Did a full Cooling system flush today and swich out the thermostat. The rubber ring was broken allowing it to constantly flow making me have no heat. Now I need to work on the A/C. I'm pretty sure it's either the control knobs, or whatever inside makes it switch from Heat to Cold. There is no sound of a changeover when turning the knob. Anyone had this issue, or know how these systems work so I know what parts to look at to narrow it down?