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.
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
Reading the result safely
- Tank heaters: DOE guidance sizes by first-hour rating (FHR) for your peak hour, not by tank gallons alone. Look for an FHR near your calculated peak demand.
- Tankless: Size to simultaneous fixture flow (GPM) and the temperature rise from your coldest incoming water to the desired outlet temperature. Colder groundwater means fewer GPM from the same unit.
- Safety note: Setting and working on gas, electric, or propane water heaters can involve combustion, electrical, and scald risks. Use a licensed plumber or HVAC technician for installation, gas connections, T&P valve work, and anode or element service. This page does not walk through DIY gas or electrical repairs.
- Many public health sources discuss household hot-water temperature and scald risk; follow the manufacturer and a qualified installer for setpoint advice for your household (including young children or vulnerable adults).
Sizing sources linked under Sources (DOE / ENERGY STAR / Building America). This section does not replace code or manufacturer instructions.