Short definition
The first-hour rating (FHR) is the gallons of hot water a tank water heater can deliver in the first hour, starting from a full tank. It combines stored hot water with the heater’s recovery during that hour — a more useful sizing number than tank capacity alone.
What it is
Tank capacity tells you how much hot water is sitting in the tank right now. FHR tells you how much usable hot water you’ll actually have during peak demand. A 40-gallon heater with strong recovery can deliver more first-hour hot water than a 50-gallon heater with weak recovery.
The federal EnergyGuide label (FTC Appliance Labeling Rule, 16 CFR Part 305) groups FHR into four categories:
- Small: under 18 gallons
- Low: 18–50 gallons
- Medium: 51–75 gallons
- High: above 75 gallons
To size for your household, estimate peak-hour hot-water demand: showers (10 gallons each), dishwasher (6 gallons), washing machine (15–25 gallons), and pick a heater whose FHR meets that peak. A family of four with one shower at a time, a dishwasher, and laundry on weekends typically wants 50–70-gallon FHR.
Why it matters to a homeowner
When you ask “why are we running out of hot water?” the answer is almost always: FHR isn’t matched to your peak-hour demand. Two simultaneous showers plus a dishwasher start can easily exceed 30 gallons in 20 minutes, and a 40-gallon tank with weak recovery can’t keep up.
The fix is one of three things: bigger tank (higher FHR), faster-recovery heater (gas or HPWH instead of resistance electric), or staggered demand (one shower at a time). When a plumber’s quote pitches a 65-gallon replacement instead of a 50-gallon, FHR is usually the reason — they’re sizing for your actual usage, not the existing nameplate.
Common variants and what FHR is not
- FHR vs. tank capacity. Capacity is static gallons. FHR is delivered gallons in the first hour, including recovery.
- FHR vs. recovery rate. Recovery is gallons-per-hour at steady state. FHR is the first-hour combined output.
- FHR (storage) vs. GPM rating (tankless). Tankless heaters have no FHR — they’re rated in gallons per minute at a given temperature rise. Different sizing math entirely.