What size water heater do you need?
Tank: first-hour rating (FHR)
The DOE says to size by FHR, not tank capacity. Count your busiest hour. Gallons per use come from the DOE fact sheet, which cites GAMA.
Tankless: flow rate (GPM) + temperature rise
Add up the fixtures that run at the same time. Enter your fixtures' actual GPM. Typical ranges from PNNL/DOE Building America: shower 0.8–2.5, hand sink 0.2–1.0, dishwasher fill 1.0–3.0, clothes washer fill 1.0–6.0.
AFFILIATE SLOT: tank and tankless water heaters (e.g., a home-improvement retailer program). No affiliate IDs yet.
Sources
- US DOE Water Heating fact sheet (FHR / peak-hour demand table): "choose a model with a FHR within about 2 gallons of this peak demand."
- DOE Building America Solution Center: gas-fired tankless sizing and fixture flow ranges
- ENERGY STAR FHR definition
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