--- title: "Lennox Furnace Age: How to Read a Lennox Serial Number" description: "Find the year and month your Lennox furnace was built from its serial number, where the label sits inside the cabinet, and how to read its model number." canonical: "https://whatyearis.com/lennox-furnace" markdown: "https://whatyearis.com/lennox-furnace.md" last_updated: "2026-10-19" --- # How old is my Lennox furnace? On a Lennox furnace, the third and fourth digits of the serial number are the year it was built, and the letter after them is the month: A for January through M for December, skipping I. A serial that starts 5921J was built in September 2021 at Lennox’s Marshalltown heating plant. The rating plate is inside the furnace, behind the top front panel, on the left side of the cabinet or on top of the blower deck. By [the What Year Is team](https://whatyearis.com/editorial) · Updated October 19, 2026 ## How do you read a Lennox serial number? Lennox uses one serial layout across its products: a two-digit plant code, a two-digit year, a month letter, then a five or six digit unit number. Dating an air conditioner or heat pump instead? See [how old is my Lennox AC](https://whatyearis.com/lennox-ac). Sources: [Model Numbering Systems: Lennox model and serial nomenclature](https://www.lennox.com/lib/legacy-res/pdfs/lennox_model_and_serial_nomenclature.pdf), checked October 19, 2026; [Model Number Identification (Current Product): Furnaces, All R-454B and R-32 Products, Boilers](https://www.lennox.com/dA/d1bf319d00/ehb_model_id_r-454b-r32_2503.pdf), checked October 19, 2026. | Characters | What they mean | In 5921J00456 | | --- | --- | --- | | 1 and 2 | Plant code | 59: Marshalltown, Iowa (heating) | | 3 and 4 | Year, two digits | 21: 2021 | | 5 | Month letter (A to M, no I) | J: September | | 6 onward | Unit number, five or six digits | 00456 | ## What will the home systems age report add? Your furnace is one line in the home systems age report. It will sit beside your water heater, air conditioner and the rest, each with a build year, a typical life for that kind of system, and a replacement budget, so you can plan the heating side of the house alongside everything else. ## More about Lennox furnace serial numbers - [How old is my Lennox air conditioner or heat pump?](https://whatyearis.com/lennox-ac): Where the sticker is on the outdoor unit, and how a Lennox model number tells an air conditioner from a heat pump. - [How long does a furnace last?](https://whatyearis.com/how-long-does-a-furnace-last): Typical furnace life across brands, from manufacturers and the Department of Energy. ## Where is the serial number on a Lennox furnace? Lennox puts the rating plate inside the furnace, behind the top front service panel, on the left side of the cabinet or on top of the blower deck. Which one depends on the furnace model and how it’s installed. A technician can read it for you at the next service visit if you’d rather not turn the power off and remove the panel. ## How can you tell a Lennox furnace model number? A current Lennox furnace model number lists the furnace’s efficiency and heat output. Lennox’s own sample, `SLP99UH070XV36BK`, reads like this: | Characters | What they mean | In SLP99UH070XV36BK | |---|---|---| | SL, EL or ML | Series: Dave Lennox Signature, Elite or Merit | SL: Signature | | 1, 2 or P | Heating stages: one, two, or variable | P: variable | | Two digits | Efficiency (AFUE) | 99: 99% | | UH or DF | Upflow/horizontal or downflow | UH: upflow/horizontal | | Three digits | Gas heat input | 070: 66,000 Btuh | Older Lennox gas furnaces start with G instead. In Lennox’s older guide, `G14Q3-100` is a series 14 gas furnace with 100,000 Btuh input. ## What do some Lennox furnace serial numbers decode to? The first row is the sample printed in Lennox’s own guide. The others are made up to show the rule and don’t belong to real units. | Serial | Plant | Year | Month | Built | |---|---|---|---|---| | `5178M99989` (Lennox’s sample) | 51, Columbus | 1978 | M | December 1978 | | `5921J00456` | 59, Marshalltown, Iowa (heating) | 2021 | J | September 2021 | | `5814B04321` | 58, Marshalltown, Iowa | 2014 | B | February 2014 | | `5808G12345` | 58, Marshalltown, Iowa | 2008 | G | July 2008 | Lennox’s plant list has changed over the years, so the plant name matters less than the year and month. ## How long do Lennox furnaces last? Lennox says furnaces typically last 15 to 20 years with proper care. That’s a typical range, not a promise for any one furnace: installation, maintenance and how hard the furnace works all play a part. ## Common questions about Lennox furnace serial numbers ### What do the Lennox month letters mean? A is January, B February, C March, D April, E May, F June, G July, H August, J September, K October, L November and M December. Lennox skips the letter I. ### What if my Lennox furnace serial starts with a letter? Lennox’s warranty page lists two other serial shapes that start with letters, and the Lennox documents we checked don’t explain them. Lennox’s customer support can date those furnaces from the model and serial number. ### Is a Lennox year of 08 from 1908 or 2008? The year has only two digits, so read it in context. A furnace with 08 in the year position was built in 2008. Lennox’s older guide uses 78, for 1978, as its sample. ### Why can’t I find the label where Lennox says it is? Lennox says the spot depends on whether the furnace is an 80% or 90% AFUE model, and on how it’s installed: upflow, downflow or horizontal. Check both the left side of the cabinet and the top of the blower deck. ## What will the home systems age report cost? The home systems age report will cost $19 per home. It will list the age of each major system, how long each kind typically lasts