Cold Air Flow When Furnace Not Running

[ujx51]

Our house is a year and a half old.  It's a two story on a slab.  The upper story covers only a portion of the lower floor so there are basically two attics – an upper and a lower.  The HVAC is a single unit installed in a mechanical room on the upper floor.  There are two thermostats (up and down) and two zones.  The zones are controlled by normally open dampers and a Honeywell HZ432 controller.    A single feed goes to a distribution box in the upper attic and then is distributed to the registers on the upper floor.  The lower floor is fed by zone one thru two main feed ducts.  One feed goes to a distribution box at the rear of the house and one to the front half of the house.  The front half of the lower floor is the problem.  It's fed by a distribution box (call it Box A) in the lower attic and has 9 ducts out of it to registers in the ceiling of the downstairs.  Three of those 9 go back to the upper attic, down thru the mechanical room and then to registers on the back side of the house.  They did it this way to get around a the stairs.


The duct work is insulated (R-8) flexible foil covered and is basically hanging wherever they could find room.  There is no other insulation covering the ducts.  The distribution box is sheet metal and has some kind of insulation on the inside of it – maybe a couple inches thick.


Now to the problem.  During cold weather, it appears that warm air is going up the stairwell and into the registers and duct work in the upper attic then getting forced (or pulled) back thru the furnace plenum and back up to the attic and fed to Box A.  By the time the air gets back to Box A the air is cold and a significant amount of cold air comes out several of the registers coming out of Box A.  Also, when we're using the fireplace air also gets sucked back up those ducts from Box A that go up to the mechanical room before dropping back to the first floor.

I used a smoke pencil to follow the air flows as I described so I'm pretty sure this is what's going on.  The HVAC installer has checked all kinds of things.  He has verified that there is cold air coming out of the lower floor registers but seems to be at a loss as to how to address this.  Any and all ideas would be greatly appreciated.