My T84F1015 is wired to only 1 stage--which wire do I need to switch to turn it back to 2 stages? I think (hope) the first stage was to take advantage of solar preheat which wasn't functioning so only blew out cold air. Solar should be working now so I would like to go back to using both stages. At one point an HVAC tech who was here switched some wires in the thermostat but that was several years back and I can't remember what he did. I know it was a simple change and prefer to do it myself.