What should I look for when buying sizing your water heater?

Use first-hour rating (FHR), not just tank size, to compare. Cold climates need tankless sized for higher temperature rise. Heat pump water heaters recover slower -- size up.

What are common mistakes when buying sizing your water heater?

Sizing by tank gallons alone -- FHR matters more. Buying tankless sized for warm climate in a cold climate. Replacing 40-gal with 40-gal without checking if the old one was adequate.

Sizing Your Water Heater | The Plumbing Directory

Overview

An undersized water heater runs out during showers. An oversized one wastes energy. Tank heaters are sized by first-hour rating (FHR). Tankless by flow rate (GPM) and temperature rise. Getting it right means never running cold.

What to Know

Tank Water Heaters

First-hour rating (FHR) is the key metric -- how much hot water delivered in the first hour of heavy use. Calculate peak-hour demand: showers (20 gal each), dishwasher (6 gal), clothes washer (7 gal), faucets (4 gal each). Match FHR to that number.

Tankless Water Heaters

Sized by flow rate (GPM) and temperature rise. Add GPM of all simultaneous fixtures. Calculate rise: desired output (110-120F) minus incoming water temp (40-70F by region). Match unit to both numbers.

Quick Sizing Guide

1-2 people: 30-40 gal tank or small tankless (6-7 GPM gas). 2-3 people: 40-50 gal. 3-4 people: 50-65 gal or mid tankless (8-9 GPM gas). 5+: 65-80 gal or large tankless (10-11 GPM gas). Heat pump: size up one tier.

Comparison

30-40 gal: 1-2 people, small households

40-50 gal: 2-3 people, moderate use

50-65 gal: 3-4 people, most families

65-80 gal: 5+ people or high demand

Tankless: sized by GPM and temperature rise, not gallons

Buying Tips

Use first-hour rating (FHR), not just tank size, to compare.
Cold climates need tankless sized for higher temperature rise.
Heat pump water heaters recover slower -- size up.
When in doubt, size up. Slightly oversized costs marginally more but never runs cold.

Common Mistakes

Sizing by tank gallons alone -- FHR matters more.
Buying tankless sized for warm climate in a cold climate.
Replacing 40-gal with 40-gal without checking if the old one was adequate.
Forgetting simultaneous use -- two showers need 4-5 GPM total.

Bottom Line

Tank: match FHR to peak-hour demand. Tankless: match GPM and temperature rise. Size up when in doubt.

Sizing Your Water Heater