Forgot to add: normally, the valve on a Rheem should have power any time
the thermostat is turned to the heat position, regardless of the
temperature setting, Possibly your tinkering with the thermostat is
keeping the unit from running steadily enough to need to go into
defrost, preventing the problem. Ony other way to insure constant heat
is to run it in Emergeny Heat. That is what it is there for. Larry