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New Nest Thermostat C-Wire on Taco SR502

I recently bought 3 of the new Nest thermostats (not Nest-E or Nest Learning) and want to replace my heat-only thermostats in two of my zones and add a powered C-wire to each to avoid issues related to the battery running down I’ve read about.

I have a gas boiler that serves my hot water tank and 3 zones: (1) Basement radiators, (2): Main floor baseboard, and (3) 2nd floor hydro-air. The second floor also has AC that runs through the same air handler. Everything on the 2nd floor works fine, and I successfully replaced a 5-wire thermostat (with functional C) with the Nest. The basement and main floors have only heat and the thermostats I want to replace are simple, battery powered, and use only the R and W wires. In the bundle of wires behind each, there are extra unused wires, including blue, which I hope to use as a C-wire.

I know very little about HVAC wiring, so I had a guy come out to help me install. He located the blue wires associated with each thermostat and tried connecting them to the “comm” port on the Taco, but could not get a voltage reading (and the Nest could not detect the C-wire). He then attached the C-wires directly to the nearby transformer that runs the zone valves. This seemed to have done the trick, as the Nests now recognize the C-wire and I don’t get any power-related error messages. However, as soon as the Nest tries to call for heat, the “No Power” message flashes on the screen and the zone valve does not engage. If I go to the power menu on the Nest, I see ~30V reading, and as soon as heat is called for it drops to ~2V. When heat is no longer called for it goes back up to ~30V.

My uneducated guess is that the transformer is diverting all of its power to flipping open the valve and leaving the tstat unpowered. However, it’s a 40VA transformer, and it services all 3 zone valves, and only 1 is being called for, so I really don’t understand why it wouldn’t be able to continue to service the tstat. My HVAC guy pretty much threw his hands up at that point and said maybe a 60VA transformer would give better results, but he really didn’t know (nor was he really interested in helping to continue to debug).

My questions are:

1. Is it expected that comm on the taco should be dead? (I assume not), and are there any common reasons or remedies?

2. Is it okay to connect the C-wires directly to the transformer (connected to the same contact as the white wires of the zone valves, which I believe makes contact with the white tstat wires.

3. Where would you go from here if you were me? Bigger transformer? Would a separate plug-in transformer work? And if so, how should I connect it? (I have 24V doorbell wires I can borrow directly behind the tstat for testing, but I'm afraid to hook them up in a way that will damage something).

I attached a photo of the controller in case a high-level view is helpfu, but it’s just a rats nest with all the tstats and zone valves. Zone 2 is unoccupied on the Taco, which seems odd to me, given I have at least 2 tstats that terminate here. I wish I could tell you more (and I will try my best to answer any questions) but I’m hoping from the descriptions someone can at least give me some possible directions of investigation. Thanks in advance!


    EBEBRATT-Ed Member Posts: 12,328

    See the post "NEST Wiring" currently on the 2d page here. I think you will find what you need there. Nest wiring is one of the most common questions on here.

    IMHO a separate transformer and relay are best but there ar a few work arounds. The Nest steals power and don't work well on a two wire set up. And they draw a lot of power
  • Jamie Hall
    Jamie Hall Member Posts: 19,383
    To @EBEBRATT-Ed 's comment, I would add that the voltage change you are seeing indicates that somewhere in that rat's nest you quite literally have your wires crossed, with what should be common being power, and vice versa. Hard to say where.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SteveSan
    SteveSan Member Posts: 123
    @fusilli_jerry Please give us a call at 401-942-8000 and ask for Tech Support. We are here Mon-Fri 8am-5pm EST.
  • brnrman1
    brnrman1 Member Posts: 32
    Jamie Hall for the win we see this issue all the time the polarity matters when playing with the WFI thermostats
    Roger Mitchell
    Senior Technical Representative
    Energy Kinetics
  • fusilli_jerry
    fusilli_jerry Member Posts: 2
    Thanks all. @Jamie Hall if my wires are crossed as you suggest, is it potentially as simple as switching the C and RH wires at the thermostat? Naively that doesn't make a ton of sense to me given the old dumb thermostat worked off the of RH and W wires. If anything is switched it seems like it should be those two, but I quickly lose track of the logic of the thing when we get to 3 wires.
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,781
    Here's some light reading...