Where should my humidistat on my whole house humidifier be located
Two points. First, be sure you are using a humidistat and not a dehumidistat. With a humidifier you need a control (humidistat) that shuts off the unit when the humidity gets too high. When you have a dehumidifier or HRV you need a control (dehumidistat) that shuts off the unit when the humidity gets too low. From your description your control just keeps the humidifier going and going, like a dehumidistat telling the humidifier to keep running. Second, the humidistat should be in a location that reflects the actual humidity of the dwelling space. An open wall near the return vent would work.