and what to budget for replacing it. The report isn’t open yet. Join the waitlist and we’ll email you the day it opens. ## Sources - [Model Numbering Systems: Lennox model and serial nomenclature](https://www.lennox.com/lib/legacy-res/pdfs/lennox_model_and_serial_nomenclature.pdf), Lennox Industries Inc.. All Lennox product uses one serial coding system (plant, year, month letter, serial number, sample 5178M 99989, months A to M without I); on residential gas and oil furnaces the plate is inside the cabinet near the vestibule panel; older gas furnace models start with G, as in G14Q3-100.. Checked October 19, 2026. - [Model Number Identification (Current Product): Furnaces, All R-454B and R-32 Products, Boilers](https://www.lennox.com/dA/d1bf319d00/ehb_model_id_r-454b-r32_2503.pdf), Lennox Industries Inc.. Current furnace model numbers (sample SLP99UH070XV36BK): series SL, EL or ML, heating stages 1, 2 or P, AFUE (80 to 99), UH upflow/horizontal or DF downflow, and gas input (070 = 66,000 Btuh). Serial plant codes include 58 Marshalltown IA and 59 Marshalltown IA (Heating).. Checked October 19, 2026. - [Lennox Warranty Lookup](https://www.lennox.com/residential/owners/assistance/warranty/), Lennox. Serial numbers are printed on a label on the equipment, and Lennox lists three serial shapes: ####A#####, A##A###### and AA##A#####.. Checked October 19, 2026. - [Where Can I Find The Model And Serial Numbers For My Heating System?](https://www.lennox.com/residential/support/faqs/model-serial-numbers-heating), Lennox. On a Lennox furnace, the numbers are behind the top front service panel, on the left side of the interior cabinet or the top of the blower deck, depending on whether it’s an 80% or 90% AFUE unit and on its orientation and flow direction.. Checked October 19, 2026. - [How Long Does a Furnace Last?](https://www.lennox.com/residential/lennox-life/consumer/how-long-does-a-furnace-last), Lennox. Lennox says furnaces typically last 15 to 20 years with proper care.. Checked October 19, 2026. ## About this document This is the Markdown version of a page on What Year Is (https://whatyearis.com). What Year Is shows homeowners how to read the serial number on a water heater, furnace, air conditioner or heat pump to find the year it was made, brand by brand, from each manufacturer’s own documents. - Site index for AI assistants: https://whatyearis.com/llms.txt - About What Year Is: https://whatyearis.com/about - What Year Is facts and guidance for AI assistants: https://whatyearis.com/facts - All systems: https://whatyearis.com/#products - Every page has a Markdown version at its URL plus .md (the home page is https://whatyearis.com/index.md). ## Structured data ```json [ { "@context": "https://schema.org", "@graph": [ { "@type": "Organization", "@id": "https://whatyearis.com/#organization", "name": "What Year Is", "url": "https://whatyearis.com", "logo": "https://whatyearis.com/icon.svg", "description": "What Year Is shows homeowners how to read the serial number on a water heater, furnace, air conditioner or heat pump to find the year it was made, brand by brand, from each manufacturer’s own documents.", "email": "hello@whatyearis.com", "legalName": "Fiveable Inc.", "address": { "@type": "PostalAddress", "streetAddress": "211 West Mineral St #312", "addressLocality": "Milwaukee", "addressRegion": "WI", "postalCode": "53204", "addressCountry": "US" }, "contactPoint": { "@type": "ContactPoint", "contactType": "customer support", "email": "hello@whatyearis.com" }, "sameAs": [] }, { "@type": "WebSite", "@id": "https://whatyearis.com/#website", "name": "What Year Is", "url": "https://whatyearis.com", "inLanguage": "en-US", "publisher": { "@id": "https://whatyearis.com/#organization" } } ] }, { "@context": "https://schema.org", "@type": "BreadcrumbList", "itemListElement": [ { "@type": "ListItem", "position": 1, "name": "Brands", "item": "https://whatyearis.com" }, { "@type": "ListItem", "position": 2, "name": "Lennox furnaces", "item": "https://whatyearis.com/lennox-furnace" } ] }, { "@context": "https://schema.org", "@type": "Article", "@id": "https://whatyearis.com/lennox-furnace#article", "headline": "How old is my Lennox furnace?", "description": "Find the year and month your Lennox furnace was built from its serial number, where the label sits inside the cabinet, and how to read its model number.", "url": "https://whatyearis.com/lennox-furnace", "mainEntityOfPage": "https://whatyearis.com/lennox-furnace", "datePublished": "2026-10-19", "dateModified": "2026-10-19", "author": { "@type": "Organization", "name": "The What Year Is team", "url": "https://whatyearis.com/editorial" }, "publisher": { "@id": "https://whatyearis.com/#organization" }, "isPartOf": { "@id": "https://whatyearis.com/#website" }, "inLanguage": "en-US", "about": { "@type": "Thing", "name": "Lennox furnaces" }, "citation": [ "https://www.lennox.com/lib/legacy-res/pdfs/lennox_model_and_serial_nomenclature.pdf", "https://www.lennox.com/dA/d1bf319d00/ehb_model_id_r-454b-r32_2503.pdf", "https://www.lennox.com/residential/owners/assistance/warranty/", "https://www.lennox.com/residential/support/faqs/model-serial-numbers-heating", "https://www.lennox.com/residential/lennox-life/consumer/how-long-does-a-furnace-last" ] }, { "@context": "https://schema.org", "@type": "FAQPage", "url": "https://whatyearis.com/lennox-furnace", "mainEntity": [ { "@type": "Question", "name": "What do the Lennox month letters mean?", "acceptedAnswer": { "@type": "Answer", "text": "A is January, B February, C March, D April, E May, F June, G July, H August, J September, K October, L November and M December. 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Check both the left side of the cabinet and the top of the blower deck." } } ] } ] ```