Why is my AC Doing this (Upstairs/Downstairs)???

Christo2

I just bought a two-story house built in 2006. There is one 5-ton York AC unit outside and everything runs up to the attic which I have not explored much yet.

Here's the issue: My downstairs thermostat (Honeywell Digital 7 day) was likely installed by the previous owner and it seems to control the whole house (cold air coming out of all registers upstairs and downstairs). The upstairs thermostat (Honeywell digital, non-programmable) was installed during the original build and it controls only the upstairs (cold air coming out of upstairs registers only and it comes out with much more force than it does when the downstairs t-stat is controlling the air coming out of all registers). Why would this be the case? Shouldn't the downstairs t-stat in a system like this just control the downstairs (or shouldn't the upstairs t-stat control the whole house like the downstairs one?) Does it have something to do with the install of the downstairs t-stat by the previous owner?

I don't know much more than this as we just moved in and the previous owner got foreclosed on, so I don't know much about the history. I'm willing to do some sleuthing but I don't really know where I should start. Thanks in advance!

statman

Sounds as if you have a zoned system where either thermostat should send a call for cooling or heating on the level that it is on. Is there a zoning panel beside your furnace in your house or do you see any dampers (usually low voltage) in your ducting off the furnace? It could be that the damper for the main floor is stuck open (if there is one at all) or that they just tried to control the system to cool the second floor ? You probably need to call in someone who is familiar with reidential zoning system control and troubleshooting.

biggles

if the stas are not exactly them same they must have different purposes.i would say upstairs is volume only with the downstairs cooling the stat upstairs throttles the cool air only with the first floor running.to see if it is a volume control put the FAN only on the 1st floor stat then work the 2nd floor stat lowest setting see if the air changes(drops) on the first floor all the way up air should increase on the first floor.your setting to acommedate the upper zone is to always run the system cool/fan ON not in AUTO.if you move thesecond floor stat up and down and hear the air changing with or without the cooling calling it is a volume zone controlled supply to the upstairs.....basically a the setting will limit the air if temp.is reached during the cooling cycle

Christo2

Ok...here's an update:

Statman: Its been too hot in the attic for me to spend anytime up there (Phoenix area = 112 outside during day, 90 at night) but I'll check up there ASAP.

Biggles: I tried your suggestion and I think we're kind of getting somewhere. Until the ac compressor kicked on, the air was coming through both the upstairs and downstairs registers being controlled by the stats as per your suggestion. When the compressor kicked on, all air to the downstairs stopped. Hmmm.

Thanks for all your help...please keep the suggestions coming!

biggles

here's your possibilities on what controls whateach stat controls the AC.....if the floor isn't calling it shuts down as the 1st did when the 2nd started to cool.....you have to have ZONE damper motors within view some place down from the supply duct in the attic that is the only way air is being shutoff.if you feel ambitious pull the 1st floor and 2nd floor stats off thier subbases ,and count the wires on each, and the lettering and color the are wired too.a modulating motor might have 3-4 wires max and the main stat would have no less then 5 (R,G,Y,C,W)