Can I run a portable AC in a completely sealed room?

No. The exhaust heat must go somewhere. Running a vented portable AC in a sealed room actually increases room temperature because the compressor generates additional waste heat.

Do ventless AC units exist?

There is no such thing as a ventless refrigerant air conditioner. Products marketed as 'ventless AC' are evaporative coolers — they use water evaporation, not refrigerant, and only work in dry climates.

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