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C wire help

sscerberus
sscerberus Member Posts: 7
I'm not that experienced with HVAC wiring to start to I apologize but I'm wondering if I could get some input regarding installing a C wire to convert a 2 wire system to to a three wire for a programmable thermostat like the EcoBee3. The power extender kit they include does no good for me they said as I would still need to have a third wire at the thermostat.

First question is this. At the thermostat when not actively heating (it is a heat only system) I get a voltage reading of 26.5v across my Rh and W. When the unit is heating I get 0v which makes sense to me because the mercury thermostat installed now shorts it. At my boiler 24v transformer I get 26.5v across R and C. I also get 26.5v across R and G. I get no voltage across C and G. When the unit is running I get 0v across all of them. So I know R is my 24v
I'll also say that in my wiring the 24v transformer terminals go to the follow. C goes to W at the thermostat. And R and G are combined to go to Rh at the thermostat.

So if I add a third wire for the C at the thermostat that these new thermostats require... What terminal am I supposed to wire it to? I would have guessed R but when the unit runs it loses its 24v. I don't have a constant 24v output on any terminal. I also only have R C G (there is no Y or W on the transformer in the boiler... Well there is but it is a Honeywell R8285D transformer and there are no connections to Y or W)

I was also looking at the fast-stat common maker but I don't know if that would work again being that i only have R C G at the transformer.

Comments

  • hvacfreak2
    hvacfreak2 Member Posts: 500
    edited February 2016
    Terminals R and C represent the secondary side of the transformer with R being Hot and C being Neutral. In commercial work we usually ground the Neutral side of the transformer as it is often shared with DC voltage control signals. Any load device will require hot and neutral to energize , weather it is a relay coil or an electronic circuit in a digital display thermostat. Some electronic thermostats are designed to " work around " not needing a neutral wire by referencing ( stealing ) neutral from a non energized terminal that is connected to neutral by default ( electro mechanical loads , these will not work with triac or other solid state loads ).

    Terminal G at the boiler is due to the repurposed fan centers that are often used by manufacturers to isolate burner and control transformers. Your description of the current wiring seems to indicate a dead short on each call for heat. The correct wiring at the stat would be R , W , C going to R , ? , C at the boiler. The only question being what will W from the stat do ? It should go to G at the fan center to energize the relay but it seems that something else is being done with it.



    hvacfreak

    Mechanical Enthusiast

    Burnham MST 396 , 60 oz gauge , Tigerloop , Firomatic Check Valve , Mcdonnell Miller 67 lwco , Danfoss RA2k TRV's

    Easyio FG20 Controller

    Steve Minnich
  • sscerberus
    sscerberus Member Posts: 7
    Nope C terminal is defiantly going to W. I made this wiring diagram when I was previously talking with Nest.
  • sscerberus
    sscerberus Member Posts: 7
    So you think I could just connect a new c wire to my c terminal? How come I can never get a voltage reading from C then?
  • hvacfreak2
    hvacfreak2 Member Posts: 500
    edited February 2016
    If you notice in your diagram C is grounded , G energizes the burner relay. C is neutral for the entire control circuit. W connects to the zone valve that it does now.

    Also , the power to the stats is broken at the aquastat. Those stats will require power all of the time. The aquastat may need to break one of the wires on the contact side of the fan center.
    hvacfreak

    Mechanical Enthusiast

    Burnham MST 396 , 60 oz gauge , Tigerloop , Firomatic Check Valve , Mcdonnell Miller 67 lwco , Danfoss RA2k TRV's

    Easyio FG20 Controller