Reading the Label on Your Water Heater

Why this matters

The label on the side of your water heater answers most of the questions a plumber will ask before they come out, and most of the questions you will have when the hot water runs out halfway through the second shower. It says what fuel the heater burns, how much it holds, how fast it heats, and how much hot water it can deliver in a busy hour. It also carries the model and serial numbers that identify your exact unit for parts and warranty.

This card walks through a typical label field by field, then uses one household's morning to show what the numbers mean in practice.

Before you go near it

You only need to look. Reading the label does not require removing any cover or touching any control. Two cautions while you are there:

• If you smell gas near a gas water heater, leave the house and call 911 or your gas utility from outside. Do not read anything first.
• The pipe on the side coming down from a valve near the top is the discharge pipe from the temperature and pressure relief valve. Do not stand where it points or try the lever. If water is dripping from it, call a plumber. If it is spraying or running steadily, turn the heater off first (gas control to OFF, or the breaker for an electric heater), then close the cold water valve above the tank, and leave both off until the plumber has found the cause.

The label, field by field

The rating label is usually a large sticker on the side of the tank, at about eye level or lower. Next to it there is often a yellow federal energy label. The example below is made up; your label will have the same fields in a different layout.

+------------------------------------------------------+
| RESIDENTIAL STORAGE WATER HEATER                     |
| Model: GW-40-T6           Serial: XXXXXXXXXX         |
| Fuel: NATURAL GAS         (not for propane)          |
| Capacity: 40 U.S. gal                                |
| Input: 40,000 Btu/hr      Recovery: 40 gal/hr        |
|                           at 90 F rise               |
| First Hour Rating: 67 gal                            |
| Uniform Energy Factor: 0.64   Draw pattern: Medium   |
| Working pressure: 150 psi   Test pressure: 300 psi   |
| Install with clearances shown in manual.             |
+------------------------------------------------------+

Model. What the heater is. The model number often encodes the capacity and fuel. A plumber uses it to look up the manual and the right replacement parts.

Serial. Which unit you own. Warranty claims are made against the serial number, and on many brands it also carries the manufacture date. See related: Finding Out How Old Your Home Equipment Is.

Fuel. Natural gas, propane (sometimes written LP), or electric. A gas heater built for natural gas cannot simply be run on propane or the other way round; it needs a conversion done by a qualified technician, and the label says which fuel it is set for.

Capacity. The nominal size of the tank in gallons. You do not get all of it as hot water at the tap. As you draw hot water out, cold water comes in at the bottom and mixes, so the water leaving the top cools before the tank is empty. That is why the next field exists.

Input. For gas, the burner's heat input in British thermal units per hour (Btu/hr). For electric, the label lists the elements in watts, often two elements such as 4,500 watts each, with a note that they do not run at the same time. Bigger input means faster heating.

Recovery. How many gallons per hour the heater can heat, at a stated temperature rise. "40 gallons per hour at 90 F rise" means it can take 40 gallons of water up by 90 degrees Fahrenheit in an hour, for example from 50 F incoming water to 140 F. At a smaller rise it heats more gallons, at a larger rise fewer, so compare recovery figures only at the same rise.

First-hour rating. The most useful number on the label for a household. It is how many gallons of hot water the heater can deliver in one hour starting with a full tank of hot water: what is stored, plus what it can reheat during that hour. It is measured under a U.S. Department of Energy test procedure at set temperatures.

Uniform energy factor (UEF) and draw pattern. The UEF is an efficiency rating; higher is more efficient. The draw pattern (very small, low, medium or high) says what size household the test assumed. Compare UEF only between heaters with the same draw pattern.

Working and test pressure. The highest water pressure the tank is built to operate at, and the pressure it was tested to at the factory. Your home's water pressure should be well below the working pressure. See related: The Water Pressure in Your House and the Valve That Controls It.

How the input and the recovery fit together

The two figures are linked, and seeing how lets you trust them. Heating one gallon of water by one degree takes about 8.3 Btu, because a gallon of water weighs about 8.3 pounds and one Btu heats one pound by one degree.

