Basement humidity affect on Humidistat?

mbear2k

Hello all - I have a question on the relation between basement humidity, living space humidity and the system humidistat.

I have an Aprilaire RP-58 humidistat installed on the upper plenum of my cold-air return of my up-flow gas furnace in our unfinished basement. The humdistat is about 12" above the whole house by-pass humidifier (April-aire 600). The humidistat also has an external temp sensor installed. This was all installed per spec and when we had a new furnace installed a year ago, the installtion company checked it out and said all was well.

What I'm noticing is that the living space (1st floor) is indicating about 19% RH on a couple of spot stand-alone hygrometers I set up in two rooms. However, the reading on the humidistat read-out on the basement (RP-58) indicates the RH is 42%.

I have the fan set to run all the time as I also have a UV system installed. So, I guess my question is - how would the humidistat ever call for water if the RH is at 42%? I do have a basement dehumidifier that runs as needed (set for normal). I also understand that a cold outside temperature will affect when humidity is called for.

Do I need to move the humidistat in to a cold air return located in the actual living space?

Any feedback appreciated!

M