A Hot Room Is A Load ProblemHeat gain is arithmetic, not opinion. The hallway thermostat, meanwhile, reads 73 and is perfectly satisfied, so the blower shuts off and the office keeps climbing. Say a three-ton changeout with corrected return air runs $9,500 here and trims $60 a month across four cooling months, with another $80 a month coming off the bill when the portables get unplugged. A larger system is the right answer only when the whole-house load genuinely exceeds what is installed. If a quote arrives without those, it is a guess, and a guess is what put a fourth portable unit in the spare bedroom.