In article ,
says...

On Thursday, July 27, 2017 at 3:46:15 PM UTC-5, Taxed and Spent wrote:
I have a thermostat that allows a setting for fan only, which I wish to
try to try to push some air around during the warm weather. There is no
AC. Furnace is 1960's vintage, and had only two wires to the
thermostat. I added a C wire for the smarter thermostat. If I add a
FAN wire, will the old furnace have a place to hook it up and respond
appropriately?


It should have. It's where the green wire hooks up. It will be marked with a "G" which is for the fan relay. ?(?)?

https://highperformancehvac.com/ther...g-colors-code/

[8~{} Uncle Fanny Monster

Well, wouldn't you know it. I just replaced a theromstat on my heat
pump. The wiring did not exectally match up old to new. I think I
have it correct as the cooling part is working. Won't know for about 3
or 4 months if the heat part works.

I could have used the explination of the letters about 8 hours ago..

I did think to take a pix of the old one and labled the wires with the
old letters.

I copied that web page and will file it with the other heat pump info.