We have a Chrysler Airtemp furnace, model 4216-50, serial number 2B395979. The serial number makes me think it was made in 1979; the house was built in 1962, if that is a factor.
Does anyone know the age and efficiency of this furnace?
Tom-
The info I have says that Airtemp started a two letter serial # system in 1975. It seems like your unit may preceed that date.
The efficiency of a unit is usually measured by a technician at the unit when it is in operation.
divide output btu by input btu on the manufacturer label.
ex.
output btu = 75,000
input btu = 100,000
75,000 \ 100,000 = .75 or 75%
I always take a closeup pick of the furnace, ac, and water heater labels, if possible.
CC- That will give you the manufacturers expected efficiency, but isn’t a test under operating conditions the true measure (especially on a unit that old)?
How about we simplify this and say…
Old and inefficient! 
Thanks for the useful information.
On efficiency, I’ll have to settle for Blaine’s comment of “inefficient.” The plate on the furnace says input of 160,000 Btu/hr, but only lists a “bonnet capacity” of 128,000 Btu/hr. I don’t know what bonnet capacity is, but it’s not output, since that would give a totally unrealistic efficiency of 80%.
Again, thanks to all for your help.
absolutely agreed
1967- 1972 prestons is sad… 80k btu
actually you used the correct numbers and your furnaces is 80% efficient. Hoever there is a catch. that is the steady state efficiency, meaning after your furnace has warmed up for 10 minutes, and the blower is running, you are getting 80% efficiency. Modern furnaces are rated in AFUE or annual fuel utilitzation efficiency. it takes into acount off cycle losses like pilot lights and chiminey losses which don’t figure into the steady state efficiency. your funace probably has a AFUE of 65 to 72%