unit. I'd size up to the Rinnai RU180iP or go with a two-unit parallel setup.

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u/pex_pioneer • 1 days ago

Minnesota winter is brutal for tankless sizing. The 38°F groundwater is your bottleneck. Same family in Florida with 70°F groundwater would be fine with 199k. Consider talking to the customer about lowering their simultaneous demand expectations OR budgeting for a two-unit setup.

u/drain_dave • 1 days ago

Also make sure you have adequate gas supply. A 199k BTU unit needs a properly sized gas line — many residential services can't support it without upgrading the meter or running larger pipe. Check the existing gas line size before the sale.