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.enter image description here enter image description here Final Configuration on Nest Final Configuration on Nest

  • What model furnace & heat pump do you have? @ThreePhaseEel raises a good question as to how the P117 is controlling the furnace. If it is acting as the control board of the furnace rather than "a fancy thermostat" then you're out of luck. But if it is essentially a fancy thermostat then my "move the wires" idea should work. – manassehkatz-Reinstate Monica Oct 23 '18 at 4:18
  • The furnace is KeepRight and heat pump from Lennox. I don't have specific models as I recently purchased house and previous owner didn't provided this info. Contacted one Nest Pro, he said he would do rewiring but is charging double of what I paid for Nest retail. :( – user3813989 Oct 23 '18 at 15:47
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    Yes, thats why its time to take matter in my own hands. Just scared before I rip any wires and screw up the HVAC unit. for this reason, I am looking for advise from experts here who can understand this circuit. – user3813989 Oct 23 '18 at 15:57
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    If you can post pictures of the wiring going between the P117 and the furnace & heat pump, that may help us figure this out. – manassehkatz-Reinstate Monica Oct 23 '18 at 16:07
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    Yes, a photo showing the wiring from the P117 board to the other units more clearly would be helpful – ThreePhaseEel Oct 23 '18 at 22:52

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:

  • Heating: one, two, and three stages (W1, W2, W3)
  • Cooling: one and two stages (Y1, Y2)
  • Heat pump: with auxiliary and emergency heat (O/B, AUX, E)
  • Fan (G)
  • Power (C, RH, RC)
  • Humidifier or dehumidifier (HUM, DEHUM)

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:

  • Find and label all connections from your HVAC equipment to the P117.
  • Replace all the connections from the P117 with the appropriate thermostat cable wires.
  • Connect the NEST to the matching wires.

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.

  • sorry,could please you explain line again "You will likely need to find a way to run a new cable ....". The wire has all standard colors inside but just that they are not connected to Thermostat or board. – user3813989 Oct 23 '18 at 2:37
  • Aha! Based on your limited description, I assumed that you just have a 3-wire black/white/red cable. I will update my answer. – manassehkatz-Reinstate Monica Oct 23 '18 at 2:40
  • It appears that the P117 board is a combustion controller, so it's probably not so trivial to replace... – ThreePhaseEel Oct 23 '18 at 4:14
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    @ThreePhaseEel , I am assuming you maybe right, I haven't checked if there any other control boards for Furnace and Heatpump and P117 could be just thermostat. Will go to the Furnace area today and check it – user3813989 Oct 23 '18 at 15:54
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    I removed P117 and ET117 all together and directly connected to furnace/Heat pump. After some hiccups ( Sleeping in cold for one night) and things seem to be working. Could only check Heating from Heat pump and Emergency Heating. Nest wouldn't allow to test cooling as outside temp is 1 Degree Celsius. I am shocked that Nest does not support Defrost cycle of thermostat so had to let go that feature . Thanks for your help it help greatly. Will add some pictures in original question. – user3813989 Oct 26 '18 at 15:11

It appears your system does not support the Nest, and likely cannot

The P117 control board uses a proprietary protocol to talk to the thermostat, it appears. As it's the primary combustion control for your indoor unit, I would hesitate before throwing it out -- it may be the case that your system is not compatible with more "generic" furnace or boiler controls, even. Or in other words, the Nest may have to go, because the P117 board can't.

  • I'm not so sure about that. The fairly standard wire labels - e.g., C, R, O/B, Y, W, etc. - seem to imply the E117 + P117 set are together in place of a normal thermostat, and therefore could be replaced by a Nest. – manassehkatz-Reinstate Monica Oct 23 '18 at 4:16
  • @manassehkatz -- look more closely at that -- the standard labels are only for the connection from the P117 to the outdoor unit – ThreePhaseEel Oct 23 '18 at 4:22
  • Not 100% clear to me. My french pretty much starts & ends with noir/blanc/rouge. My wife knows french but knows nothing about HVAC. If the OP lets us know the model # of the furnace & heat pump it should be easy to figure this out (and you may turn out to be right). – manassehkatz-Reinstate Monica Oct 23 '18 at 4:33
  • The key is the symbols in the top right of the posted diagram, and what they connect to, @manassehkatz – ThreePhaseEel Oct 23 '18 at 4:44
  • I checked the wiring, I am leaning toward the idea of removing P117 and directly using Nest. The question now is, I have 2 C wires one from each heat furnace and heat pump and two re R wires, can jump them and use in Nest? – user3813989 Oct 24 '18 at 12:24

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