The $4,000 unit that did not need replacing.
A business owner's AC went down and the landlord's answer was to change the whole unit for four thousand dollars. We went out, found the actual fault, and fixed it the next day. The unit is still running and the owner kept the money.
Same story on rooftop units. We have replaced RTU compressors and fan motors and brought commercial systems back rather than writing a replacement quote because it is easier. EPA Universal (Section 608) certification covers the refrigerant work on all of it.


