Quick answer
Water heater first-hour rating (FHR) is the federal test estimate of the most hot water, in gallons, that a storage water heater can deliver in one hour starting from a fully heated tank, and it depends on both the tank’s storage volume and its recovery rate.
Recovery rate is how many gallons per hour the burner or elements can heat by a set temperature rise. FHR is printed on the EnergyGuide label and is the number to compare against your household’s busiest hour.
Quick facts
The key facts about Water Heater First-Hour Rating, with sources:
Key takeaways
- FHR answers the practical question: how much hot water can this tank give in its busiest hour?
- It combines two things: what is already stored, plus what the heater can reheat during that hour.
- Recovery rate is that second part, in gallons per hour at a stated temperature rise.
- Two tanks of the same size can have very different FHRs if one has a bigger burner.
- Size a tank heater by matching FHR to your peak-hour use, not by tank gallons alone.
What first-hour rating measures: stored plus reheated water
A tank water heater starts the morning full of hot water. As showers run, cold water enters and the burner or elements start reheating.
The first-hour rating captures both: the federal test procedure defines it as an estimate of the maximum volume of hot water the heater can supply within an hour that begins fully heated, and says it is a function of both the storage volume and the recovery rate 1.
That is why FHR is often larger than the tank itself. A 40-gallon tank does not stop at 40 gallons; it keeps adding heated water as it goes. One maker’s 40-gallon gas model is rated at 75 gallons in the first hour 4.
FHR applies to storage heaters only. Tankless heaters store no hot water, so they are rated by maximum gallons per minute at a 67°F rise instead 12. Tankless sizing works from flow per minute, not stored volume.
How the federal FHR test works
A plain-language summary of section 5.3.3 of the DOE test procedure 1.
- Set up: Inlet water is held at 58°F, and the thermostat is set to deliver 125°F, plus or minus 5°F 1.
- Start full: The test begins after the tank has fully heated and the burner or elements have shut off 1.
- Draw: Hot water is drawn at 3.0 gallons per minute for tanks of 20 gallons or more, or 1.5 gpm for smaller tanks 1.
- Stop and wait: Each draw ends when the outlet temperature falls 15°F below its peak; the next draw starts when the heater has recovered enough 1.
- Finish at one hour: A final draw happens at the one-hour mark, and the hot water from all draws is added up as the FHR 1.
- Tank fully heatedThermostat satisfied 1
- Draw at 3.0 gpmTanks ≥ 20 gallons 1
- Stop at 15°F dropHeater recovers 1
- Repeat drawsUntil the hour ends
- Sum the gallonsThat total is the FHR 1
Source: [1]
Which FHR bands DOE and the FTC use
The FHR also decides which daily use pattern DOE uses to test a tank’s efficiency, and the FTC uses the same bands to group models on the EnergyGuide label 12.
Example: same tank, bigger burner, more hot water
One maker’s published ratings for four atmospheric-vent gas models show how burner size, not just tank size, drives FHR; the chart below plots each model’s rating 4. The table shows the recovery side, with a check using the formula in the next section at 80% recovery efficiency and a 90°F rise.
| Model (one maker) | Tank and burner input | Listed recovery at 90°F rise | Formula check |
|---|---|---|---|
| RG140T6N | 40 gal, 34,000 BTU/hr | 37 GPH 4 | About 36 GPH |
| RG240T6N | 40 gal, 40,000 BTU/hr | 43 GPH 4 | About 43 GPH |
| RG150T6N | 50 gal, 34,000 BTU/hr | 37 GPH 4 | About 36 GPH |
| RG250T6N | 50 gal, 40,000 BTU/hr | 43 GPH 4 | About 43 GPH |
Recovery follows the burner, not the tank: both 34,000 BTU models recover the same 37 GPH whatever their size.
That is why the 40-gallon tank with the larger burner matches the 50-gallon tank with the smaller burner at 75 gallons in the first hour 4. If space is tight, a smaller tank with faster recovery can deliver the same peak hour.
All four meet the medium or high FHR band, which changes which efficiency target applies 1.
Recovery rate: the reheating half of the FHR
Recovery rate is how many gallons of water a heater can raise by a stated temperature in one hour. Spec sheets usually list it at a 90°F rise, for example from 30°F incoming water to 120°F 4.
It depends on the burner or element input and the recovery efficiency, which federal rules define as the ratio of energy delivered to the water to the energy content of the fuel used 1.
You can check a gas heater’s listed recovery with basic physics. Heating one gallon of water (about 8.33 pounds) by 1°F takes about 8.33 BTU. So recovery in GPH is roughly burner input × recovery efficiency ÷ (8.33 × temperature rise).
For a 40,000 BTU burner at the maker’s stated efficiency of up to 80% and a 90°F rise, that gives about 43 GPH, which is the figure the maker lists 4.
Recovery rate alone is not on the EnergyGuide label; the FHR is 2. Use recovery when comparing spec sheets, or when your hot-water use is spread over several hours.
