How to read the rating plate on your water heater, furnace or AC

By Dana Whitlock. Updated October 13, 2026. How we work.

One label tells you what you own

Somewhere on your water heater, furnace, air conditioner, heat pump or boiler is a factory label with the model number, the serial number and the electrical and fuel ratings. Every unit sold in the US has one. Installers call it the rating plate, data plate or nameplate. It is the best record of what's in your basement, and it's the first thing a contractor, a warranty department or our serial decoder asks for.

Reading it takes a flashlight, a phone camera and a few minutes. No tools. Nothing on this page involves opening a cabinet that is held shut with screws.

Where to find it on each type of equipment

Each product type keeps its plate in roughly the same spot, though brands have their habits. Start below, and look for a metal or foil label covered in a dense block of small print.

• Tank water heaters: on the front of the tank jacket, usually close to the gas control or thermostat cover. A. O. Smith's recall notice describes its data plate as located next to the gas control valve/thermostat.
• Tankless water heaters: on the side or front of the cabinet. Many makers also print the serial on the shipping box.
• Gas furnaces: often on the inside of the blower compartment door, or on a white label on the left or right side of the cabinet. Trane's recall notice says its model and serial numbers are printed on a white label on the left or right-hand side of the unit.
• Outdoor AC condensers and heat pumps: on the cabinet, often on the back near where the copper refrigerant lines go in. Goodman says its plate is generally found slightly above the refrigerant valves on the back side of the unit.
• Boilers: on the jacket. Some brands add a second tag. Weil-McLain, for example, uses a separate CP number that its online lookup turns into a manufacture date.
• Indoor coils and air handlers: their own plate on the coil cabinet or air handler. Write it down too. An AC or heat pump is rated as a matched pair.

What each field means

Layouts vary. The same handful of fields shows up on nearly every plate, and each one answers a question a contractor will ask.

• Model number: product family, size and options packed into one code. Parts, ratings and matching equipment are all looked up by it.
• Serial number: this one unit. Most brands tuck the manufacture year and month somewhere inside it, which is how you date the equipment.
• Capacity: gallons for a tank water heater, input in BTU per hour for furnaces and boilers, and cooling capacity in BTU per hour or tons for AC and heat pumps (12,000 BTU per hour equals one ton).
• Voltage, phase and amps: the electrical supply the unit expects. Outdoor units list minimum circuit ampacity and maximum fuse or breaker size, which an electrician uses to check the circuit.
• Refrigerant type and factory charge: shown on AC, heat pump and some packaged units as a refrigerant name (R-22, R-410A, R-454B, R-32) and a weight in pounds and ounces.
• Fuel and gas pressure: natural gas or propane, plus input rating and manifold pressure on gas appliances. A unit set up for one fuel won't run on the other without a factory conversion kit.
• Efficiency rating: some plates print UEF (water heaters), AFUE (furnaces and boilers) or SEER2 and HSPF2 (AC and heat pumps). Many skip it. Then the figure comes from the model lookup or the yellow EnergyGuide label.
• Manufacture date: some brands print it outright, such as MFD followed by month and year. When it's there, it beats any decoding.

How model numbers encode size

No industry-wide standard governs model numbers. Most brands still follow a similar pattern: letters for the product family and fuel, a group of digits for size, then letters or digits for voltage, design revision and options. Spot the size digits and you can answer the first thing everyone asks, which is how big the unit is.

Trane's 2026 product handbook makes a clean example because it publishes the full breakdown. Its outdoor AC and heat pump model numbers have 15 positions. In the printed example 5TTR5024A1000AA, the first digit (5) means R-454B refrigerant, positions 7 and 8 (24) mean a nominal 2.0 tons, and position 10 (1) means 208 to 230 volt single phase. The handbook maps 36 to 3.0 tons, 48 to 4.0 tons and 60 to 5.0 tons.

The same handbook's furnace codes give heating input in thousands of BTU per hour. In the printed example TUHMB080CCV3VAA, positions 6 through 8 (080) mean 80,000 BTU per hour input, and V3 near the end means the blower is sized for 3 tons of cooling. In the S-series example S8B1B080M4PSCCA, the 8 in position 2 is the 80 percent AFUE code.

Other brands use their own positions. Look for a two or three digit group that matches a standard size: 018, 024, 030, 036, 042, 048 or 060 on AC and heat pumps (thousands of BTU per hour), 040 through 140 on furnaces, and 30, 40, 50, 75 or 80 on tank water heaters (gallons). That's a strong hint. Confirm it against the capacity field elsewhere on the same plate or the maker's spec sheet, since a few brands encode size with letters and the digit habit is only a habit.

Taking a photo you can read later

One good photo saves a second trip into the attic. It also lets you send the exact model and serial to a contractor or warranty line without retyping a long string of letters and digits.

• Turn on the phone flashlight and shoot at a slight angle so foil labels don't throw glare back.
• Take one wide shot showing where the plate sits on the unit, then one close shot where every character is sharp.
• Photograph every plate: outdoor unit, indoor coil or air handler, and furnace. They are often different ages.
• On a faded label, try raking light from the side. Embossed or heat-printed characters stand out.
• Type the model and serial into a note right away. O and 0, or I and 1, blur together later.

Stay safe while you look

Read only what you can see from outside, or behind a door that lifts off without tools. Furnace blower doors often have a safety switch that cuts power when removed. The wiring behind it can still be live, and the burner area gets hot. A label behind a screwed-on cover can wait for a licensed HVAC technician at the next service visit.

Don't reach into an outdoor condenser or pull its electrical service cover. Keep your hands off the gas control on a water heater. Smell gas at any point and you leave the house, then call the gas utility from outside.