Sizing a window AC for bedroom, but with extenuating circumstances

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I'm looking to install a window AC in my son's bedroom, which is about 120 sq ft. The guides that I'm reading suggest a 5000 BTU unit due to the size. However we live in the high desert where the summer temperature gets > 100 degrees (F) at times during the summer. Also he has several reptile terrariums in his room that require lamps which also heat up his room. In this case would using a 6000 BTU AC unit be more appropriate?

My 180 sq ft. home office/spare bedroom is in the room next to his. I use a 6000 BTU unit and it chills this room no problem. So thinking of getting the a simular sized unit, but don't want to oversize if that's going to cause problems.

Thanks in advance for the advice.

Best Answer

I would go larger, I have installed many heating and cooling systems and when the systems are running properly the AC will cycle more often, icing is more of a problem with very high humidity and or low air flow and this can happen even in the desert on a hot day if the filters are dirty reducing the air flow while the system is working at its maximum cooling capacity.

I find customers are happier being able to turn the thermostat up a little verses when the room is still hot with the AC at its lowest setting running full blast. An over sized unit may cost a little more because it may cycle the compressor on and off more often but this cost will be a small percentage of the annual cost to run and you have a better chance of keeping the room comfortable.