How Old of Unit

How old of boiler would you guys say this is: HYDRO THERM - Gas Fired - Model #R108 Ser# 6600

Very, very old.
Probably original with house.
When was the house built?
When one finds serial numbers with just 2, 3, or 4 digits, look to the 1960s and earlier.

Hydrotherm does not have a deciphering reference tool and never has. The newer models now have the year contained right in the Serial number.

As for the older models, Hydrotherm owns a miscellaneous list that they utilize as a reference tool but they will not distribute a copy. (It must be Top Secret information). You must call Hydrotherm, every time you need a date of manufacture for the older boilers.

Here’s the number…