Sizing method and sources
Method v1.0. Source and arithmetic checks completed October 14, 2026. These worksheets support residential planning. They do not provide installation designs, contractor quotes or a model-fit guarantee.
Fixture flow and mixing
Hot-side flow equals simultaneous count × entered GPM. For mixed fixture rows, hot fraction = (mixed target − inlet) ÷ (heater setpoint − inlet). Multiply that fraction by the fixture count and mixed GPM, then sum the rows. Valid inputs require inlet below mixed target and mixed target no greater than the setpoint.
The calculator evaluates both inlet endpoints with both flow bounds. It takes the minimum and maximum from those paired scenarios. It never combines a mixed hot fraction from one inlet with the rise from another. Bounds must be positive and ordered, with a positive whole-number count. Liquid inlet water must be above 32°F.
Heat duty and units
Thermal output Btu/h = 8.34 lb/US gal × 60 min/h × hot-side GPM × temperature rise in °F, using approximate water heat capacity of 1 Btu/(lb·°F). Thus the working constant is 500.4. Thermal kW = Btu/h ÷ 3412.142. This approximation omits distribution loss and adds no generic safety multiplier.
Gas burner input is thermal output divided by a supplied steady-state thermal efficiency. Electric input kW is thermal kW divided by a supplied heating efficiency. The efficiency must be greater than zero and at most one. UEF is a draw-pattern test metric, so it is not substituted. An efficiency of one is available only as an explicitly hypothetical ideal assumption.
Electrical scope
Aggregate current = input kW × 1000 ÷ selected voltage. The tool covers resistive single-phase scenarios at 208 V or 240 V. The result is not an individual circuit rating, breaker selection, conductor size or service-load calculation. An exact model's voltage-specific ratings and circuit schedule remain separate checks.
Reproducible calculation checks
- 3 GPM hot-side, 120°F outlet and 50 to 60°F inlet: rise 60 to 70°F; output 90,072 to 105,084 Btu/h; about 26.4 to 30.8 thermal kW.
- 2 GPM mixed to 105°F, 50°F inlet and 120°F heater outlet: hot fraction 55/70; hot-side flow about 1.5714 GPM; output 55044 Btu/h; about 16.1 thermal kW.
- Adding a 1 GPM hot-side draw to that mixed example produces 90,072 Btu/h, about 26.4 thermal kW.
- 3 GPM at a 70°F rise, ideal efficiency one: about 128.3 aggregate A at 240 V, or 148.1 A at 208 V. These are scenario calculations, not model ratings.
Primary sources and limits
- DOE Building America transition guideline, September 2012: historical demand, rise and retrofit context. Its historical prices and code references are not treated as current.
- Stiebel Eltron Tempra flow table: a model-specific example of flow at rise, not a universal curve.
- Tempra Trend/Plus installation manual: exact voltage and circuit requirements must follow the applicable model row. No table entry is converted into permission to use an existing service.
- ENERGY STAR criteria: definitions of UEF, GPM and first-hour rating.
- CPSC Publication 5098: tap-water scald caution. The 120°F example is not a universal sanitation recommendation or a guarantee against burns.
No live manufacturer curve is loaded into the tool, so no automatic product fit is reported. Actual pressure, water quality, pipe losses, delivery delay, fuel supply, electrical service and installation conditions remain outside its calculation.
Corrections and changes
October 14, 2026: v1.0 introduced paired inlet/flow endpoints, mixed fixture heat balance and separate thermal-output and input-power results. To report an error, send the page, entered values and source through contact. Do not send a phone number or address. A method correction is checked against primary evidence and applied to the tools and derived examples together.