I couldn't find any help on forums or websites if anyone may have tried to install Nest by replacing old Viconics P117 Board and E117 thermostat. I have an Electric Furnace and Heat Pump configuration. I have already brought the Nest 3rd generation and couldn't get it running. My system have only 3 wires connected in Thermostat(All other colour wire are note connected). I am adding the system diagram here so that you can look and suggest if I can make any wiring changes. Please help. Could only find manual in French.
Final Configuration on Nest
Installing Nest on Viconics P117 Board
home-automationhvacnest
Related Solutions
According to the Manufacturer's documentation, the common wire from the transformer connects to the board at pin 6 of the Molex connector.
According to the wiring diagram, there should be a junction off the transformer, where one wire goes to the Molex connector, and another goes to the ground (chassis). This junction may be directly at the transformer terminals. This would likely be the best place to connect your new C wire.
Make sure you use a voltmeter, to verify that those are indeed the correct pins.
If there's not an extra wire in the cable that runs up to the thermostat. Then you'll have to pull a new wire/cable.
Had someone come look at it last night. It was the capacitor on the unit. (Said that the top was bulged meaning it was bad) He replaced it, works like a charm! $100. I was convinced the whole thing was going to need replacing!
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Best Answer
It looks like the P117 + E117 was an intelligent system. Sort of like a NEST, but in 2 parts. This is different from the typical (in the US at least) thermostat where the thermostat - whether an old Honeywell dial, a typical digital setback thermostat, or a newfangled NEST - communicates to the HVAC equipment using a fairly standard set of wires. From the NEST manual:
You can actually see some of these wires in the P117/E117 diagram. For example, the upper right shows connections to C/X (the /X doesn't match NEST), R, G, W2; the right shows C, R, O/B, Y, W.
The problem is you can't just rip out the P117 and E117 and put in the NEST because you only have a 3-wire connection (which is all two computers need to talk to each other - equivalent to an old serial port send/receive/ground). You will likely need to find a way to run a new cable with more wires from the actual HVAC equipment directly to the NEST and then you can remove BOTH the P117 and E117.
Based on your comment that you already have a cable with the standard colors, you need to:
One critical piece is power. If I understand the diagram correctly, power is supplied on C/X and R in the upper right. The NEST needs power and that is listed as C, RH, RC. So C/X --> C and probably R to your choice of RH or RC. But I can't say for sure. The good news is that it looks like the C/X - R is 24V AC, which is standard for the US.