First hour rating
First Hour Rating of a 66 Gallon Gas Water Heater
Researched from the DOE test procedures, NFPA 54 and NEC code tables, and manufacturer specifications. Updated .
Quick answer
First hour rating is the gallons of hot water a heater delivers in one hour starting from full. It is the usable part of the storage plus one hour of recovery, and it is the number that decides whether a household runs out.
For a 66 gallon gas tank that works out near 85 gallons. Gas tanks carry a high first hour rating relative to their size because a burner recovers roughly twice what a standard element does.
Where the 85 gallon figure comes from
| Component | Figure | Basis |
|---|---|---|
| Tank capacity | 66 gal | Nameplate |
| Usable fraction | 70% | Planning convention |
| Usable storage | 46.2 gal | Computed |
| Recovery at a 90 F rise | 38.4 gal per hour | Computed from the input rating |
| Estimated first hour rating | 85 gal | Storage plus recovery |
Convention Source: Usable fraction is a planning convention. The recovery figure is computed from the published energy relation. The published DOE first hour rating is a MEASURED test result. This is an estimate of it, for the case where the label is missing or unreadable.
How the temperature rise changes it
Recovery rate falls as the temperature rise climbs, because the same energy has to lift each gallon further. A tank in a cold inlet region has a genuinely lower first hour rating than the same tank in a warm one.
| Temperature rise | Recovery | Estimated first hour rating |
|---|---|---|
| 60 F | 57.6 GPH | 104 gal |
| 70 F | 49.4 GPH | 96 gal |
| 80 F | 43.2 GPH | 89 gal |
| 90 F | 38.4 GPH | 85 gal |
| 100 F | 34.6 GPH | 81 gal |
Published standard Source: Energy relation Q = m x c x dT, with the usable fraction convention applied to the storage term. The DOE test procedure uses a defined rise, so a label figure is not directly comparable to a row here at a different rise.
Is 85 gallons enough?
Compare it against your peak hour demand rather than against your daily use. A four person, two bathroom household uses about 54 gallons in its busiest hour, so a first hour rating of 85 covers it with room to spare.
Work out your own with the peak hour demand calculator, then check the tank against the tank size pages for your household.
What raises a first hour rating without a bigger tank
Three things, in order of how much they help. A higher input burner raises recovery directly, within what the gas line can supply. A thermostatic mixing valve lets the tank store hotter and deliver at a safe temperature, which increases the usable gallons out of the same storage. And reducing the peak itself, by moving an appliance out of the busy hour or fitting a lower flow shower head, works on the other side of the equation.
Gas tanks and the parts that raise usable output
GE RealMAX Choice 40 Gallon Natural Gas
$1,051.49An atmospheric tank vents on buoyancy alone through an existing flue, which is why it is the cheapest gas tank to install and the one most exposed to backdrafting.
Best for: A like-for-like swap into an existing masonry or B-vent flue
Check price on Amazon
Rheem PROG50-36P 50 Gallon Propane
$1,207.00A 36,000 BTU propane atmospheric tank. The input figure is what sets the recovery rate, and 36,000 BTU is the usual figure on a residential 50 gallon.
Best for: Propane properties replacing an atmospheric tank
Check price on Amazon
Cash Acme 3/4 in Tank Booster with TMV
$145.00A tank booster mixes cold back into the outlet, so a tank stored at 140 F delivers at 120 F and yields more usable hot water from the same gallons.
Best for: Getting more usable hot water without a bigger tank
Check price on AmazonFrequently asked questions
- What is the first hour rating of a 66 gallon gas water heater?
- About 85 gallons with a 36,000 BTU burner at a 90 F rise: 46.2 gallons of usable storage plus 38.4 gallons of recovery during the hour. The exact figure depends on the input rating of the specific model and on your temperature rise, and the measured DOE figure is printed on the yellow EnergyGuide label.
- Why is first hour rating higher than the tank capacity?
- Because the heater keeps working while you draw. The burner or element runs the whole hour, reheating incoming cold water as it arrives, so the total delivered exceeds what the tank holds. This is why a gas tank with a fast burner can show a first hour rating well above its nameplate gallons.
- Is the first hour rating on the label the same as this estimate?
- No, and where the label figure exists it governs. The DOE first hour rating is measured under a defined test procedure with a defined rise and draw pattern. This page estimates it from the tank size and the input rating for the case where the label is missing, painted over or unreadable, which is common on an older heater.
- Would a larger tank help more than a faster burner?
- It depends on the shape of your demand. A larger tank helps a household with one big concentrated draw, because storage covers it. A faster recovery rate helps a household with repeated draws through the morning, because the heater catches up between them. Counting the peak hour tells you which shape you have.
- Does storing water hotter increase the first hour rating?
- It increases the usable gallons, which is part of the same thing. Storing at 140 F and mixing down to 120 F at a thermostatic mixing valve yields more usable hot water from the same tank than storing at 120 F does. That comes with a scald consideration, which is why the mixing valve is the required part of the arrangement rather than an optional one.
Gas and flue work belongs to a licensed plumber or gas fitter. An undersized gas line starves a burner, and a starved burner produces carbon monoxide. Backdrafting on an atmospheric vent is real, it is invisible, and it kills. Nothing on this page is a substitute for the sizing done by the person who signs the permit, and a working carbon monoxide alarm belongs on every floor of a house with a gas appliance.