Downstairs thermostat is kicking the heater on but there is heat coming out of the upstairs zone to the extent that the upstairs is getting quite warm because it has to help heat downstairs. I flipped the two thermostats, no change. I checked the controller on the furnace and no errors. I'm a lay person, but due to a tight budget I need to try and resolve this myself.

Anyone have any ideas how to troubleshoot this next? What actually controls the signals from the thermostats to determine what zone should have heat pushed to it. My guess would be this is the culprit.

I have EXACTLY the same problem - I have a tech coming tomorrow and I'll update the post.

Any updates since your post?

Nope, you are the first to respond to my question. Timely as the last two day's the weather has turned cold and I'm again going nuts with this issue. In the past I've had problems with thermostats going bad but am pretty certain this isn't the issue this time.

As you know, surfing the web hasn't paid off in finding an answer, so, I'd really appreciate you sharing what you find out. I still may end up hiring someone but I'd have to imagine the solution isn't difficult to implement, just difficult to diagnose from a lay person's point of view.

Good luck

@Rigity - Did you have any luck getting your zones working as they should be? Anything you can share that would help me? Cheers

The tech determined the problem to be the damper part. (ULTRAZONE model #: MA-ND) It's located in the hvac unit - in the attack in my house.

The damper is the motor that controls the air flow ventilation in the zones. The part wasn't working and the last setting was only allowing air-flow to the upstairs zone. The tech was able to manually change the air flow to the downstairs zone and we verified it worked. This part is about $300. My house is new enough that this is under warranty so the builder will need to repair this. Once they completely fix this I'll know exactly the root cause. Hope this helps!

$300, that's quite a bit for a furnace part. Yes, this does help to guide me and I appreciate it. Kind of figured it was something like this. At least now I have an idea of what the part may look like and can dig in more. If you get more details I'd appreciate the share. Cheers