Hello. I have a fairly new air conditioner / furnace unit (3yrs old). The AC has been running non stop on hot days , (set at 72 degrees- not keeping up). I understand this may be normal for some homes, however, my father lives 4 miles away and his AC runs intermittently set at 71 degrees, and his unit maintains the set temperature . I think I may know the problem in my home. I live in a ranch with a basement. The staircase to the basement is OPEN. The cool air being pumped into the house is probably sinking into the basement, that is why the AC unit is running constantly. Do you guys think I wold benefit from closing off the staircase? What do you think?

Might also be as simple as the filter(s) being too dirty.  The system can ice up because of that and need to melt off after replacing the filter(s).

Design temp in MI is between 81 and 88 degrees.

For example, in Marquette it is 81 degrees. In a perfectly designed system, the AC would run non-stop when the outdoor temp is 81 degrees and maintain an indoor temp of 75 degrees.

Exceed either one of these design conditions and the AC will not keep up. E.g. if you want to keep the house at 74 degrees when the outdoor temp is 81 degrees, a perfectly designed system could not do that. Likewise if you wanted 75 degrees inside when the temperature is 82 degrees outside, the system would not keep up.

99% of all of the HVAC companies will oversize the system to avoid callbacks for a properly designed system. Many of the times it's unavoidable-- if the calculations say you need 3.1 tons, your choices are 3 or 3.5 tons, so your system is oversized 10% just due to available equipment.

If it's not system size, check the filter. If filter is clean I would call the installer to check the refrigerant level and/or try and explain what the problem is.

Check filter also see if you can block off the basement. Does that change anything?

AFAIK - a system that runs for long times and keeps the temps where they should be is supposed to be the most efficient and long lasting. When a system runs for a few minutes and shuts off and runs etc usually means it's wearing out the compressor and not running efficiently (something wrong with set up)

Unless you bought from a "run and gun" installer - your unit should still be under warranty. Call them and let us know what they find out. I'm always interested to learn more about AC systems

There are many variables but that is one of the bad ones.  Start there.

Is it pumping out cold air? 

Our AC just crapped out Thursday night - the blower ran 24/7 but no cold air. The tech explained it as wiring gremlins - throwing a code stating the furnace had overheated and as a result, the furnace and AC were disengaged and the blower ran constantly trying to cool the system down. 

Once the wiring dilemma was fixed it was back to on/off and cold air. 

Sounds like a bad limit switch.

Ours at work had the same problem. The air filter had not been changed since late 2015 which cause a strain on the motor which blew a capacitor I think is what the HVAC guy said. Under warranty fixed free of charge.

BTW, are there any return ducts in the basement???  Assuming you have one system for all floors..