Go to the old equipment before anyone disconnects it. Find the nameplate. It is usually a metal or printed tag on a side panel, the back, or the inside of an access door. You are looking for the model, the serial, and the rating line. Photograph each one flat and close enough to read every character.
Read the tag from outside the cabinet. Do not open panels that are not meant for you. Write the same fields in a notebook or on the folder cover while the unit is still in place. Once the cabinet is carried out, the tag is harder to reach and sometimes gone.
Decode the Install Year
A serial number often carries the build period somewhere in its characters. The format differs by manufacturer and has changed over the years, so do not guess from memory. Ask the installer or the manufacturer which part of the serial holds the build code, and ask them to confirm the year in writing. Write that year beside the serial.
That year is the starting point for the planning window in the matching system guide. If the old unit has no tag left and no install record exists, say so in your notes. A planning window built on an uncertain year should be treated as a rough one.
Match the Plate to the Quote
Put the old nameplate next to the replacement quote. Check the capacity line, the voltage line, the fuel type, and the equipment category. If the quote lists a different size or a different fuel, ask why in writing before you sign. A change in size or fuel changes the install scope, the permit, and the work on the rest of the house.
Repeat the same photographs on the new unit when it arrives. Compare its model and serial to the delivery ticket and to the signed order. If they do not agree, note the difference before the crew leaves the site, not after.
Carry the Record Forward
Put the old and new plates in the same folder. Add the installer's name, the permit reference, and the startup day. Keep the photographs on your phone and in the folder too. The next service visit will start with the model and serial, and a technician who has them ready can begin work without a call-back.
Keep the startup readings the installer records, and keep the operating notes that come with the unit. These pages belong with the nameplate photographs, not in a separate drawer.
Use the Record for Planning
The install year is an input, not a verdict. Take it to the matching guide, such as Gas Furnace: typical service life and when to plan, and read the window it produces. Use that window to decide when to start comparing options and when to set money aside. It does not tell you when a unit will fail, and it is not a reason to replace equipment that is still doing its job.
When the next walk-through comes around, bring the folder. Compare the nameplate, the install year, and the planning window against what you see in the room. Note anything that does not match, and settle the difference with the installer or the technician before you decide anything else.