Weil-McLain 120-24 Volt Transformer /with relay


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Old 04-19-21, 02:33 PM
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Weil-McLain 120-24 Volt Transformer /with relay

Can you tell me what each of the terminals on the transformer are used for and what the voltage out of each terminal should be Terminals R , C , W , G, Y
I have a low voltage on my Rc terminal on my thermostat and I am trying to locate the problem
I have a BurnhamGas fired hot water boiler with a Trane A/C unit
 
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Old 04-19-21, 03:40 PM
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Welcome to the forums.

If you have a typical boiler setup.... the transformer is attached to a mounting plate with a relay.
That is called a fan center.

You will have 24vAC from R to C.
You will have 24vAC from R to G.
To activate boiler.... a short is placed across R and G. That will close the relay.
At the fan center.... G is used. At the stat.... it is on W.

W and Y are blind terminals used as a splice and have 0v on them.
There is nothing connected to W and Y inside the fan center.
 
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Old 04-19-21, 06:26 PM
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Low voltage at Rc terminal at thermostat

I am trying to replace my old Honeywell thermostat with a new WiFi thermostat but I only get 16 volts
at the Rc terminal , I have a gas fired hot water Burnham boiler and a Trane central A/C unit.
The transformer on the boiler is a Weil-McLainn 120-24 volt model # 510312167 can you tell on the transformer what the voltage should be at the R,C,W,G and Y terminals should be and what is terminal is used for , where does the Rc wire go and come front ? any help in troubling showing this would be greatly appreciated Thank You
 
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Old 04-19-21, 08:13 PM
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Welcome to the forums.... again.

I combined your threads. One thread per topic is allowed.

Rc is the 24vAC source.
On your fan center..... G is boiler run.
C is common.

So you'll need a wire from Rc to Rh on the stat.
A wire from G to W on the stat.
A wire from C to C on the stat.

If you have A/C in the mix.... there are more wires.
If you have A/C shoot a picture or two of the wiring and post them.
How-to-insert-pictures.

 
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Old 04-20-21, 01:59 AM
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Pictures

The old Thermostat does not show the wire I connected to the C terminal on the transformer and at the thermostat ( yellow wire ) I check with a volt meter at the thermostat C terminal to
Rc to C 16 volts
R to C 0.15 volts
W to C 27.42 volts
Y to C 9.16 volts
G to C 9.12 Volts









 

Last edited by PJmax; 04-20-21 at 10:25 AM. Reason: cropped/resized/labeled pic
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Old 04-20-21, 10:32 AM
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Thanks for the pictures. They gave me a 100% view of your system.
Here we go......
You have one cable that goes to the boiler.
Currently white is on W, red is on R and green is unused.

You have a second cable that goes to the A/C air handler.
Currently white is on Y, red is on Rc and green is on G.

With a wifi or a smart stat..... the C must come from the A/C system.
That means you cannot get C from the boiler. That is not system C.... it's boiler C.

It would be best to run a larger cable to the A/C air handler. If that can't be done..... you can convert the G wire to a C wire. That means the A/C fan will only come on when the A/C is calling for cooling. The FAN ON position will no longer work.
 
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Old 04-20-21, 01:20 PM
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When you say convert the G wire to C wire do you mean just put the G wire on the C terminal of the thermostat ?
 
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Old 04-20-21, 02:26 PM
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No. You need to move the green wire to the C location at the air handler.
Then the wire that was connected to the green now goes to the white wire.

 
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Old 04-20-21, 08:10 PM
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Thank you for your help, Now I have to find the C terminal at the air handler
Thanks
JK
 
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Old 04-21-21, 10:16 AM
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It is common that both heating and AC have separate 24 volt transformers.

Heating transformer 24 volts wires go to C and Rh..eating.

AC transformer wires goes to both C and Rc ..ooling.

When one transformer is used for both heating and cooling Rh and Rc are jumper-ed. Thermostats often come with jumper.

Multlple transformers can cause problems. DH uses one 70 VA 24 volt transformer for everything. Is fused and has 24 volt pilot light to show active.
 
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Old 04-21-21, 05:48 PM
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Can't use one transformer here without major rewiring.
We're talking two completely independent systems in separate locations.

If you have trouble with the air handler wiring.... post a picture or two where the stat wires connect.
We'll help you ID them.

Typically, but not always, blue is C. Many times you can trace what looks like the C wire and find that it connects directly to the air handler chassis. Most air handlers have either specifically colored wires or tags on wire leads.

Both your stat and condenser wiring should meet at the air handler. The condenser will connect with two wires. One will go to Y (white stat wire) and the other will go to what is C.
 
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Old 04-22-21, 01:37 PM
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Wiring diagram found inside Wire handler, I see the blue wires and they also go








 
Attached Images
File Type: pdf
Air Handler Wiring Diagram.pdf (397.8 KB, 7 views)

Last edited by PJmax; 04-22-21 at 03:12 PM. Reason: trimmed pics
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Old 04-22-21, 03:18 PM
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In your pictures.... the blue connection confirms C.
The wiring in the yellow circuit is two cables. Stat & condenser. That's what I need to see.
You will have red, white and green from stat connect.
Probably only two wires on the other cable connected.

Wiring diagram is very handy.
 
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Old 04-22-21, 03:46 PM
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Do I just add a wire to the transformer at the Blue wire labeled C at the low voltage field Connection
 
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Old 04-22-21, 03:55 PM
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You can if you have it. I thought you only had three wires and all were used.
 
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Old 04-22-21, 04:00 PM
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I will have to run a wire from the air handler transformer at the blue wire down to the C terminal at
the thermostat is that correct ?
 
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Old 04-22-21, 06:44 PM
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That is one alternative.
The other was losing the FAN ON mode and make a wiring change.
 
bossjohn voted this post useful.
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Old 05-02-21, 05:53 PM
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Thank you so much for helping me out, I just finished running the wire from the air handler C terminal to the C terminal on the thermostat and it works just fine.

Thank you for your time and help

Bossjohn
 
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Old 05-02-21, 07:11 PM
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Good job. You're welcome. Thanks for letting us know how you made out.
 
 

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