Recovery (GPH) ≈ Input (BTU/hr) × Recovery efficiency ÷ (8.33 × Temperature rise °F)
Input- Burner rating on the plate, BTU per hour
Recovery efficiency- Share of fuel energy that reaches the water 1
8.33- BTU to heat one gallon of water by 1°F
Temperature rise- Outlet minus incoming temperature, often 90°F on spec sheets 4
Check: 40,000 × 0.80 ÷ (8.33 × 90) ≈ 43 GPH, matching the maker’s listing 4.
Where to find the FHR on a label or spec sheet
Look at the yellow EnergyGuide label on a new heater. Federal labeling rules require storage water heaters to show capacity as both rated storage volume (“tank size”) and first hour rating 2. The same label groups models by FHR band when it compares operating costs 2.
On an installed heater, the label may be gone. Find the model number on the rating plate and look it up on the maker’s spec sheet, which lists FHR and recovery in a table 4. Contractors’ quotes often list it too; if a quote shows only tank gallons, ask for the FHR.
Why FHR is the better sizing number
What FHR does well
When the rating and real life differ
The test uses 58°F inlet water and a delivery setting of about 125°F 1. In a cold climate, winter groundwater can be colder, so the heater has a bigger temperature rise to cover and delivers less hot water than the label suggests. In warm climates it can do better.
Your thermostat setting matters too. Setting the tank lower to cut scald risk, as CPSC urges at 120°F 5, changes how far stored water stretches when mixed at the tap. And a tank with heavy sediment on its bottom may fall short of its original rating 6.
How a higher FHR changes what you pay
We do not print prices, because fair numbers need dated local quotes; see what drives plumbing prices.
In general, a higher FHR comes from a bigger tank, a bigger burner or element, or both, and each of those can raise the price of the heater and sometimes the venting, gas line or electrical work needed to install it.
Undersizing has a cost too: cold showers, or replacing a heater early. Oversizing means paying to keep more water hot. FHR band also decides the efficiency target a model is tested against 1, so compare tank models within the same band. Our water heater sizing tool helps estimate the FHR you need.
ENERGY STAR requires at least a 6-year warranty on certified gas storage heaters 3; check the maker’s warranty for lifespan terms.
How to use FHR to size a tank heater
The common method is to estimate your peak hour of hot water use and match it to FHR. Installing the heater is licensed work in most places, with a permit, so check the plumber’s license first.
- Pick your busiest hour, often the morning, and list everything that uses hot water in that hour: showers, a dishwasher, a washer.
- Estimate gallons for each use; your fixtures’ flow rates and run times are the inputs (our sizing tool walks through it).
- Add them up. That total is the minimum FHR to look for.
- Compare models by FHR on the EnergyGuide label, not tank gallons 2.
- In a cold-water climate, leave extra margin, because the rating assumes 58°F inlet water 1.
- Have a licensed plumber confirm venting, gas or electrical capacity for the model you choose.
Keeping the heater close to its rating
- Drain and flush the tank as the manual says; A.O. Smith suggests a first flush at six months and at least yearly after 6.
- Inspect the anode rod at the same time 6.
- Keep the thermostat at a safe setting; CPSC urges 120°F 5.
- On electric tanks, a failed lower element cuts recovery sharply; have it checked if hot water suddenly runs short.
Signs your FHR is too low or falling
- Hot water runs out partway through the morning routine.
- Back-to-back showers turn lukewarm.
- Less hot water than the heater used to give, which A.O. Smith links to sediment on the tank bottom 6.
- Long waits for the tank to recover between uses.
Do not raise the thermostat to stretch hot water
Turning the tank up to make hot water last longer raises scald risk. CPSC says most adults suffer third-degree burns in six seconds at 140°F and in thirty seconds at 130°F 5. If you need more hot water, a larger FHR or a mixing valve installed by a plumber is safer.
If a heater that used to keep up no longer does, have it checked before replacing it. Our water heater repair and replacement page covers the service.
The federal rules behind the number
Test procedure: DOE’s Uniform Test Method for Measuring the Energy Consumption of Water Heaters, 10 CFR Part 430, Subpart B, Appendix E, defines first-hour rating and recovery efficiency and sets the test conditions 1.
Label: the FTC’s Energy Labeling Rule, 16 CFR 305.17, requires storage water heaters to show both rated storage volume and first hour rating on the EnergyGuide label, and groups cost ranges by FHR band 2.
ENERGY STAR: certified gas storage heaters need an FHR of at least 51 gallons per hour; certified heat pump water heaters need at least 45 3.
Local codes govern installation, not FHR. Rules checked 2026-10-13.
How FHR differs from recovery, UEF and max GPM
| Rating | What it measures | Used for |
|---|---|---|
| First-hour rating (FHR) | Gallons of hot water in the first hour from full 1 | Sizing a tank heater |
| Recovery rate | Gallons heated per hour at a set rise 4 | Comparing burners and elements |
| Rated storage volume | Tank capacity as certified 2 | Physical size, label |
| Max GPM | Tankless flow at a 67°F rise 1 | Sizing a tankless heater |
| Uniform Energy Factor (UEF) | Overall efficiency 1 | Energy use and ENERGY STAR 3 |
FHR myths about tank size and tankless heaters
- Myth A 50-gallon tank gives 50 gallons of hot water.
