Question about adding C Wire for Thermostat

bmma

I'm new to the forum and tried searching for the answer but couldn't quite find what I was hoping to so here goes. I have a Weil McLain PEG-50 boiler with single pipe steam rads. I currently have a Honeywell programmable thermostat but want to replace it with a WiFi thermostat (most likely the Ecobee 4), primarily for the ability to monitor the temp in the house remotely for when we're traveling. I know the Ecobee, and most if not all WiFi thermostats require a C wire. Current wiring to the thermostat is just two-wire (red & white). I can easily get new wire to the thermostat but I'm curious where on the boiler I should be connecting that C wire. Attached is the wiring schematic from Weil McLain. I'm hoping someone can confirm that the area I circled in red is the correct point to connect the C wire, or point me in the right direction if not. Thanks!

wpmike

I have a two wire boiler and recently connected a Ecobee 4. I got a generous discount purchasing the stat thur my utility company. I purchased a 24 volt transformer and connected it to an electric outlet and used the two 24 V leads from the transformer to power the stat. There is some setup you will need to do on the Ecobee to tell it what wiring scheme you are using. The instructions give some direction on using a transformer if there is not a C wire available.

Here is the type of transformer you may need?
https://www.amazon.com/Packard-Control-Transformer-Class-Mount/dp/B01HPJT7C0/ref=sr_1_5?ie=UTF8&qid=1544468854&sr=8-5&keywords=24-volt+transformer

Voyager

It looks like you are in the right spot.

Jamie Hall

Voyager said:

It looks like you are in the right spot.

I agree.

Steamhead

We install an isolation relay that can work off the 24V transformer on the boiler. This requires a 3-wire cable between the thermostat and the boiler. It creates a separate 24V circuit for the thermostat, and the relay is wired to switch the boiler's "thermostat" leads on and off. This way, the thermostat always has power feeding it as long as the transformer is energized.

Solid_Fuel_Man

You are correct in the C location.

Voyager

Steamhead said:

We install an isolation relay that can work off the 24V transformer on the boiler. This requires a 3-wire cable between the thermostat and the boiler. It creates a separate 24V circuit for the thermostat, and the relay is wired to switch the boiler's "thermostat" leads on and off. This way, the thermostat always has power feeding it as long as the transformer is energized.

Can you explain more the rationale for this approach? I am not seeing the purpose at first blush. My Lennox furnace provides power to the thermostat via R and C as long as the furnace has power.

KC_Jones

Voyager said:

Steamhead said:

We install an isolation relay that can work off the 24V transformer on the boiler. This requires a 3-wire cable between the thermostat and the boiler. It creates a separate 24V circuit for the thermostat, and the relay is wired to switch the boiler's "thermostat" leads on and off. This way, the thermostat always has power feeding it as long as the transformer is energized.

Can you explain more the rationale for this approach? I am not seeing the purpose at first blush. My Lennox furnace provides power to the thermostat via R and C as long as the furnace has power.

The way a lot of these boilers are wired, if a safety kicks in such as the LWCO or pressuretrol, it can kill power to the thermostat. The OP has a Weil Mclain EG series same as mine so they would have this situation.

I never hit any of my safeties so I don't really concern myself with it.

SuperJ

It looks like the the common is grounded on that transformer, so technically the whole backplate would be a C connection.

Voyager

KC_Jones said:

The way a lot of these boilers are wired, if a safety kicks in such as the LWCO or pressuretrol, it can kill power to the thermostat. The OP has a Weil Mclain EG series same as mine so they would have this situation.

I never hit any of my safeties so I don't really concern myself with it.

OK, that makes sense, particularly if you have a Wifi thermostat that you want to be able to send you alerts or such. Thanks.

bmma

KC_Jones said:

Voyager said:

Steamhead said:

We install an isolation relay that can work off the 24V transformer on the boiler. This requires a 3-wire cable between the thermostat and the boiler. It creates a separate 24V circuit for the thermostat, and the relay is wired to switch the boiler's "thermostat" leads on and off. This way, the thermostat always has power feeding it as long as the transformer is energized.

Can you explain more the rationale for this approach? I am not seeing the purpose at first blush. My Lennox furnace provides power to the thermostat via R and C as long as the furnace has power.

The way a lot of these boilers are wired, if a safety kicks in such as the LWCO or pressuretrol, it can kill power to the thermostat. The OP has a Weil Mclain EG series same as mine so they would have this situation.

I never hit any of my safeties so I don't really concern myself with it.

KC_Jones said:

Voyager said:

Steamhead said:

We install an isolation relay that can work off the 24V transformer on the boiler. This requires a 3-wire cable between the thermostat and the boiler. It creates a separate 24V circuit for the thermostat, and the relay is wired to switch the boiler's "thermostat" leads on and off. This way, the thermostat always has power feeding it as long as the transformer is energized.

Can you explain more the rationale for this approach? I am not seeing the purpose at first blush. My Lennox furnace provides power to the thermostat via R and C as long as the furnace has power.

The way a lot of these boilers are wired, if a safety kicks in such as the LWCO or pressuretrol, it can kill power to the thermostat. The OP has a Weil Mclain EG series same as mine so they would have this situation.

I never hit any of my safeties so I don't really concern myself with it.

Is this true if I’m connecting the C wire to the C terminal on the transformer? Wouldn’t the transformer always have power, as long as the main power to the boiler was still connected? You may be absolutely correct, but I’m just trying to understand the functionality of the system and making sure I have a grasp of how it all works to avoid situations like you described. I appreciate all the help.

