TH8320ZW install

I replaced a “White-Rodgers” TS with a Honeywell TH8320ZW. I had to connect the blue wire at the furnace to the common post, and connected it at the TS, other than that, all wiring was the same. I powered it up and set it to cool and set the temp well below the room temp. 5 minutes later, the system begins to cool. Everything is fine for about 20-30 minutes at which time heat starts coming out of the registers instead of cold air. I went down to the furnace and the furnace is running (gas furnace), so I go outside to the condenser and it is also running and I feel the hot exhaust coming from the PVC out of the furnace. My furnace/AC is an “Achiever 90 Plus”. Standard furnace and single speed condenser. I have cut and stripped fresh wire to make sure I have no “shiners”. I’ve verified nothing is shorted across terminals, etc… I’m kind of at whit’s end. Did I get a faulty thermostat or is something else going on? It is not a heat pump system, so the reverse valve isn’t an issue.

I hooked the old TS back up (without the Common wire) and it runs fine. I went out and verified the condenser is running, but no exhaust out of the furnace.

Thanks!
(this is cross posted on another forum)

I found that the thermostat is defective. ohm meter shows the R and W are shorted in the thermostat when removed from the wall.

I got the replacement thermostat and installed it last night. Same issue. I’m beginning to think there is a setting wrong in the thermostat.

Anyone have any ideas?

Ok. It appears I have figured out and resolved this issue. There is a setting in the Installer Menu 0190 (Changeover Valve) that is set to 0 by default. I changed this to be a 1 and it fixed the issue. I had read several people who have Heat Pump Systems that needed to do this, but I didn’t find anywhere saying that people with conventional 1h1c needed to do this. I just decided to give it a try and it works great.