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T-Stat "G" wire question

Bucky
Bucky Member Posts: 47
edited October 2021 in Thermostats and Controls
Hi all, I amongst the hordes trying to find a "C" wire for a smart thermostat. In the past I've run new wire from the t-stat controlling my radiators- no problem there. I did all of the control wiring on my floor/boiler/indirect/fan coil system, so I *thought* I was smarter than the average DIY bear! (Note: I only used the term "smarter" to match the yogi quote!)

So now I want to replace the A/C t-stat with an ecobee. This is the one thing I didn't wire myself- the HVAC company installed the ductwork and air handler in my crawlspace, I showed them the 3/4" hydronic loop (h/t to "Pumping Away" for teaching me how to set that up!) and the T/T wire from an SR-504, and the 5 wire from the t-stat location and let them do their thing- Installing the Spacepak air handler w/hydronic coil. That was 16 years ago. (Despite my initial conversations about how many wires I needed to not need batteries in my t-stat, I need batteries!).

So now when trying to install an ecobee, I have a better understanding about what happened way back when. My SR-504 of course has a 24vAC transformer to feed the T-T wires. And the air handler/compressor control of course requires its own 24vAC control circuit. So they had to take the jumper out of the t-stat that was between R and Rc and create two separate circuits: R and W and Rc and Y. That's the part I understand. But what I don't get is how they wired up the the fan relay. it works- whether I'm using it for A/C or heat (it's actually almost "emergency heat", as the radiant floor heating handles any day unless it drops below 10F), or even turning on "fan only". It seems like the G wire from the t-stat goes to a fan relay and not the spacepak control panel. And likewise, there's another wire (I think from the outdoor A/C) that's also connected to the fan relay. (I just had rotator cuff surgery, so it's not easy to go into the crawlspace and figure stuff out for more than a few minutes at a time!).

So short story long: in this scenario, where would the G terminal get its hot leg from? R or Rc? The W seems to go to both the SR-504 and the relay- (disregarding the terminal block in the spacepak control unit). The Y wire does go to the control block, which does seem to go to the relay somehow.

I guess at this point I'm resigned to the fact that I have to pull another wire if I want to hook up the ecobee, but I'm also confused about how the fan operates with only one wire G (presumably a "neutral") in the t-stat with the funky wiring in place. If the G was hooked up to the terminal block, I would assume that the t-stat would connect either R or Rc to it whenever appropriate, but given that the G circumvents the terminal block I'm really confused....

Comments

  • GW
    GW Member Posts: 4,493
    Ecobee is fussy with the G terminal 

    Once, I’m pretty sure I just spliced onto the Y, and connected the G. That seemed to satisfy the thermostat. That was too many jobs ago for me to remember the exact details.

    Perhaps I did not thoroughly absorb your entire message. But when I install a system that connects to an air handler and a boiler, The thermostat wire is going to the air handler. Then, there’s a relay that kicks on the Taco zone control.

    In your situation, I would not run a wire to RC and a separate wire to rH.

    Are you saying the thermostat is controlling a hydronic loop, plus, a hydronic coil and air handler?

    Are you saying your radiant floor heat is acting as an auxiliary function?
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
    Bucky
  • Bucky
    Bucky Member Posts: 47
    Thanks for the response Gary! This t-stat controls a Spacepak A/C which has an auxiliary hydronic coil. The radiant floor is on a separate t-stat and a separate "zone" on the SR504. The two separate R and Rc wires are in place today- trying to determine how to best deal with that.

    I guess my main question (last night!) was how does a "G" wire function? Looking at the below schematic, seems it needs 24v applied to it and it will then activate the Fan Relay. But where does it get its 24v? The way mine is wired today I think it *has* to be Rc, as R is from a different xfromer (the SR504).

    I did do a bit more digging. I came up with the "preferred" wiring diagram for this Spacepak unit that has a hydronic coil. In this form, it's pretty simple. But..since my "circulator relay" (on the bottom) is actually its own 24v source (from the SR504), they scrambled when they hooked it up and left a bunch of spaghetti. (Spaghetti that works, but pasta nonetheless!)

    Seems like the simplest thing to do would be to add a 24v relay where "circulator relay" is. So the control voltage for the relay would be derived from the Spacepak xformer, and the switched circuit would be the SR504 TT. That way, everything could be wired just like in the diagram (C wire and all). But for some reason (maybe they just didn't have a relay?) they pulled an audible. R/W from the t-stat are wired directly to the SR504. Rc is wired to R on the control block, Y to YT. G goes to a wire nut and connects to the fan relay somehow. There are some other leads from the fan relay in the spaghetti. I need to take some time to figure out exactly what is what. My thinking right now is to add that relay onto W, remove all the jury-rigged fan relay circuitry, and wire it up just like the diagram shows. Biggest problem will be figuring out if those fan relay wires were added and can be disposed of, or were taken from somewhere else and need to be restored to their original spot. The physical location of this lends itself to sitting cross-legged and working on the control block, but recent new knees have ruled that out. So I'll have to make do on my side somehow without being able to lean on my right arm...too many years of sports has caught up to me in 2021!!


  • GW
    GW Member Posts: 4,493
    Bucky OK it’s a little hard to figure this out over the text. My brain works better with less information. It’s nice that you’re going to long descriptions, but less is more when it comes to technical and wiring controls 

    I cannot think of any reason that you would have an additional transformer. Makes zero sense. The Taco control requires a simple switch to activate that zone. Also called a dry contact. 

    just to confirm, the floor heat has nothing to do with this conversation? That helps me know what to talk about and not talk about. 

