HVAC is not shutting off

Hi HVAC exeperts,

Looking for your expert advice on this AC behavior.

My home is 1 year old and the home inspector identified AC supply and return ducts interchanged in Master Bedroom, Laundry, and Master bath. I have communicated the issue to AC company as the unit is under warranty. They have interchanged ducts and after, we were experienced slow cooling and the second-floor thermostat never hits 72 degrees and keep on running. I have called the AC company several times as the machine keeps on running and all the time the company tried to educate me that it is all because of ambient temperature, wall radiates heat etc. They said according to Manual J if the home cools 25-degree difference that ambient temperature would be enough.

Before the duct switch the home cools relatively fast but now each one-degree drop take 60-75 minutes and when it closer to 72 degrees it takes 2 hours or more, mostly don’t shut off.

I have a doubt that whether supply and return air mixed together as I see a lot of carpets dust accumulated in various places which we don’t see this before. Is there any chance that this could happen? If so how could this will be identified?

So could you please advise how to identify the root cause of the issue as the AC company said everything is perfect which is really not.
What are the potential reasons do you think the system is running like this?
I have observed that when they switched the duct, the return duct is placed on top of the supply duct.

Please help as I am not an HVAC expert.

Regards.
DP

How hot has it been where you’re located? There is only so much an AC unit can do. It might simply not have enough cooling capacity to cool it down to 72F up there if the weather is hot enough, especially if it’s humid as well, and especially if there is significant supply flow to rooms below. 25 degrees below outside temperature is pretty good, especially if your house is not shaded, so upper levels get more heat from sunlight. If a house is not well insulated or very air-tight (e.g., old windows) that can also have a great impact on cooling capability.

If the master bedroom and bathroom are downstairs, now they would be getting more AC flow now if they only had returns before, and the upstairs rooms less. If the same unit is serving all rooms, you can adjust the registers in the 1st floor rooms, and of course if there is a basement, make sure supply registers down there are fully closed. Cool air is heavier than warmer air, so it will sink, so the lower rooms will get some cooling from the upper ducts. What is the coolest air temperature reading you see at the thermostat?

As for the dust, if they had supply and return ducts switched, I’d expect some dust to be blown out of supply ducts that were formerly return ducts that had sucked in such dust. Unless the ducts were cleaned really well.

Based upon this; I would say you have disconnected air ducts.

You need to have a duct leakage test performed to determine this.

Next, you have nowhere near enough information to take a wild *** guess.

Where do you live?

You talk about ambient air, what is its temperature and relative humidity?

So it was working OK, then it was ‘fixed’ and now it does not work OK?

“My home is 1 year old and the home inspector identified AC supply and return ducts interchanged in Master Bedroom, Laundry, and Master bath. I have communicated the issue to AC company as the unit is under warranty. They have interchanged ducts and after, we were experienced slow cooling and the second-floor thermostat never hits 72 degrees and keep on running.”