How old is my Weil-McLain boiler?

By Dana Whitlock. Updated October 2026. Official format.

A Weil-McLain boiler's CP number contains no date at all. It is a running production count printed on its own CP tag. Match it against Weil-McLain's year-by-year range table for the approximate year, or enter it in the CP Lookup for the exact manufacture date.

Both the range table and the lookup tool come from Weil-McLain, so this is an Official format. Our decoder applies the table and gives a year. For a precise day, or a boiler near a year boundary, use the lookup.

Where to find the label on a Weil-McLain boiler

The CP number is not on the main rating plate. Weil-McLain puts it on its own CP tag or sticker on the boiler jacket, and the spot varies by model, so walk around the jacket with a flashlight and look for a small label reading CP followed by six or seven digits. Basement boilers are often close to walls and piping. Look, but don't climb on pipes or pull insulation. Keep jacket panels on while the boiler is running. Anything inside the jacket is a job for a licensed heating technician.

How to read the code

Write down the digits after CP. If there are only six, Weil-McLain says to add a leading zero to make seven. Then find the year whose starting and ending CP numbers bracket yours in the CP# Date Range Guide, which covers 1988 to 2026. CP5075477 to CP5221234, for example, all fall in 2005, and a federal recall confirms that run was built from April through October 2005. Numbers below the 1988 starting value are older than the table, and inspectors note that six-digit CP numbers generally point to the early 1980s or before.

Real serials in this code

When the format changed

Because CP numbers count up continuously instead of encoding a date, there is no letter cycle and no era change to untangle. The limits are at the edges. A boiler built in late December and one built in early January can be a few numbers apart and land in different years; recall records for Weil-McLain's Ultra boilers show a CP range made from June 2012 to March 2014 that the table assigns partly to 2013. Weil-McLain also warns that AquaBalance CP numbers are not sequential, so the table may not fit those, and the CP Lookup has no data for EG and EGH boilers. The lookup is the final word. One InterNACHI member posted a lookup result showing CP 2720501 built on 8/24/1994.

Who makes Weil-McLain

Weil-McLain was founded in Chicago in 1881 as Weil Brothers. It is headquartered in Burr Ridge, Illinois, and builds boilers in Michigan City, Indiana, where it has made them for more than a century, and in Eden, North Carolina. The CP system is unique to Weil-McLain; no other brand in our database uses it.

How sure is this?

Official format. Weil-McLain publishes how its serial numbers encode the date. We tested the weil mclain rules against 4 serials, 3 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 Weil-McLain's serial lookup before you spend money on the answer.

Quick notes

• The CP number lives on a small separate tag. The big rating plate won't have it.
• With only six digits after CP, add a leading zero before using the lookup.
• For an AquaBalance, EG or EGH boiler, rely on install records or call Weil-McLain with the CP number.
• DOE's analysis puts the average gas hot-water boiler at about 27 years, so a cast-iron Weil-McLain from the 2000s may have many years left if it has been serviced yearly.