How old is my Ducane furnace or air conditioner?

By Dana Whitlock. Updated October 2026. Widely documented.

Check the fifth character of a Ducane serial before anything else. A letter there means the newer Allied Air layout, where digits three and four are the year and the letter is the month. Ten digits with no letters means the older Ducane layout, and the last four digits hold the year and week.

Anything older than both styles, or a label that fits neither, is outside what the tool reads. The printed date or the installation paperwork will do the job there.

Where to find the label on a Ducane furnace

Ducane furnaces keep the rating plate inside the cabinet behind the access door. Turn the furnace off before you open it. On Ducane outdoor units, check the side of the cabinet near the electrical panel. Ducane condensers from the 1990s can be weathered badly; if the plate is unreadable, the indoor furnace or coil may have a cleaner plate, and the two were often installed together.

How to read the code

Newer Ducane (the Lennox family layout): two digits for plant, two for year, a month letter from A to M with I skipped, then five digits. Older Ducane: ten numbers where only the last four matter. Digits seven and eight are the year and digits nine and ten are the week. A Ducane email quoted by InspectAPedia described the pattern as XXXXXX9933, meaning week 33 of 1999. The tool reads that older style for units built roughly 1990 to 2004.

Layout	What it means	Years used
PP YY M #####	Plant, year, month letter	1999–2026
###### YY WW	Older Ducane: year and week are the last four digits	1990–2004

When the format changed

Lennox bought the heating and air conditioning division of The Ducane Company in October 1999, and Ducane serials moved to the Allied Air style after that. The exact changeover date was never published, so a unit from 1999 to the early 2000s could carry either style. Because the two are easy to tell apart by whether a letter appears in position five, the overlap rarely causes a wrong answer. If a numeric serial produces a week above 53, it is not the old Ducane format; recheck the plate or look for a printed date.

Who makes Ducane

Ducane is part of Allied Air Enterprises, the Lennox International division that also sells Armstrong Air, AirEase, Concord and Magic-Pak. Lennox's filings describe these as two-step brands, sold to distributors who then sell to local installing dealers. Allied Air consolidated its manufacturing in South Carolina starting in 2006.

How sure is this?

Widely documented. Ducane doesn't publish this code; it comes from recall notices, service literature and inspector references. We tested the lennox allied rules against 14 serials, 7 of them with a date known from a recall notice, a printed label date or the maker. None decoded to a wrong year; codes whose year mark repeats show every possible reading. Always confirm with Ducane's serial lookup before you spend money on the answer.

Quick notes

Position five decides it. A letter means the newer format, a digit means the older numeric one.
Older numeric Ducane serials only use the last four digits for the date; ignore the first six.
Allied Air runs the extended warranty lookup for its brands; have the full serial ready.
If the outdoor plate is unreadable, date the indoor furnace or coil instead and note that it is an estimate for the pair.

Next steps for a gas furnace

Common questions

Who makes Ducane furnaces?

Allied Air Enterprises, part of Lennox International since Lennox bought Ducane's heating and cooling division in 1999.

My Ducane serial is all numbers. How do I read it?

Use the last four digits: the first two of those are the year and the last two are the week.

Why won't my old Ducane serial decode?

It may predate both documented formats or be a component label. Look for a printed date or the original invoice.

Is Ducane related to Armstrong Air?

Yes. Both are Allied Air brands, and newer Ducane serials use the same layout as Armstrong.