Installing new thermostat on 2 wire system that requires common “C” wire

irakli8997 · October 25

Hello. I have new Weil Mclain forced hot water oil burning boiler, that has new Beckett burner Genisys 7565. Setup is as follows: boiler, indirect super stor water heater, grundfos circulating pumps, Tekmar 260 boiler control with DHW. I have old thermostat that has only 2 wires going to it. I would like to upgrade to smart wifi thermostat that requires common “C” wire that I don’t have. I already run new 18/5 wire from boiler room to the thermostat. I know I will only be using 3 wires (R, C, W) because there are no fans or A/C it is heat only system. I have few thoughts where to attach C wire but my expertise is not enough to make sure it will be a safe connection. First thought is to tap into 24vac 40va transformer C terminal, not sure if itnis the right way. Second thought is Tekmar control but I beleive it does not have dedicated C terminal. Third thought is to connect straight to the Genisys 7565 front desicated Tc terminal as in the manual it says it is for thermostat Common wire. But the question I have is if I go this route will it be safe because as I checked and looker around I found out that T T dry terminals are going straight to Tekmar control and tbere is a jumper between Genisys front exposed terminals between Tr and Tw. My thought was it is done this way because on boiler everyrhing is closed it to be always in ready mode and Tekmar acts like a switch to turn boiler on/off based on thermostat needs. I need help to make sure this connection is the right way to bring Common wire to new smart thermostat without complications. If any photos needed I can provide them as well.

irakli8997 · October 25

thank you all for helping in this

Jamie Hall · October 25

There are so many different ways to do this…

The easiest and most general way is to get and install a separate power supply — you can even get wall warts for this — and hook it up to power the thermostat (R and C) and control the system as usual with R and W.

irakli8997 · October 25

I already purchased 24V outlet plug in transformer and run the wires for that as well but when I was going to hook it up to my thermostat I saw that my plate has one R terminal that is going to be used for already existing R wire and one transformer wire as you said will be plugged in R and C terminals but I was not sure if R can have those two wires hooked up one from wall wart and one from boiler in a same terminal on the thermostat, would it cause some damage? Some say it is not a good idea it can blow the fuse but I am not sure. I have a Nest Thermostat will attach picture of a base plate. If I had Nest learning thermostat I would feel more comfortable using it but they are so pricey, that one would have separate R and Rc terminals and I would connect one wore from wall wart to C and one to Rc. Could Genisys C connector (labeled as Tc) be used for this application? As it is desicated for C? Also checked with multimeter the voltage and it is constant. I will attach some photos.

Thank you

irakli8997 · October 25

I ment it is Common

Jamie Hall · October 25

You can hook the R wire from the transformer to the R connection on the Nest, along with the R wire from the boiler. However, I would suggest that better practice will be to splice the two together with a third pigtail to the Nest, as putting two wires under one screw is not the best idea. Then W to the boiler, and C to the transformer. If you want to be doubly safe, you can wire a 1 amp fuse in the R wire to the boiler.

irakli8997 · October 25

I like your idea! Thanks a lot! What are your thoughts on Tc terminal?

irakli8997 · October 25

since I already have 18/5 wired from boiler room to Thermostat and illuminate need of wall wart

Jamie Hall · October 25

https://forum.heatinghelp.com/discussion/comment/1816200#Comment_1816200

And put the independent transformer or wall wart in the basement… no problem.

irakli8997 · October 25

thank you Jamie

EdTheHeaterMan · October 26

Before you go and start connecting any C wire to your new thermostat, you need to know what R and W from your old two wire thermostat is/was connected to. That R and W from the thermostat could go to the Tekmer, or the GeniSys, or to the boiler aquastat/high limit/circulator relay. Depending on the age of the WM boiler, that aquastat relay could be a Hydrostat, 3200 or 3250 control, or a Honeywell/Resideo L7224 or a Honeywell L8148A

The place where the thermostat is attached will determine where you get your C terminal from.

EdTheHeaterMan · October 26

https://forum.heatinghelp.com/discussion/comment/1816197#Comment_1816197

Since the Tw and the Tr are jumped to run whenever there is a 120VAC from the boiler limit control, the Tc will do you no good for your smart thermostat C terminal. That transformer inside the Beckett Genisys is not sending power to the thermostat ….it can not be the return path to the Tc

You need to have the same transformer be the source and the return path. That is why tou need to know what the thermostat wires are connected to.

