Have a friend who has a furnace in his house and no a/c. House is about 800 square feet. He has a 75,000 btu furnace in there. He asked if I can put an a/c in for him. Not really sure what size unit I should put in there. I’m not familiar with residential too much since all we do is commercial. Can anyone reccomend what size unit I should put in without doing the whole manual j.