- Reality It gives more in the first hour, because it reheats as you draw 14.
- Myth Bigger tank always means more hot water.
- Reality A smaller tank with a bigger burner can match it 4.
- Myth FHR tells you efficiency.
- Reality Efficiency is the UEF, a separate number 1.
- Myth Recovery rate and FHR are the same thing.
- Reality Recovery is one input to FHR; storage is the other 1.
Cold incoming water changes the math
Incoming water temperature is the regional variable. The federal test assumes 58°F inlet water 1.
Where groundwater runs colder, as in northern states in winter, the heater must lift water further and will deliver fewer hot gallons per hour than its rating; in the South it can beat it.
Use the recovery formula above with your own temperature rise, or ask your plumber to size for winter conditions.
What FHR your household should look for
Match the FHR to your busiest real hour, then add margin for winter inlet water. If space limits tank size, look for faster recovery. If your peak lasts several hours, weigh recovery rate as heavily as FHR.
Next step: write down your morning hot-water routine, then compare three models’ FHR on their EnergyGuide labels.
Ask for the FHR on every quote
Quotes often list only tank gallons. The EnergyGuide label must show FHR for storage heaters 2, so ask for that figure for each option and compare like with like.
Questions about Water Heater First-Hour Rating
What is a good first-hour rating for a water heater?
One that covers your household’s busiest hour of hot water use, with some margin. The FTC groups ratings into very small (under 18 gallons), low (18 to 50.9), medium (51 to 74.9) and high (75 or more). ENERGY STAR gas tanks need at least 51 gallons.
How is first-hour rating calculated?
It is measured, not calculated from tank size. Under DOE’s test, a fully heated tank is drawn at 3.0 gallons per minute with 58°F inlet water. Draws stop when the outlet drops 15°F and restart after recovery. The hot water delivered in one hour is the FHR.
What is water heater recovery rate?
Recovery rate is how many gallons a heater can warm by a set temperature in one hour, usually listed at a 90°F rise. It depends on burner or element size and efficiency. One maker’s 40,000 BTU gas models recover 43 gallons per hour at a 90°F rise.
What is the difference between recovery rate and first-hour rating?
Recovery rate is only the reheating speed. First-hour rating adds the hot water already stored in the tank to what the heater can reheat during that hour. Federal rules describe FHR as a function of both storage volume and recovery rate.
Why is the first-hour rating higher than the tank size?
Because the heater keeps heating while you use water. A 40-gallon tank starts with stored hot water and adds more as the burner runs. One maker’s 40-gallon gas model with a 40,000 BTU burner is rated at 75 gallons in the first hour.
Where do I find the first-hour rating?
On the yellow EnergyGuide label, which must list both tank size and first hour rating for storage water heaters. If the label is gone, take the model number from the rating plate and find the maker’s spec sheet, which lists FHR and recovery rate.
Do tankless water heaters have a first-hour rating?
No. Tankless heaters do not store water, so they are rated by maximum gallons per minute at a 67°F temperature rise. The EnergyGuide label for a tankless model shows flow rate instead of a first-hour rating.
Does cold weather change my water heater’s first-hour rating?
The rating does not change, but real output does. The test assumes 58°F incoming water. When winter water is colder, the heater has to raise it more, so it delivers fewer hot gallons in an hour than the label shows.
Does a heat pump water heater have a lower first-hour rating?
It can, because a heat pump heats water more slowly than a gas burner. ENERGY STAR sets the minimum FHR for certified heat pump water heaters at 45 gallons, compared with 51 for gas storage models. Compare FHR on the label when switching types.
Sources
- 10 CFR Part 430, Subpart B, Appendix E, Uniform Test Method for Measuring the Energy Consumption of Water Heaters (via Cornell LII), retrieved .
- 16 CFR 305.17, Energy Labeling Rule: labeling for water heaters and other products (via Cornell LII), retrieved .
- ENERGY STAR, Water Heater Key Product Criteria, retrieved .
- Bradford White, Residential Atmospheric Vent Gas Water Heater specification sheet 1101-E-1020 (distributor copy), retrieved .
- U.S. Consumer Product Safety Commission, Avoiding Tap Water Scalds (Document 5098), retrieved .
- A.O. Smith, Residential Gas Water Heater Installation Instructions and Use & Care Guide (Lowe’s edition, 100283020), retrieved .
Expert review
Written by the HyrePlumber Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: FHR and recovery efficiency definitions, test conditions (58°F inlet, 125°F ±5°F delivery, 3.0 and 1.5 gpm draws, 15°F cut-off, one-hour end) and draw-pattern bands are from the current 10 CFR 430 Appendix E on Cornell LII.
Label requirements and FHR cost bands from 16 CFR 305.17. ENERGY STAR minimums and warranty from energystar.gov. Model FHR and recovery figures from one Bradford White spec sheet (single maker, labeled).
The recovery formula uses the standard 8.33 BTU per gallon per °F and was checked against that maker’s listing. Scald data from CPSC; flush interval from A.O. Smith. No prices.
Suggest a correction. We fix errors and say what changed.