What thermostat model do you have? Does it use batteries and if so have you changed them recently?

The thermostat may display 73 but it may not shut off until it reaches 72.5 degrees. That is a low setting which your AC might be struggling to reach and causing it to run for hours. You could try a setting of 75 and see if cycles on and off properly to verify the thermostat is working correctly. If you have not had a maintenance check in a long time then I recommend you have that performed by a qualified HVAC tech. A temperature of mid 80s is below the outdoor design temperature for Chicago. The indoor temperature should be down to 70 degrees if it is running for hours.

I agree with Mike‘s advice and having a PM check on your condenser especially checking your refrigerant charge. You might want to see what your supply temperature reads at several locations in your home.

IMO

@mike_home I have Pro T705 thermostat and I have also changed the batteries. My unit is new. Just installed a month ago. I asked same question to contractor who installed it and without even checking he said my thermostat is bad. This week I am getting new thermostat and will try out that. But besides thermostat what else could be wrong. Because manually raising or lowering the temp AC unit turns off and on accordingly.

@tigerdunes I will have the refrigerant charge checked. As far as temp goes what should be the ides temp difference between supply and return vent?

that‘s called Delta T. The ideal split should be 15-20 degrees. I would want to see inside supply temperature no higher than 60 degrees. I just checked mine several weeks ago and my split was 21 degrees...

TD

Given that the thermostat is brand new, I am skeptical it is bad. It is possible so let the contractor change if he thinks that is the problem without doing any other diagnosis. If you are not getting a temperature split of 15-20 degrees then there may be an installation problem. How large is the house, and what was the size of the previous AC?

I am upgrading to smart thermostat so will know soon if thats the issue.

my house is 1700sqft. The older unit was 3 ton too. I am yet to measure the temp at supply and vent

Common Causes of a Constantly Running System.

https://www.air-conditioning-ductless.com/

Dirty evaporator coil. Did you forget to have your air conditioner serviced this spring before you turned it on for the summer? If so, the evaporator coil may be covered with dirt and debris, impeding the cooling process and making the unit work harder to cool your space. This can happen even if it was cleaned a few months ago, and the unit is in a location where it picks up a lot of grime, like near a restaurant kitchen.

Frozen evaporator coil. If your unit is blowing warm air from the supply vents in addition to running constantly, it may have frozen up. Turn the air conditioner completely off, and call in a professional right away to diagnose the cause and prevent further damage to the system.

Clogged air filter. When your system’s air filter is full of dirt and debris, the flow of air is restricted, which can impede the functioning of your air conditioner. Without enough air flowing over the coils, the system can’t remove humidity and cool the air to the set temperature, so it keeps running when it shouldn’t.

Restrictive filters. Even if your air filter is new, you may be using one that’s too restrictive for your system (such as one designed to remove pollen and allergens). For better air flow, use a filter with a lower MERV value.

Low refrigerant charge. If you don’t have enough refrigerant in the system, probably due to a leak in the coils, your unit can’t cool the space. When this happens, the unit will keep running as it works harder. In this case, you need an experienced HVAC service professional to find and repair the leak and restore the refrigerant to the correct level.

Faulty thermostat. If your thermostat is not working correctly, the unit doesn’t know to turn off even when the temperature is reached. Check your thermostat to see if the actual temperature in the space is below the set point. If it is, there’s a good chance that the thermostat may need replacing.

Leaky ducts. Especially if you have older ductwork in your building, you may be losing cool air through leaks in the ducts. The joints may not be sealed, or the insulation may be gone. If enough cool air escapes, the temperature in your space never reaches the set point and the system will keep running.

Dirty or blocked condenser. Just like the evaporator coils on the inside, the condenser coils on your outdoor unit also need to be cleaned, possibly even more so since they are exposed to the elements. The flow of air can be impeded by dirt and debris from the air, but also from accumulated leaves or even animal or insect nests. A good cleaning using professional products and tools will take care of this problem.

Blower motor and fan issues. If your blower motor is not producing enough air for your system, or the fan is running at a low speed, that can cause the system to lose enough efficiency to make it run too much. Adjusting the fan speed can solve the problem. A layer of grime on the fan blades can also slow it down, so again you may just need a good cleaning.

Incorrectly sized unit. Have you recently renovated your space or rearranged the layout without changing your air conditioner? It’s possible that your unit is no longer adequate for the needs of your space. Bring in a certified HVAC professional to do a load calculation, taking into account the design, size and needs of your building as well as windows and sun exposure, insulation and more.

Many of these issues can be taken care of, and even prevented entirely, with regular maintenance of your equipment. What’s more, neglecting this task can be costing you in many ways that you haven’t considered. To learn more, download a copy of our helpful guide to Calculating the Hidden Costs of Poor HVAC Maintenance.

Hi all. Thank you for taking time answering my question.

I upgraded to new thermostat and my AC is now cycling fine 10-15 min on and off on avg hot day and little longer on very hot and humid day.