Is the date on the plate the installation date?

No. A date printed on the plate, or coded in the serial, is when the unit was made. Installation can follow months later, so check the invoice or the permit for the installation date.

The serial is worn and I cannot read one character. What can I do?

Photograph the plate in raking light, from the side, which often shows embossed or faded characters. Some units also carry the serial on a second label or on the carton paperwork. Do not guess a character: one wrong letter can change the decade.

Which plate should I read when the furnace and the air conditioner were installed together?

Each unit has its own plate and its own date. The outdoor unit, the furnace or air handler and the indoor coil can all be of different ages, so read each one you care about.

Where to find the data plate and what it tells you

Every water heater, air conditioner, heat pump and furnace sold in the United States carries a rating plate: a metal or printed label with the model number, the serial number and the ratings the unit was built and tested to. It is the one document that stays with the equipment for its whole life, which makes it the best place to start when you want to know how old a unit is or what size it is.

Water heaters

On a tank water heater the plate is a label on the outer jacket of the tank. It names the model, the serial number, the capacity in gallons and the gas input or the electrical rating. Bradford White and Rheem both publish pictures of where the label sits on each of their heater types. On tankless heaters the label is on the cabinet.

You never need to open the burner compartment or an element cover to read the serial. If insulation or a blanket covers the label, fold it back carefully rather than cutting it.

Outdoor air conditioner and heat pump units

The condensing unit outside carries its plate on the cabinet. American Standard publishes an instruction sheet that shows where the label is on its equipment; check the side and back panels of other brands before opening anything. Besides the model and the serial, the plate states the refrigerant and its factory charge, which is the reliable answer to the question of whether a system uses R-22, R-410A or a newer refrigerant.

Furnaces and air handlers

Indoor units carry the plate on the casing or just inside the access door. If you can only see it with a door off, switch the unit off first and leave sealed electrical boxes, the burner and the gas valve alone. A control board inside a furnace may have its own small date label; that date belongs to the board, not to the furnace.

What the plate lists

Model number: the product line and size; for cooling equipment it usually contains the nominal capacity.
Serial number: unique to the unit; most brands code the date of manufacture into it.
Ratings: gallons and input for water heaters, voltage and current for electrical parts, heating input for furnaces.
Refrigerant: on air conditioners and heat pumps, the refrigerant type and the factory charge.
Date line: some plates print a manufacture date outright; when they do, it overrides any decoded reading.

Copying the serial correctly

Take a sharp photo of the whole plate, then zoom in on the serial.
Copy every letter and digit in order; spaces and dashes do not matter to the decoder.
Check the characters that look alike: the letter O and the digit 0, the letter I and the digit 1, the letter S and the digit 5.
Note the brand printed on the plate, which may differ from the brand on the front of the cabinet.

Then choose the family on the decoder, or browse the water heater and HVAC pages.

Page updated October 27, 2026. Decoding rules last checked against their sources on the date shown with each source.