Model numbers shared by furnaces or boilers indicate which parts the system uses. Since different models of furnaces and boilers often look similar, experts rely on model numbers to ensure that replacement parts are compatible with the system.
Different manufacturers use model numbers in various ways, but model numbers often show which year the unit was designed or manufactured. They are often located near the service panel, which makes them easy for experts to find. Model numbers are sometimes engraved on metal to keep them from fading over time, although it is worth writing down the model number and storing it. If experts can't find the model number of a unit that needs repair, it might take some detective work to determine which parts the unit uses.