How do you find the age of a Mars Furnace. I have a 200,000 Input BTU with output of 160,000. It has a label attached to front that says Size 2-600 and no model but a serial number of H 18220. I looked for Pacific Gas and Heat Co. online and only got hits for PG&E which is Pacific Gas and electric company. It does have rigid pipe with asbetos wrapped around it. I just want to know how old this puppy is? Brent

Mars Furnace was manufactured by Pacific Gas Heating Co, San Francisco Calif. According to the plate on my furnace.
It was installed in October 1961.
I have been told to never replace it unless I have to because it was one of the best furnace ever made.