Where's the C Wire????

AdmiralYoda

This may be painfully obvious to you pros...but I'm having a bit of difficulty figuring out where to wire the c-wire.  

I'm installing a new thermostat and running a new 18/5 thermostat cable back to the boiler.  I have a 4 terminal 24VAC transformer...120V in, 24V out with a red and a white wire.

Below is a picture of my control circuitry.  Where would be the best place to put the C wire?  Thanks!!!

Big Ed_4

24V 1.

mattmia2

C is 24V(1) R is 24V(2). There are usually either quick connect or screw terminals available to connect to at the transformer. There should be an overall schematic for the appliance somewhere inside a cover or something too.

HVACNUT

Yeah that's the ignition control. You want the transformer. Post those pics.
Is it a... Nest?

AdmiralYoda

Thanks guys!  My boiler is a bit of a mongrel.  It is a Peerless G461 which to my understanding was the predesessor to the Series 61 with very few changes before the company changed hands back in the 80's.

To make it more confusing, my access cover is from a series 61 and the diagram is in pretty rough shape.

Here is the wiring diagram out of the Series 61 manual which is about as close as I'm going to get.  Pretty sure I don't have an isolation relay like the diagram suggests either.

AdmiralYoda

@HVACNUT I can take pictures of the transformer but it's attached to a small enclosure with a bunch of wire nuts inside.  I'm pretty sure the wires on the transformer go directly to 24V 1 &2 though I need to confirm that.  The transformer on initial inspection seems to have flying leads vs screw terminals.

Oh, and it's an Ecobee, whatever the newest model is.

mattmia2

Try not to make a mess out of it. Installing a terminal strip might not be a bad idea.

EdTheHeaterMan

If you experience problems with your thermostat after you make all the proper connections, you may want to look at this info.

Some thermostats that require 24 Volts and a Common may not operate properly and issue an error code or lockout if the 24 Volt power supply is interrupted during a call for heat. The wiring diagram on the control indicates such a problem. Check to see if the actual furnace or boiler wiring diagram has this situation or has been modified with this issue.




A heating pro can usually do a workaround for the problem (if needed).

AdmiralYoda

Thanks @EdTheHeaterMan!  I'll have to dig around a bit.

Here are some pictures of the transformer.  Not exactly much to see except most of the wires go to the junction box where the ignition control module lives.

HVACNUT

On the diagram, the wire labeled "O (Y)" at the transformer. It continues to a splice with Y to the vent damper and W to terminal 5 on the ignition control. That whole mess is Common. Splice, or crimp, or screw your thermostat wire wherever you can.

EdTheHeaterMan

From the looks of your diagram, (which was not available when I posted my comment) your thermostat does not lose power by way of any limit control. You should not have that problem. Follow @HVACNUT instruction above.

AdmiralYoda

@EdTheHeaterMan I have the G-461 which is the predesessor to the 61 series which I posted above.  I'm 90% sure I don't have an isolation relay and my ignition control is a Honeywell S86F vs a S8610F in the diagram.  Probably some other differences too.

I can confirm that my T-Stat loses power when the Vaporstat or P-trol kicks in.

mattmia2

AdmiralYoda said:

@EdTheHeaterMan I have the G-461 which is the predesessor to the 61 series which I posted above.  I'm 90% sure I don't have an isolation relay and my ignition control is a Honeywell S86F vs a S8610F in the diagram.  Probably some other differences too.

I can confirm that my T-Stat loses power when the Vaporstat or P-trol kicks in.

Does that make it unhappy if you give it C?

dabrakeman

I had to have an isolation relay put in when an Ecobee and new C wire was installed on my boiler.

Steve_Wheels

Why not just purchase a Taco SR501, has R/W/C connections for your new Ecobee and an isolated end switch to tell the boiler you have a call for heat.

AdmiralYoda

Thanks guys, these are all great suggestions!  I'm not an HVAC guy, just a DIY'r and tinkerer but I do have a quick question.  

Wiring a C wire directly off the transformer or 24V-1 on the ignition module is very easy (and free).  Is there a reason this is a bad idea?  Sure, I'm adding 150mA extra draw to the transformer but I'm sure it can handle that.  Am I missing something?

mattmia2

You have to make sure the way the operating control chain is wired so that r is connected directly to the transformer or at least that anything between r and the transformer doesn't open during a normal cycle, safety limits and rollout switches would be ok but the pressuretrol and anything involving flame proving needs to be between w at the thermostat and the gas valve, not between the transformer and r at the thermostat. If that is not the case, an isolation relay is the simplest and safest way to solve it.

AdmiralYoda

Thank you @mattmia2, that answers my question.  It can be done wiring directly ...just has to be done right.

Is there a common isolation relay you pros typically use?  

EdTheHeaterMan

This diagram is from the book you have on the boiler from 2014. I believe it should be accurate. I have added a Blue wire for the common. I have also highlighted the Red wire from the thermostat directly from the Transformer.

EdTheHeaterMan

If you do the wiring correctly, you should not need an isolation relay. (because there is already one there.
You will need to find the wire from the thermostat and determine which of the 2 conductors is connected directly to the transformer (That must go to R on the thermostat). Determine which of the 2 conductors goes to the relay. (That must go the W on the thermostat.)

When using only 2 wires, it does not matter. When using a Common, you must know which one is which.

Once you are sure that the R is connected directly to the Transformer, then you can connect the C to the other side (the common side) of the transformer.

Does that Help?

AdmiralYoda

Super clear @EdTheHeaterMan !  My R wire is black and I do know that it bypasses the limit controls and makes its way to the LWCO.  I'll have to see where it goes from there.  My W wire goes through the limit controls and is interrupted when a limit is reached.

If it gets confusing I'll whip up a schematic of my actual wiring and post it here.  Thanks again!

HVACNUT

This isn't rocket surgery. The Common is just a ground. You can pick it up anywhere from terminal 5 on the ignition control on back.

mattmia2

If the pressuretrol/vaporstat is in the w the you shouldn't need an isolation relay ( @EdTheHeaterMan , somewhere it says his boiler is the predecessor to that one and does not have the isolation relay). Those other controls might interrupt power to the t-stat but those will only open if something else is preventing it from heating for an abnormal condition anyhow, they won't open during a normal cycle.

AdmiralYoda

Thank you @EdTheHeaterMan, @mattmia2, and @HVACNUT.

Since a schematic does not exist...I decided to make my own.  Here ya go!

AdmiralYoda

@mattmia2 you are correct.....but as I understand, without the C wire the newer thermostats will get a bit angry if one of the limits is reached and the circuit is opened.

My boiler is oversized and will cycle on pressure when its very cold or if I use a setback, so this will happen often enough to warrant a permanent C wire.

For now I plan to install the C wire to 24V(1) in the above diagram.

mattmia2

i thought i replied to this. Your schematic doesn't match the newer model. white and red are reversed to your t-stat. you will need an isolatuion relay or to insert the t-stat between the transformer and the lwco in this case.

EdTheHeaterMan

You can rewire the thermostat this way?

notice the thermostat R is on the Blue wire directly from the transformer.


This should be an easy rewire.

AdmiralYoda

Thank you @EdTheHeaterMan this is exactly what I plan to do. Super easy.