Tankless Water Heater GPM

Achievable flow from burner kBTU and inlet/outlet temperatures.

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Formula and source

Required heater output (BTU/hr) = 8.34 × ΔT × GPM × 60. Solved for GPM given the heater's rated input × thermal efficiency.

ASHRAE Handbook (HVAC Systems and Equipment) by name; manufacturer ratings on the heater nameplate.

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