Anyone with HVAC or Knowledge Regarding Installing a Common Maker?

[Burnin Since 1991]

Hi, I have electric baseboard heat (yeah, I know. Working on a plan for the Spring to switch to a NG solution) and we were using a Nest Wi-Fi Thermostat, but google decided to EOL it and stop supporting it.

I purchased a Sensi Classic Thermostat and it needs a C- Wire (Common) to run. My thermostat wiring only has two wires and no common. I traced the wiring back to the electric panel and found the 240v/24v transformer. It only has two wires leaving the case that run directly to the thermostat. There are other connections inside the transformer housing but nothing is marked. I do have a schematic and pics of the transformer "box" that I can post, later today.

Any help would be appreciated.

I contacted the manufacturer and they told me to hire an HVAC company. And no disrespect to any in the HVAC industry, My HVAC Co charges a minimum $300 to walk in the door. It would probably take them 5 minutes to hook up the Common Maker. I have no issues installing the sender to the thermostat. That is pretty straight forward.

 

[EbS-P]

Hi, I have electric baseboard heat (yeah, I know. Working on a plan for the Spring to switch to a NG solution) and we were using a Nest Wi-Fi Thermostat, but google decided to EOL it and stop supporting it.

I purchased a Sensi Classic Thermostat and it needs a C- Wire (Common) to run. My thermostat wiring only has two wires and no common. I traced the wiring back to the electric panel and found the 240v/24v transformer. It only has two wires leaving the case that run directly to the thermostat. There are other connections inside the transformer housing but nothing is marked. I do have a schematic and pics of the transformer "box" that I can post, later today.

Any help would be appreciated.

I contacted the manufacturer and they told me to hire an HVAC company. And no disrespect to any in the HVAC industry, My HVAC Co charges a minimum $300 to walk in the door. It would probably take them 5 minutes to hook up the Common Maker. I have no issues installing the sender to the thermostat. That is pretty straight forward.

Remember these are 24v AC systems. Think common as hot and then you need to make the neutral connection back to the transformer.

 

[EatenByLimestone]

Thermostat wire is all color coded. Your baseboard isn’t asking for specific wire colors? While electricity doesn’t care about the color, the next guy who works on it will thank you if you keep it standardized!

I had a HVAC guy suggest/warn me to use the 18g vs 20g wire. It probably saves frustration.

 

[Burnin Since 1991]

Remember these are 24v AC systems. Think common as hot and then you need to make the neutral connection back to the transformer.

Thanks. I have that same diagram. On the transformer end, the purple from the C Maker to the white thermostat. The red from C Maker to the red from thermostat and to the transformer.

Here's where I'm not sure about. The white and black from the common maker go to the TT Control. The 24V transformer is inside a small metal box. After I cut the power and open that box, it looks like the red and white wires both go to the transformer. I will open it back up in the morning and re-check the wiring inside and double check what is connected to what.

 

[Burnin Since 1991]

Thermostat wire is all color coded. Your baseboard isn’t asking for specific wire colors? While electricity doesn’t care about the color, the next guy who works on it will thank you if you keep it standardized!

I had a HVAC guy suggest/warn me to use the 18g vs 20g wire. It probably saves frustration.

My HVAC system is nearly 60 years old. The thermostats only run a 2 in one wire with a red and white. No common. The Thermostat is on finished walls, but I might be able to pull a new wire through, if I can pull it with old wire. I imagine it's stapled and running through studs but the utility room is only about 15 feet away. The baseboards are 240V and I have a 240/ 24V AC transformer that is joined to the bottom of my breaker box with the transformer in a small metal enclosure. I will get better pics tomorrow and post them so you guys can see what I'm talking about.

The thing is to, that I'm shutting down the baseboards in the living room in the spring and going with a NG stove which due to house design will also heat the upstairs. So basically what I'm doing right now is just to get me through the next 4 months or so.

