I have a natural gas furnace and central AC system, with two zones, one for the 1st floor and one for the 2nd floor. The first floor has a thermostat with two switches on top, one for cool/off/heat, and one for fan on/auto. The 2nd floor thermostat has no switches, just the dial. See pictures below.

1st floor zone thermostat: back of dial, wall piece:
http://picasaweb.google.com/andy.den...91746862384642
http://picasaweb.google.com/andy.den...91748599289442

2nd floor zone thermostat: back of dial, wall piece:
http://picasaweb.google.com/andy.den...91734458029218
http://picasaweb.google.com/andy.den...91737514904114


The master control is an TrolATemp MasterTrol Mini-zone model MM-2.
http://picasaweb.google.com/andy.den...91753514683666

The wires from the TrolATemp lead to two damper motors, one for each zone. The left one controls the 1st floor, right controls the 2nd floor.
http://picasaweb.google.com/andy.den...91756318147138

OK, here's what's happening. When the system is NOT running, both of the damper motors are ON, holding the dampers open. That seems illogical to me right from the start; I know nothing about this stuff, but I would have expected them to be off, such that the dampers would be closed. Then, when one or both of the zones needed air, they'd get a signal to open the damper(s) for those zone(s). But that's not what I'm seeing.

Scenario 1: first floor zone only.
With everything off, if I adjust the 1st floor thermostat to start the AC for that zone (e.g. set the mode to "cool", fan to "auto", and turn the dial to a temp that is below the current ambient temperature), the AC system fires up, the fan starts running, and the motor controlling the damper for the 2nd floor disengages, thus closing that damper. So, this has the desired effect (1st floor damper open, 2nd floor damper closed), but not the way I would have expected.

Now, if I turn the 1st floor thermostat back up above the ambient tempurature, the damper motor for the 2nd floor opens up that damper again, the AC and the fan shut off, and we're back where we started.

Scenario 2: 2nd floor zone only.
Everything works similarly if I engage only the 2nd floor zone -- that is, the damper motor for the 1st floor zone turns off and closes that damper, so the air flows only to the 2nd floor. Reset the 2nd floor thermostat and the first floor damper opens again.

So, functionally, this is producing the desired result for these two scenarios (only using one zone at a time).

Scenario 3: first floor, then 2nd floor.
If I get the first floor zone going and then engage the 2nd floor zone via its thermostat, the damper for the 2nd floor zone attempts to open the damper, but it only gets part way open. The motor is still pushing, but not hard enough to fully open the damper. If I manually help it along by rotating the damper with a screwdriver, the motor will continue to turn and will finish opening the damper and hold it open. But if I don't intervene, what happens is the 2nd zone to engage (regardless of which one I start with) doesn't completely open the damper and thus that zone only gets a weak air flow.

I've removed the damper motors and manipulated the dampers myself, and they open and close smoothly with no perceptible "sticking point" , so my guess is that the dampers themselves are not the problem. I suppose the resistance of the damper spring varies imperceptibly throughout the range of movement, so I could be wrong, but I'm skeptical (why would the motor be able to open it sometimes, but not others, and in a consistent pattern?). Could one motor not be getting enough juice from the controller when the other one is already engaged?

What about the seemingly backward behavior of the damper motors in the one-zone-at-a-time scenario? Is that the way it should work, or does it indicate something is wired incorrectly? And if the latter, how can I tell if it is the wiring of the motors, the wiring in the controller, or the wiring of the thermostats?

I'm over my head, so I appeal to the HVAC gods for help!

Sorry, my images didn't appear above, let me try that again...

1st floor thermostat:
http://picasaweb.google.com/andy.den...91746862384642
http://picasaweb.google.com/andy.den...91748599289442

2nd floor thermostat:
http://picasaweb.google.com/andy.den...91734458029218
http://picasaweb.google.com/andy.den...91737514904114

Controller:
http://picasaweb.google.com/andy.den...91753514683666

Dampers & motors:
http://picasaweb.google.com/andy.den...91756318147138

OK, either this image feature doesn't work or I'm too dumb to figure it out. Probably the latter. Anyway, the images can be seen at:
Picasa Web Albums - Andy - HVAC

Here is about everything you need to trouble shoot that toy. There is really no way to do it from here. All testing to be done with meter or a 24 volt light bulb to simulate operation. LOOK for the simple items first. (that means do not tear into the system just test it to find out the problem). Good luck

https://www.askmehelpdesk.com/heatin...de-237360.html

Some zone system will operate that way. They sill satisfy the main control or zone one first than they will satusfy the second zone...
Might want to break out your manuls and see if that is the case here.

Thanks for the link, hvac1000. I can see right away from a quick look at the manual that the one-zone-at-a-time scenarios are working as designed. So now I can focus on why the damper motors aren't opening all the way sometimes.

They're doing exactly what they are supposed to. The dampers are set to 'normally open' which means that when the thermostats are not calling the dampers remain open. You can have the fan on, for instance, and circulate air ( which will actually help the comfort level in the entire house ) OR you can set the control panel on the furnace such that the dampers are 'normally closed" which will produce the effect you are expecting. Leave them the way they are, you'll be happier.