    I really like the RIBrelay. Super simple and it has a cool little led light

    The picture below is an air handler with a hot water coil. The ecobee thermostat is only using RC. Sounds like you need to clean up that rats nest, get the wiring under control. 

    The coil side of the rib relay would connect to the C-terminal of the air handler, and the W terminal of the ecobee. The relay side of the rib would use common and NO (normally open). Those to connect to the TT on the Taco






    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
    Bucky
  • Bucky
    Bucky Member Posts: 47
    edited October 2021
    Thanks again Gary! I was eventually going to ask for a relay recommendation!

    So I just dove into my SR504 and came back with a repressed memory :smile: . When I envisioned all this, I had studied "Pumping Away" and designed a single-pipe primary secondary system and hired a steamfitter to install all the piping. I left two stub-outs for this hydronic loop. When I got back from my day job, I noticed that they had used only one of the stubouts for the supply and cut into the existing return line to add that return there. Guy told me I didn't know how a primary/secondary worked. (In fact, he had just never seen a single-pipe primary/secondary before). I told him well, the way you piped it, whenever any of the other zones call for heat, the system circulator will send hot water through my hydronic coil- even in the middle of A/C season. He said try it, no it won't! Well, guess who was right. They came back out and their remedy was to add a zone valve which only opens up when that zone calls for heat- NOT to put the pipe in the right spot. I should've stamped me feet and insisted they do it right, but I was in the middle of building my conservatory addition that necessitated all these changes and let it stand.

    So now I see/recall what they did- there's a transformer stuck to the bottom of the SR504 that is wired to the t-stat (through the spacepak) and feeds the zone valve. the zone valve looks like it has an end switch that trips when open because terminals 2 and 3 are wired back to T-T on zone 3 of the SR504.

    So that solves the mystery of the second transformer. I thought the SR504 supplied 24V to T-T, but you have corrected me there.

    I guess step #1 is to get someone out here to re-do that return into the right spot so I can get rid of that zone valve and transformer. You're not in the Philly area I guess, Gary?? :wink:

    (and you are correct- the floor heat is incidental to this conversation. I threw it in there to illustrate why this hydronic coil only gets used rarely).
  • GW
    GW Member Posts: 4,493
    I'm in Mass, gotta be someone around there that can wire stuff. Controls is a little but wild west- some electricians don't have a clue. Some heating guys don't have a clue.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • Bucky
    Bucky Member Posts: 47
    Ah bummer. I think I've figured out the wiring part. I just need someone to do the plumbing part so I can 86 the zone valve. I could use a relay to get the spacepak side sorted out, but I think it's time to bite the bullet and correct the atrocity that was hoisted upon me 15 years ago :) .

    Thanks for engaging! Although I am long winded (I always think I need to provide every bit of detail!) your replies educated me and made me keep searching for the real answers!
    GW
  • mattmia2
    mattmia2 Member Posts: 5,323
    Sounds like that is worth learning how to sweat copper.
    Bucky
  • Bucky
    Bucky Member Posts: 47
    edited October 2021
    I'm not half bad at sweating, but very slow. The problems with this scenario are (1) that since the indirect DWH is on this circuit, it can't be spread out over a couple days. And (2) the new tee have to be added in very close proximity to an existing tee (the 1.5 diameter rule). And I'm not confident that I can do a new sweat that close to another sweat without compromising the existing. I'm not very good at delineating when I should leave something to a pro instead of doing it myself, but I think this is one of those time!

    *- I didn't disclose (in order to not confuse things further) that there are TWO p/s circuits that weren't plumbed correctly. The first was when I had the steamfitter put the whole system together. Despite much communication, and many drawings, when he finished up the return for the DHW circuit was not next to the supply- it was on the bottom pipe (which in this case is the return path). So while I'm fixing things, that should get corrected also. That's where I first learned the "zone valve" trick. Shortly after putting this system into service, I was getting superheated DHW. I then realized what was happening- whenever the system circulator was running, the DHW heater was getting boiler-heated water pushing through it, whether the zone was active or not. So I came up with the concept of a zone valve. Which, for posterity's sake, I wired up much simpler (IMO better) than the way the HVAC company did when correcting their problem (which I told them would happen because I had just seen it happen!). I wired the transformer for the zone valve off of the circulator circuit- so when the circ was running, the zone valve would open. I'm considering a preliminary "correction" of the spacepak circuit so I can simplify that circuit prior to having the plumbing re-done in the proper manner (and add the "C" wire to the t-stat). My only misgiving with that plan is that it eliminates the urgency to finally get the root cause connected, and, knowing me, I'll procrastinate!
  • Bucky
    Bucky Member Posts: 47
    To show you what I'm dealing with, this first pic is the incorrect return from the DWH. The blue valve bottom left is the return (on the return loop). The top right is the supply side for that.



    Here is a better view of where I have to put that return. There's space to get it in there, but I don't trust that I can solder a tee in there at the appropriate minimum distance without un-soldering the supply tee:



    The empty stub on the right there is where the HVAC company should've put the supply for the spacepak loop, which they put on what should be the return loop immediately to the right. So the supply line has to come off and connect to that empty stubout- and then the return line has to be re-routed from behind the boiler to the stubout that the supply is currently in. Flow is from left to right on the top loop, then after traversing the p/s runs from the radiant control panel (pre-built Weil McLain jobby) the loop does a u-turn and returns via the bottom pipe, where the flow is (obviously) right to left.

    Probably not expert level stuff, but seems to be a little above my skill and speed level.