KC_Jones

Everything that controls the burner is in series, this would include the pressuretrol, LWCO, and thermostat. The thermostat is the last in the chain. For power the thermostat has the ground (C) and the hot. Since the thermostat is last, if the LWCO opens it kills the hot feed for everything after, so the thermostat loses it's power. The wiring diagram is a bit misleading as it doesn't show all the physical connections, just how things are connected electrically.

Remember these are all just switches they don't need or use any power for normal operation, they just interrupt the flow to the load, in this case the gas valve.

bmma

KC_Jones said:

Everything that controls the burner is in series, this would include the pressuretrol, LWCO, and thermostat. The thermostat is the last in the chain. For power the thermostat has the ground (C) and the hot. Since the thermostat is last, if the LWCO opens it kills the hot feed for everything after, so the thermostat loses it's power. The wiring diagram is a bit misleading as it doesn't show all the physical connections, just how things are connected electrically.

Remember these are all just switches they don't need or use any power for normal operation, they just interrupt the flow to the load, in this case the gas valve.

Okay, that makes sense now. Final question I think, let’s say the pressure in the systems rises and the pressuretrol does its job and shuts the boiler down, would the power to the thermostat be restored once the pressure dropped and the the boiler cycled back on? If that’s the case the power to the thermostat would likely only be cut off for a very short period of time.

I’ll probably install the relay you discussed, but again I’m just trying to learn more about how the whole system works. I appreciate your insight and assistance.

SuperJ

You can likely power the thermostat from the boiler's transformer, but drive a relay coil (connected to the C and W from the thermostat) that will connect the boiler's R and W terminals to maintain the series arrangement with the safeties, and maintain power to the stat if/when the a safety trips.

Or, in other words you can use a relay without relying on a second transformer to keep the wiring intact and maintain thermostat power (as long as the boiler is powered).

bmma

Okay, I'm revisiting this thread now that I have some time in my schedule to actually install a new thermostat. As luck would have it I was given a new Honeywell Vision Pro 8000 Wi-Fi thermostat from a friend (long story), and intend to install that. I also happen to have a 24-volt transformer that I can use:

https://www.homedepot.com/p/Honeywell-24-Volt-Transformer-AT72D/202264717

I'm hoping someone can tell me if this would work. I can install the transformer separate from the boiler, although I'd keep it on the same circuit for the sake of consistency. The transformer would then power the thermostat and I wouldn't have to worry about the LWCO or Pressuretrol killing power to the thermostat, and I also wouldn't have to make any changes to the wiring at the boiler itself. Assuming this is a viable option, it's not clear on what terminals in the thermostat I would land the 2 wires form the transformer. There are "R" and "C" terminals on the transformer. I assume the C on the transformer wires to the C on the thermostat, but where would the R land? I believe it would land on "RC" and I'd remove the jumper wire between "R" and "RC" but I couldn't find anything definitive on that. Thanks in advance for the help.

ratio

If the old stat only had two wires going to it (no cooling or anything like that), then yes. RC & C to power the stat from the new xfrmr, remove the jumper between RC & R (or is it RH?), & land the two boiler wires on the R (or RH) & W1 terminals.

bmma

Yes, the current thermostat has just two wires (R and W) and works fine. I'm only changing to the WiFi thermostat so that I can monitor things remotely when I'm away and make sure I don't come home to a frozen house. Thanks for your help.

Solid_Fuel_Man

You need to be able to get the third conductor "C" there. I recommend pulling out the Old Wire and installing a minimum of three conductors to the thermostat from the boiler. Get it to that point and then we can draw you a diagram.

bmma

I have five conductor wire between the existing thermostat and the boiler, but just the red and white are being used. My plan is install the new thermostat and land the red on the R terminal and the white on the W terminal. I'll then use two of the other conductors to connect the R of the new transformer to the RC terminal on the thermostat, and the C of the new transformer to the C on the thermostat.

NY_Rob

I would isolate the t-stat from your existing wiring (disconnect all wires) then connect only the new Xformer to the RC and C terminals and see if your t-stat powers up. I tried to do what you're intending on an old 2-wire setup with a new electronic t-stat and the t-stat would only power up from the R and C terminals, the RC connection was not usable as a power terminal.

bmma

NY_Rob said:

I would isolate the t-stat from your existing wiring (disconnect all wires) then connect only the new Xformer to the RC and C terminals and see if your t-stat powers up. I tried to do what you're intending on an old 2-wire setup with a new electronic t-stat and the t-stat would only power up from the R and C terminals, the RC connection was not usable as a power terminal.

I'll give that a try before removing the old thermostat. Thanks for the tip.

Le John

If there is a jumper between RH and RC remove it.

bmma

Le John said:

If there is a jumper between RH and RC remove it.

I plan to but thanks for the reminder.

bmma

Just a follow-up for anyone that may be interested. I tested the thermostat to see if it would power up from the second transformer. I connected the C of the transformer to the C on the stat, and the R of the transformer to the RC on the stat (first removing the jumper between the R and RC) and the stat powered up fine, and I was able to go through many of the steps in setting it up. I left it powered this way for a while just to see if any issues would become evident in the short term and so far so good. So it appears for this stat at least (Honeywell Vision Pro 8000 WiFi) the RC can be used to power the stat. I'll finish installing it this weekend and get it setup and running. Thanks to everyone that offered your thoughts and assistance.

NY_Rob

Connect the new Xformer to RC and C, and wait till the t-stat powers up. Now connect one lead of your voltmeter to R and the other to C... you should see 0 volts. Make the setpoint above room temp till you heat the t-stat's relay click on. You should still see 0 volts between R and C. We want to make sure RC and R are electrically isolated all the time.

bmma

I did as you suggested and saw 0 volts between R and C with the stat set below room temp, and 0 volts again when I put the setpoint above room temp. Based on that I went ahead and finished installing the stat and getting it set up and all is working fine. Thanks again for the help.