Additional Tips for Determining the age of HVAC system or Water Heater

There are instances where the serial number may be missing or not legible, or the date code was simply not encoded within the serial number of the HAVC system or water heater. This is especially true of very old systems.

Below we have compiled some helpful information to help you arrive at an estimated year range or era of manufacture.

Gas Control/Regulator Valves

4 digit date code printed on the body

Most gas control valves (square-ish aluminum body) have a four digit date code printed or ink-stamped on the metal body of the valve itself. The first 2 digits indicate year, and last 2 digits indicate week of manufacture (YYWW).

While it certainly happens, we find that it is not very common that the gas valve has been replaced during the service life of the HVAC or water heater system. If you can locate the date code stamp or inking, there is a high probability this is the year (+/- 1 year) the gas-fired system was manufactured. At minimum, you will be able to determine when the gas valve itself was installed or replaced.

Gas valve date stamp

Mixed or Manufactured (Coal) Gas

Manufactured (Coal) Gas was last produced in the mid 1960's

Before Natural Gas was widely available, Mixed or Manufactured gas was produced or manufactured primarily from coal in factories known as "Manufactured Gas Plants". Manufactured gas production began in the 1800's. The last of these U.S. plants on a community supply scale ceased operation in 1966, though some plants supplying specific industrial needs remained in operation far longer.

Many older combustion appliances such as furnaces and boilers were designed for use with mixed or manufactured (MFD) gas. If such an appliance is still in operation, the data plate label will state something to the effect "For Use With MFD Gas" , "For MFD Gas", "NAT & MXD", or "For Use With Mixed Gas". If you find such a listing on the data plate, you can be assured the unit predates 1966.

MFD gas data plate

Mixed gas

Postal Zones & Zip Codes

Postal Zones and Zip Codes may help you determine an era of manufacture

2 digit Postal Zones were introduced in 1943 for primarily the major urban or large metro areas across the U.S.

Before 1943, only the city name was used. With increased demand on mail delivery services, postal zones became necessary. Philadelphia was postal zone 7 (07), and Indianapolis was postal zone 17, and East Cleveland was postal zone 12 for examples. These zones were widely adopted and some manufacturers included their postal zone on the name plate or data plate.

5-digit ZIP codes replaced the outdated postal zones in 1963.

If the data plate or name plate bears a 2-digit postal code, the era of manufacture was between 1943 and 1963.

Blower Door Safety Switch

First blower door safety switch introduced in mid-1970's

The first blower door safety (cut-out) switch for furnaces  was introduced in the mid to late 1970's (we believe Carrier was the first circa 1976). Other manufactures followed suit shortly thereafter and once it became a required safety device on forced air combustion furnaces.

If the furnace was not originally equipped with a blower compartment door safety switch, it likely predates 1976 to early 1980's.