Has the second floor AC always operated this way or is this something that is has started recently? If it has always been this way then it is probably due to poor design. If this is a new problem, then it sounds like a damper is not fully closing to shut off the air to the first floor. It could also be the damper controller not sending the signal to the damper. Replacing the thermostat is unlikely to resolve this.

Who inspected the dampers and concluded they are operating correctly? Did this person suggest a solution to resolve the problem?

Sounds like the damper. Have it checked again by someone else. The controller board for the damper has a way of going bad and needing replacement. Make certain that you have enough returns for the system. Often people focus on the vents and forget to think about the returns. Returns are equally important.

If it is indeed a poor design (which wouldn't be a surprise) a simple change of a thermostat rarely solves the problem.

Poor designs are fixed with the implementation of better designs. If you're not interested in truly fixing the problem, sure change the thermostat. The odds that fixes your problem is rather low.

Zone systems installed by builders are often considered in the line of garbage shortly after the 5 year old mark. You're at 4 years -- easily in line with what I've seen over and over since builders started incorporating zone systems in homes they build.

Hi, Karen,

Sorry you're having HVAC problems.

From your description, I infer you have a single air handler with zone control dampers and thermostats on each level that allow independent temperature control of the 1st and 2nd levels. I'm guessing it's a single-speed air handler. Let's start by having a look at zone control with a single-speed system and see if we can shed some light on your problem.

Your HVAC system is designed to heat or cool both levels concurrently. For simplicity, let's assume the heating and cooling loads are the same on both levels. So, with a single-speed system, when only one zone is calling for cooling, the demand would by 50% of design. However, the air handler can only deliver air at 100% of the design air flow. Trying to force 100% of the design air flow rate through half of the ductwork system puts larger forces on control dampers and makes the blower motor work harder. To relieve some of the force, bypass ductwork is sometimes installed which directs some of the air flow from the discharge side of the air handler back the inlet side (the HVAC equivalent of a dog chasing his tail, if you will.) If you're getting a lot of air flow through the control damper in the zone not calling for air conditioning and the damper is fully functional, I'd look for problems with the bypass--and the problem could be that your system doesn't have one.

It's not the thermostat, it's the dampers or zoning control/implementation. SIngle system zoning rarely works well (see Charles' explanation above).