My HAVC system cools down slower and slower

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Hello guys, I met a problem while implementing a simple model which cools down the 30 degrees environment to 20 degrees with a relatively realistic HVAC model (not using ideal air loads). But I found the temperature changed slower and slower. for example, changed 2.34 degrees in the first 10 mins, and 1.26 in the next 10 mins, and less and less.

My HVAC is very simple, it only has a coil cooling device with a Total Cooling Capacity of 20000W and a constant volume fan.

If you have any idea about this or need more information, please tell me. Thank you for your help!