Square D never made a furnace. You are reading the part number off the circuit breaker on the electric heat module. Since that is not a actual part number you will be hard pressed to find a diagram or replacement unit under that #, not to mention due to age of unit.

Many retrofit heat coil assemblies are out there, and based on the amperage protection of the # you listed, you are looking for one in the 7.5 KW (24,000 BTU range)

Hi, his question was like the one I have. I have the original "PT2 & E20 series installation & service instructions" for Suyn Dial Electric Heat (Square D). Mine is from the 70's as well. The # on mine is E20-45-18-A0000-3T. A guy checked for me and said one of the 2 elements is bad. The label also says Sub base model #E20, 1/2 HP 1 phase 240 volts 6.0 amps, ampacity 101

Can you tell what kind of element I need to get, like you answered him??

Do you have the e 20 wiring diagram??

Yes, Square D did make furnaces under Sun Dial from 1977-1984ish. Not sure how to age it though.

Yes, Square D did make Furnaces starting in 1977 until mid 80s

Not sure how to age it though.

(click to enlarge)

Fortunately, there are expert resources available elsewhere. The age can be inferred from the serial number:

https://www.building-center.org/square-d-hvac-age/#:~:text=%C2%AE%20HVAC%20Systems.-,The%20date%20of%20production%2Fmanufacture%20or%20age%20of%20Square%20D,located%20on%20the%20data%20plate.

I love this site. I have a subscription for my Home Inspection business. Great resource.

I'm sure Ray finds it a great resource, too!

LOL, yeah Charles museum specialties have a place here just as much as builders do.

~They provide me work. (both of them) ala great resource sandwiches.