 

[EatenByLimestone]

I found mine, I assume it had been there since 1946, was not stapled in the wall. I only needed red and white also. I attached the new wire to the old and then fished it up. I hope yours goes as easy as mine did! The hardest part for me was finding the wire at the store, lol.

 

[gthomas785]

R and C are the two leads from the transformer. Think of them as the "hot" and "neutral" of 24v land. Your thermostat has a R going to it but probably not a C. The W wire is a switch leg that comes back from the thermostat to the relay/board/whatever is controlling the heat. Then from there the relay needs another wire back to C on the transformer to complete the circuit.

So to add a C wire to the thermostat, you just figure out which side of the transformer is not R and connect it there.

 

[Whitenuckler]

Just a side note on thermostat wiring that I ran into. I had a NEST thermostat and it could run in heat or cool mode.
One day it stopped working and somehow I determined the built in battery was low.
I can't remember the details, however believe they had 3 wires hooked up to it.
I did the research and found out the battery inside is charged from the 24AC, but if you DON'T run a common, it just
uses parasitic draw. I happened to have the extra wire in the cable, so I hooked it up on both ends, and now full charging.

 

[Burnin Since 1991]

The sensi thermostat I have is also capable of heat and cool. It uses 2 AA Batteries. The old Nest thermostat ran without the Common Wire but it was also getting dimmer as time went on (internal battery(ies?).

So here is what I have now

The cover of the 240v/24v transformer enclosure

The only two wires between the thermostat and the transformer enclosure. The red and white from the thermostat connecting to the wires to the transformer enclosure

Lower right is the red and white entering.

After further examination, one side of the 24v side is going directly to the thermostat and the other is going to the circuit board where the white wire is connected.

The box above the transformer must be the TT control?

The bottom of the TT Controller has a white and black going into the breaker panel and a blue and black that connect to the 240v side of the transformer. The green is ground.

I can't really tell where these top two wires go.

Hope this helps!

 

[Whitenuckler]

The sensi thermostat I have is also capable of heat and cool. It uses 2 AA Batteries. The old Nest thermostat ran without the Common Wire but it was also getting dimmer as time went on (internal battery(ies?).

So here is what I have now

The cover of the 240v/24v transformer enclosure

View attachment 342755

The only two wires between the thermostat and the transformer enclosure. The red and white from the thermostat connecting to the wires to the transformer enclosure
View attachment 342757

Lower right is the red and white entering.

After further examination, one side of the 24v side is going directly to the thermostat and the other is going to the circuit board where the white wire is connected.

The box above the transformer must be the TT control?
View attachment 342758

The bottom of the TT Controller has a white and black going into the breaker panel and a blue and black that connect to the 240v side of the transformer. The green is ground.
View attachment 342759

I can't really tell where these top two wires go.

View attachment 342760


Hope this helps!

The sensi thermostat I have is also capable of heat and cool. It uses 2 AA Batteries. The old Nest thermostat ran without the Common Wire but it was also getting dimmer as time went on (internal battery(ies?).

That was the same issue I had - the people before me installed it but did not read all the instructions I guess. They just did a one for one replacement. The previous one probably had batteries. For the Nest the battery is built in and must be recharged manually or from the furnace. Without the common from the furnace it could become low in charge.

 

[Burnin Since 1991]

The sensi thermostat I have is also capable of heat and cool. It uses 2 AA Batteries. The old Nest thermostat ran without the Common Wire but it was also getting dimmer as time went on (internal battery(ies?).

So here is what I have now

The cover of the 240v/24v transformer enclosure

View attachment 342755

The only two wires between the thermostat and the transformer enclosure. The red and white from the thermostat connecting to the wires to the transformer enclosure
View attachment 342757

Lower right is the red and white entering.

After further examination, one side of the 24v side is going directly to the thermostat and the other is going to the circuit board where the white wire is connected.

