I purchased a Nest 3rd gen thermostat on Cyber Monday through our electrical supplier (Com-Ed) for $100. Had always wanted one but thought the $250 price tag was too steep. When I saw the $100 price I decided it was time. Love the thermostat and the ability to observe & control it remotely. So I've had it installed for a little over 3 weeks now.
Fast forward to this morning... I woke up to a 60 degree (F) house! The Nest was showing an error of E74 which indicates it can't detect the Rh wire (?) or more simply it can't detect power. My furnace system is just a basic gas forced air with A/C. Standard 4 wire thermostat connection- Rh (24VAC), W1 (Heat), Y1 (Cool), and G (manual fan). There is no common wire for the 24VAC so the Nest derives its "phantom" power through the current of the cooling, fan and heating relays.
I got my meter out and did some checking. And of course I simply jumpered the Rh & W1 wires together and the heat came on as expected. The Nest goes into a diagnostic mode when its not working correctly and it was showing an open circuit voltage of about .50 VAC. So immediately I figured there must be something wrong with the thermostat or its base as I measured approx 15VAC between Rh & W1 (heat) and 24VAC between Rh & G (fan) but 0VAC between Rh & Y1 (cool). I put my meter in current mode and the heat takes about 350Ma AC, and the fan the same. So I scratched my head to wonder what could be the problem. I tried switching wire around to see if I could "fool" the thermostat or maybe identify an input that it wasn't sensing for current all to no avail.
So then I got to wondering why the A/C connections (Rh & Y1) weren't showing any voltage/current. I did some resistance measuring and saw that there wasn't any continuity to the outside condenser unit. We recently fell to below 0 degree's overnight in fact I think is was something like -10. So then I think about my experience with refrigeration and figured that the condensing unit outdoors has reached a temperature that if the suction line were to freeze over it would shut down the compressor as a matter of safety. So I thought I'd try a little experiment- I added a 10 watt 60 ohm resistor in place of the control line going to the condenser unit outside to produce a cooling circuit current of about 400 ma AC. The Nest then came back to life.
I wonder how many calls and unsatisfied customers they get because of this issue. The thing of it is the Nest knows which wires are connected and where it gets its power from. You'd think it would have known that yesterday I was getting power from Rh-W1, Rh-Y1, Rh-G but now today I'm only getting it from Rh-W1 & Rh-G and tell me so. The other option is to install a new thermostat cable that also includes the "C" for 24VAC common. Anyway the heat is back on and working....