How Sizing Is Calculated
Peak demand is estimated from: showers (12 gal each at avg), baths (20 gal), dishwasher (6 gal), laundry (7 gal), and personal use (4 gal/person). Recommended tank = peak demand x 1.1 buffer, rounded to the nearest standard size. GPM = peak demand / 60 minutes. FHR should meet or exceed peak hour demand.
Frequently Asked Questions
The right size depends on peak hour demand — how much hot water you use in your busiest hour. The Department of Energy recommends sizing tank heaters by First Hour Rating (FHR), not just tank capacity. A family of 4 typically needs a 50-60 gallon tank or a tankless unit rated at 6-8 gallons per minute.
FHR is the amount of hot water a tank heater can supply in the first hour of use, starting with a full tank. It accounts for both the stored hot water and the water heated during that hour. The FHR is listed on the EnergyGuide label and is the best metric for sizing a tank water heater.
Add up the flow rates of all fixtures you expect to run simultaneously. A shower typically uses 2 GPM, a bathroom sink 1 GPM, a kitchen sink 1.5 GPM, and a dishwasher 1-1.5 GPM. For a shower and kitchen sink running together, you need a tankless heater rated for at least 3.5 GPM.
Temperature rise = desired output temperature minus incoming cold water temperature. In warm climates, groundwater may be 65-70°F, requiring a 45-55°F rise to reach 120°F. In cold climates, groundwater can be 40-50°F, requiring a 70-80°F rise. Higher temperature rise reduces the flow rate a tankless unit can deliver.
Tank heaters have lower upfront cost ($300-1,000 installed) and are simpler. Tankless heaters have higher upfront cost ($1,000-3,000 installed) but can be 24-34% more energy efficient for average use and provide unlimited hot water. Tankless heaters make most economic sense for low-volume users or homes replacing frequently-failing large tanks.
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