I installed a new thermostat because I believed the last one didn't work. The new tstat is a Honeywell model RTH221. My furnace is a Goodman model # GMPN080-4. when I put the tstat to "on" the heat comes on sporadically. However it gets cold w/ the fan constantly running. On "auto" the heat doesn't come on regularly at all. The tstat was set to "auto" last night at 78 degrees, I woke up this morning and it was 60 degrees. Any suggestions? When the heat is on, the furnace never reaches the target temp. I did set the new tstat to setting of 3:5 (12min (gas or oil)) setting

You probably wired the thermostat wrong. You really should trace the wires from the thermostat to the furnace and see where they hook up so you can determine what color goes where. Sometimes the wire color code is not followed and if that is your case it could also be your problem.

Your wiring diagram for the tstat is below.

http://customer.honeywell.com/techli...69-2060EFS.pdf

Take a look at what Northern Heat has left for us on this subject. https://www.askmehelpdesk.com/heatin...ons-58313.html