Originally Posted By: mlord
Thermostats can leech small amounts of current from the other 4 wires in the absence of a C wire (depending on the furnace), usually enough to operate on.

They do this by inserting a resistive load across the "fan relay" wiring, such that it doesn't supply enough current to actually turn on the furnace fan, but still causes enough current to flow to power the thermostat.


Nest doesn't do that (themostats that do want the voltage drop across this bleed resistor). The scheme I came up with has a high voltage buck feeding a current limited battery charger/smartpath power controller (actually, a generic Linear tech part trimmed slightly differently at the factory). This works pretty well in most cases, and can be software configured for different max current draws to accommodate different HVAC equipment.