Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Steam Boiler to tstat wiring

I have a weil mclain steam boiler. PEG-30A-S-PI, nat gas, single pipe/zone. Trying to wire up a wifi tstat with a C wire.

Old tstat had w from transformer wired to w and y from transformer wired to R/Rc per this schematic. https://goo.gl/photos/VLr1UA58Escsjgcy7

Just ran 18/5 wire and trying to figure out how to wire up new t stat to the transformer properly with a C wire. Should I also connect up R from the transformer? Any help is much appreciated!


    HVACNUT Member Posts: 4,562
    The Y terminal on the tranformer/relay is essentially your power (R) to the thermostat. W to W, and there should be a C terminal on the transformer/relay. Maybe existing blue wires connected to it.
    Post a pic if you need assistance.
  • Steamhead
    Steamhead Member Posts: 15,272
    edited July 2017
    What I usually do with these is wire up an isolation relay to separate the thermostat power and control wiring from the boiler's stock wiring. This makes things a lot less complicated, and runs less chance of confusing someone who is looking at the boiler's wiring diagram. Any decent relay with a 24-volt coil will do, and you can mount it in the cabinet near the transformer.

    Wire the "Normally Open" relay contacts to the boiler's thermostat leads. Use the following thermostat cable hookup to the thermostat: Red to R or RH (depending on which thermostat you will use), White to W, Green to C.

    If the thermostat also operates an A/C unit, it should have two "Red" terminals: R or RH is for heat, RC is for cooling. If there is a jumper between the two, remove it, otherwise the transformers in the two respective units may fight each other and burn out.

    Now wire the Red wire in the thermostat cable to R on the transformer. Wire the White wire to one side of the relay coil. Run a Green wire from the other side of the relay coil to C on the transformer, and wire the Green wire in the thermostat cable to C as well.

    This will provide a constant 24-volt feed to the thermostat electronics no matter what the boiler's limit controls are doing.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • dbrez8
    dbrez8 Member Posts: 5
    edited July 2017
    I'm technically inclined but this is new to me so tell me if I understand correctly.. The boiler uses R from the transformer to power the flue damper, and water level. If both of these are OK then it shorts R to Y, which hooks into the t-stat. When the tstat wants to turn the heat on, it shorts Y to W which then runs back to the boiler and fires it up..

    If i just run a C up to the tstat, that will power it, except when the boiler isnt happy and doesn't short R to Y.. So I need to run C from the x-former but also R which will always will be energized. I was just not sure where to hook R from the xformer into the tstat since the tstat already has Y hooked into R as described above..

    I believe I found the answer on the tstat manufacturers website https://sensicomfort.com/support/article/adding-a-24-vac-external-transformer?product=wi-fi-thermostat
    I'm trying to do the same thing as hooking an external 24vac to the tstat. So what I need to do is..
    Y on xformer to RH on tstat
    W to W
    R to Rc
    C to C
    Cut Rc/Rh jumper

    Does this sound right? Is there a reason I'd want to hook up a separate transformer like you stated (and is pictured on the manuf website), over just using R and C from the xfrmr on the boiler? I dont have any problem pulling wires.
  • Bio
    Bio Member Posts: 278
    That will work as well, you can have the transformer plugged at the basement or close by the thermostat

    MG ELECTRONICS MGT2420 TRANSFORMER,24VAC 20VA UL APPROVED https://www.amazon.com/dp/B0010GR07O/ref=cm_sw_r_cp_api_coGyzbN1T2M0X
    HVACNUT Member Posts: 4,562
    Your making a difficult job out of something simple.
    Y at the boiler to R at the stat, only cut the R,Rc jumper if you have A/C.
    W at the boiler to W at the stat.
    C at the boiler to C at the stat.
    You will have constant 24v at the stat unless the circuit opens due to a fault on the damper or LWCO.
  • j a_2
    j a_2 Member Posts: 1,796
    Webb sells a 2 wire wifi t stat....works great, made by lux.
  • ChrisJ
    ChrisJ Member Posts: 12,670
    On my EG series WM boiler, which I believe is the same as PEG the C of the transformer goes to ground. I connected my "C" wire ground on the boiler's chassis.

    You need to see where the C side of the transformer goes, that's all.

    Even though it's AC, for this discussion picture the R being + on a battery. Picture W being +, but switched, it's what turns the boiler on.

    C is simply the other side of the battery, -. It's what you need to get continuous power to your thermostat.

    This is an old picture, and things have changed but the red arrow is pointing to my thermostat's "C" connection at the boiler.
    Yours, would be the same assuming the C side of the transformer goes to ground as mine does.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • dbrez8
    dbrez8 Member Posts: 5
    Thanks everyone for your help!

    Simply adding the C wire didnt power the tstat
    Adding C and R (to Rc on tstat) powered the tstat but the boiler wouldnt shut off when the tstat did
    Finally clipping the Rc/Rh did the trick. All seems to be working well.

    For posterity:
    xfrmr - tstat
    W - W
    Y - Rh
    C - C
    R - Rc
    Clip Rh/Rc jumper