EdTheHeaterMan · October 26

I am going to make some assumptions about the 24 VAC transformer in the picture. That transformer is connected to power up the Tekmar 260 low voltage circuits on terminals 1, 2, 3, and 4.

Tekmar 260 terminals 5, 6, 7, 8, 9, & 10 are 120 VAC input and output and are not part of the low voltage for the Nest. Terminal 11 & 12 on the 260 signal the boiler to fire by way of the Aquastat relay 24 VAC T T terminals. The rest of the terminals are for the temperature sensors to determine boiler temperature and outdoor temperature.

This also assumes that the old 2 wire thermostat was connected to the transformer R and the Tekmar 1 terminal with the completed path from Tekmer 2 terminal back to common C on the transformer. There may be reversed or transposed wires when using a 2 wire thermostat. When using a 3 wire set up with a C for a smart thermostat you do not have the luxury of swapping R & W or R & C. That is because the thermostat’s onboard computer needs the R and the C from the transformer for powering the WiFi function. Since the R on the transformer is already connected to the R on the thermostat, then the switch on the thermostat that sends power to the control on a call for heat, (whatever control it is operating) must come from W on the thermostat and needs a return path back to the power source also. W on the thermostat then goes to 1 on the Tekmar and 2 on the Tekmar needs to have the return path to the C on the transformer.

Here is the diagram

Screenshot 2024-10-25 at 10.33.34 PM.png

This only works if you have the Transformer properly connected. You should also use @Jamie Hall's advise of connecting a fuse to the R on the transformer then connect all the other R wires to the other end of the fuse. That way all the factory installed smoke inside the transformer wont get out.

Screen Shot 2022-10-05 at 4.48.20 PM.png

irakli8997 · October 26

Ed! Thank you! so much appreciated! I spent 3 hours going through all the wires, it is 2am right now and I think I covered it all. where is what connected and attached to the image below. For now everything is connected as it was for years. Until I have the right way doing this. I also found that none of the low voltage wires are by tipical color standards and now I understand why electrical work is color blinded. Red and white wires are all mixed and do not match. But after my thorough examination I placed all low volt ones on the Tekmar picture. My boiler is new 2023 model. With new burner and hydrostat. Also will attach photo. You are correct in everything except some Tekmar terminal #’s don’t match the connections maybe when it was installed in 2014 thats how the guy did it, But the whole system is running perfectly. Other then my intention adding a new thermostat. Please see these few photos and let me know your thoughts. If I am not mistaken C wire could be attached to the C terminal on the 24v 40va transformer as well as installing 3amp inline fuse but I will wait for your review. Thank you. Let me know if any additional photos are needed. And yes in present old thermostat setup R and W are backwards, that’s what rebate company installed 5 years ago and they hooked it up and took the old mercury thermostat. Never looked closely at the connections when I replaced batteries 😃

irakli8997 · October 26

the boiler temp is actualy 174 degrees, my camera did not get the last number in the image.

EdTheHeaterMan · October 26

If this diagram matches your actual wiring

Screenshot 2024-10-26 at 12.34.47 PM.png

then this is how to connect your smart thermostat

irakli8997 · October 28

Thank you very much!

irakli8997 · October 29

Do you think I need 3amp inline fuse on both R and C terminal lines from Transformer?

EdTheHeaterMan · October 29

https://forum.heatinghelp.com/discussion/comment/1816849#Comment_1816849

NO.

Think of this as a circle that has different stops around the way. For electricity to work it needs a source (battery, power company. transformer secondary winding, etc.) a load, the thing that uses the electricity (light bulb, motor, solenoid valve coil, relay coil, etc.) and a return path back to the source.

When you open a switch or blow a fuse or have a broken wire in any of the stops along the way and the electric stops flowing. It does not matter where the opening is located in the path. If there is not a complete path with a source, load and return path, then you have no electric power being used.

irakli8997 · November 2

I hooked up everything as advised and everything works perfect. Thanks once again for your help!

EdTheHeaterMan · November 2

"I love it when a plan comes together"

Screenshot 2024-11-02 at 11.25.32 AM.png

HomerJSmith · November 3

Ed Young, I didn't know you were a member of the "A" Team. Of course, your advice is, always, "A" plus. My brother-in-law was the camera man on that series. He said that George Peppard was a true professional, unlike others.

Installing new thermostat on 2 wire system that requires common “C” wire

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