For the example label: heating 40 gallons by 90 F takes 40 x 8.3 x 90, which is about 29,900 Btu. The burner puts in 40,000 Btu in that hour. The difference, about a quarter of the input, goes up the vent with the exhaust. So 40 gallons an hour at a 90 F rise is consistent with a 40,000 Btu/hr burner that gets about three quarters of its heat into the water. Check it from the other side: 40,000 Btu/hr x 0.75 is 30,000 Btu, and 30,000 divided by (8.3 x 90, about 747) is about 40 gallons. Both directions agree.

An electric heater with one 4,500-watt element running puts in about 15,400 Btu per hour (a watt is about 3.41 Btu per hour). Nearly all of that goes into the water, so at a 90 F rise it recovers about 15,400 divided by 747, roughly 20 gallons per hour. That is why electric tanks of the same size usually have a lower first-hour rating than gas tanks: the tank is the same, but they reheat at about half the speed.

Worked example: does this heater cover the morning?

A family of four has the 40-gallon gas heater on the example label, with a first-hour rating of 67 gallons. Their busy hour is 6:30 to 7:30 a.m.: four showers and a few minutes of kitchen use.

Step 1: how much water a shower uses. Their showerheads flow 2.0 gallons per minute. Each shower runs about 8 minutes. That is 2.0 x 8 = 16 gallons of mixed water per shower.

Step 2: how much of that is hot. A shower mixes hot and cold. Say the tank is set to deliver 120 F, the cold water comes in at 50 F, and people shower at about 105 F. The hot share is (105 minus 50) divided by (120 minus 50), which is 55 divided by 70, about 0.79. So each 16-gallon shower draws about 16 x 0.79 = 12.6 gallons of hot water.

Step 3: the whole hour. Four showers at 12.6 gallons is about 50 gallons. Add about 2 gallons at the kitchen sink. Total hot water in the hour: about 52 gallons.

Step 4: compare to the label. 52 gallons needed against a first-hour rating of 67 gallons. The heater covers the hour with about 15 gallons to spare.

What changes the answer.

• Winter. If the incoming water drops to 40 F, the hot share of each shower becomes (105 minus 40) divided by (120 minus 40), 65 divided by 80, about 0.81, so each shower takes about 13 gallons of hot water instead of 12.6. The heater also has to raise the cold water further to reheat it, so it delivers less than its rated first hour. The margin shrinks in winter even if nobody changes a habit.
• A fifth shower. Another 12.6 gallons brings the hour to about 65 gallons against 67. That is right at the limit, and with winter water, the last shower will likely run cool.
• An electric tank instead. Suppose the family had a 50-gallon electric heater whose label read a first-hour rating of 60 gallons. Their 52 gallons would fit with 8 to spare, and the fifth shower would put them at about 65 against 60, short by about 5 gallons. A bigger tank, but a lower first-hour rating, because of the slower recovery above.

What the family takes from it. Their complaint, "the water runs out," is really a mismatch between their busiest hour and the first-hour rating. Spacing showers out, using lower-flow showerheads, or running the dishwasher at night all help. If they replace the heater one day, the number to size it by is the first-hour rating against their busy hour, not the tank size.

The trap. Turning the tank temperature up to stretch the hot water raises the scald risk at every tap, especially for children and older adults, and many plumbers and safety groups suggest about 120 F as a starting point. Anything above that should be paired with anti-scald mixing at the taps, which is a plumber's job. See related: How to Check Your Hot Water Temperature at the Tap.

What to write in your records

From the label, copy or photograph: model, serial, fuel, capacity, input (or element wattage), first-hour rating and UEF. Add the install date from the invoice, the warranty term from the manual, and who installed it. A clear photo of the whole label is the fastest way to give a plumber everything at once. See related: Your House Records File and What Goes In It.

When the label tells you to call

• The label says propane but the house is on natural gas, or the reverse.
• The label or the area around the burner is scorched, sooty or melted.
• You see rust or water at the base of the tank.
• The label is unreadable and you need a replacement part or warranty claim; a plumber can often identify the heater another way.

Any work on the burner, gas valve, venting, elements, wiring or relief valve belongs to a licensed plumber or gas technician.

References

• U.S. Department of Energy consumer guidance on sizing water heaters by first-hour rating and reading the energy label
• Your water heater's installation and owner's manual, for clearances, temperature settings and warranty terms
• See related: Finding Out How Old Your Home Equipment Is
• See related: When to Stop and Call a Licensed Pro Around the House