A/C flow very weak on first floor

Hey everyone,
Hopefully you can help me out. I recent moved into a newly built two story house. The air conditioning system is a dual zone system, with the first floor being one zone, and the second floor being the other zone. Each zone has its own thermostat, and they work independently of each other. Well I recently noticed the first floor cools much slower than the second floor. The other day, the first floor cooled at a pace of about 1-2 degrees an hour, and it wasn't even that hot outside (not even 90 degrees). When I put my hand up to each of the registers on the second floor, I feel a nice strong cool air current. When I put my hand up to the registers on the first floor, I feel a very weak air current, amost more like a cool draft moreso than an actually current. (It is maybe like only 10-20% of the air current that the second floor is getting, so thats at least a 80% decrease) Does anyone have ideas what might be causing the problem? All the registers are completely open. I can understand that maybe the current loses a little bit of strength making its way down to the first floor registers all the way from the attic, but this seems like a bit much. I am hoping to talk to the site foreman on Monday morning, if I can find him. I was hoping you guys might have some ideas before then. Thank you so much. Keith

The overall air-delivery system might be intentionally set to send more cool air to the second floor while expecting natural convection to balance things out a bit. Even though all registers are open, there still could be some dampers or diverters in the attic that need some adjustment. If you have easy access to the blower and its ducting connections, take a look to see how and where the air is presently being directed. I have no specific experience here, but my guess is that you should be able to reduce the upper-to-lower flow ratio simply by tweaking the air-distribution system ... just be sure you do not reduce the overall total flow of air while possibly re-directing some of it.

Lee gave a good answer but I would caution you against making any adjustments on a brand new system yourself as it may invalidate any warranty.

Generally speaking, zoning of forced air heating and cooling systems in residences is a mistake, often a big mistake. It's not that it can't be done so much as most residential HVAV contractors simply do not have the knowledge or experience to do it right. There is also the cost factor in that the proper way (which involves some rather sophisticated instrumentation and controls in addition to specialized equipment) is often more costly than simply putting in two completely separate systems which seems to be much more common.

I would suggest that you try turning the fan "Auto-Off-On" switch on your thermostat to the "On" position and seeing if that increases the overall comfort throughout the house.

Thank you everyone for your responses. I spoke with the site foreman earlier today, and he said it sounds like it could be a damper problem. The customer care department is supposed to be contacting me shortly to set up an appointment for the HVAC company to come out and check things out. Ill keep everyone updated. Keith

Hey guys,
I just wanted to post an update about my a/c problem. The HVAC guy came out and discovered a defective damper feeding the first floor a/c ducts. It wasn't opening very much when turning on the first floor a/c, so I was getting increased air flow on the second floor, and decreased on the first. They are ordering a new damper, and will be replacing the defective one. Ill let everyone know how the finished product works. Keith

Just to give a final update, the defective damper was replaced today, and the AC now works like it should. Thanks to everyone who provided input and suggestions! It was much appreciated!