@hkies , let me see if I can possibly help you understand why you might be having problems charging with your particular HVAC equipment. Newer HVAC control systems utilize a more sophisticated, electronic control board as opposed to the more original analog type control systems. In the more conventional analog time systems, the various relays in electrical contractors that are providing power to the equipment have an electrical coil in on that creates a magnetic field that will engage the relay or the contactor turning it on. One side of that coal is connected to the common of the transformer, and the other side is connected to the wire coming from the thermostat. Now the thermostat takes the power coming in for the transformer on the R wire, and connect it to say the Y wire to start cooling. That 24 V travels down the Y wire and goes to the compressors contactor, causing it to engage the contacts and start the compressor. Nowh in this type of the system that electrical coil in the contactor is a path that through the Y wire, when it’s not being energize for cooling, is it path to the common that a very low current draw can be used and charge up the battery by the nest. It doesn’t draw so much current as it caused a coil to engage.
And newer, modern digital control systems that coiled no longer exists. In fact, it’s the control board that actually controls those relays and contactors now and all it’s doing is looking for the incoming power coming on. The thermostat wire does signal that something is needed. That is most likely your situation and that’s why you need a Nest power adapter in order to get the common power you need.
If you have a common wire, this problem goes away. By bringing the transformer, common straight to the thermostat, maximum powers available between R and C to charge and maintain the battery of your nest thermostat.
I hope this helps to clarify your question and my answer.
The AC Cooling Wizard
Google Pro, Mechanical Engineer and HVAC service company owner.
