I have an old Rheem package unit model no. RPND037C was working fine all sumer but now that its colder I get no heat. I suspect the reversing valve.. but not to familiar with the Rheem units. I know on the Carrier package units the reversing valve fails in heat.
Anyone have any other suggestion that I might check?

Komar Tech

First make sure your tstat is telling the reversing valve to change over