The box above the transformer must be the TT control?
View attachment 342758

The bottom of the TT Controller has a white and black going into the breaker panel and a blue and black that connect to the 240v side of the transformer. The green is ground.
View attachment 342759

I can't really tell where these top two wires go.

View attachment 342760


Hope this helps!

anyone have any thoughts to help me get this done?

 

[Whitenuckler]

anyone have any thoughts to help me get this done?

Sensi smart thermostat | Sensi CA

Check our Sensi smart thermostat product, where to buy, installation process, mobile application download and smart home integration.

sensi.copeland.com

Do you have one of these? You mentioned a different model that needs common?

Edit - You can attach a pull wire to the existing, but it needs to be able to get through the holes. You might have to open
up the wall or run a new one along the bottom and up through the wall if you can't pull a new one in.


"After further examination, one side of the 24v side is going directly to the thermostat and the other is going to the circuit board where the white wire is connected."

You want that other wire extended, the one not going to your thermostat.

 

[gthomas785]

Yeah what Whitenuckler said. The upper part appears to be a contactor and the transformer is below it. The red lead going from the transformer to the side of the contactor is the 24v common and that's what you need to extend to the thermostat. The incoming power is the big black and white wires going to the bottom of the contactor (also jumped to provide power to the transformer). The big red and white coming off the top of the contactor are the switched power going to the heater.

Those nest thermostats claim they can steal power from the relay circuit to maintain the internal battery however I have never really seen that work properly. It's always better to have a c wire for the electronic thermostats

 

[Burnin Since 1991]

I'm going to hold off pulling a new wire from the panel to the thermostat. If I go with a NG solution, I will have to rewire the thermostat anyway so I will wait until then.

so back to the common maker

Remember these are 24v AC systems. Think common as hot and then you need to make the neutral connection back to the transformer.

from the common maker the red goes to the red on the transformer which runs to the thermostat. The black wire on the common maker connects to the other red lead from the transformer. The White lead connects to the white wire from the side of the contactor and then the purple connects to the white wire to the thermostat.

I'm good on connecting the sender to the thermostat.

Ok, I think I've got it now.

Well, thank you all very much for your assistance.

I borrowed the image from @EbS-P post above and thanks for that as well.

I won't get a chance to work on this until Monday so I will update then.

 

[Burnin Since 1991]

Do you have one of these? You mentioned a different model that needs common?

I have three of the Sensi Classic's. It's a basic wifi thermostat. I also have a sensi lite which is also wifi and touch screen. The other thermostats are for my sons house and our family vac home. All the others already have a Common Wire.

I thought it was called classic, but it's actually a Sensi UP500W (Emerson Branded) and the others are Sensi ST55U (Copeland Branded). They are look the same. I can't find any differences.

 

[Whitenuckler]

I have three of the Sensi Classic's. It's a basic wifi thermostat. I also have a sensi lite which is also wifi and touch screen. The other thermostats are for my sons house and our family vac home. All the others already have a Common Wire.

Ok let us know - I kind of avoided this "common maker hard ware" issue. I do know the secondary of the transformer is what you want, and you already have one side. It's AC 24V, so which side is "common" is determined from which terminal you select. Much easier on house wiring as it is called neutral, and grounded.

 

[begreen]

The old thermostat was just acting as an on/off switch. It didn't need power. The new thermostat does. Run at least a 3 wire, thermostat wire, or run a 5 wire if fan control is going to be desired on the NG furnace. In the new cable red is typically the C common, hot wire.

 

[Burnin Since 1991]

In Hindsight, I wish I would have just pulled a new 5 wire and been done with it. But, as the say; hindsight is 20/20.

The old thermostat was just acting as an on/off switch. It didn't need power. The new thermostat does. Run at least a 3 wire, thermostat wire, or run a 5 wire if fan control is going to be desired on the NG furnace. In the new cable red is typically the C common, hot wire.

Got it and duly noted in my upgrade notes. I keep everything in notepad. A different one for every project, big or small-ish that I work on.