Common wire question

[Theodore]

Hi,
My current thermostat is failing (LCD screen is illegible). Thinking about upgrading to a wifi enabled one. This is for hydronic heat only, no AC attached. The current thermostat has 2 active wires and a spare unused wire (before I finished the basement, I ran a 3wire cable from the double relay control panel to the to the thermostat). I've included photos of the thermostat base, and the control panel (adjacent to the boiler and indirect water heater, which is the "priority" zone on the dual relay panel). I don't see a common wire terminal anywhere in this relay panel. My preference is to power a new thermostat with a common wire, not via battery or via how some thermostats steal power without triggering the heat to come on. I'd appreciate any guidance offered on what I should do or add to get a common wire terminal available.
Thanks!
Theodore

 

[Stuff]

First: for each zone I believe that R is on the left and W on the right. With a two wire connected thermostat it doesn't matter but with a common you need to get it right.
There is no terminal for C so you will have to tap it off of a component on the board. The easiest should be the left side of the top blue resistor just above/left of the left LED.

If you have a multi-meter it would be good to check that you have 24vac (a bit higher) between R and C.

 

[Sponsor]

 

[Theodore]

Hi!
Digital mutli-meter readings with respect to resistor, terminals, and ground, are per the markup photo below.

Does this help confirm whether I have the red & white wires wrong?

I'm a little wary of how to connect to the left side of the blue resistor without physically damaging it. How would you typically do this? Just loop a wire in there and twist a little? Solder it? How can you be sure I should connect to the left (and not the right) side of the resistor?
Thanks for all advice.

 

[Stuff]

Your measurements confirm that for both R is on the left, W on the right, and C on the left of the resistor. So, yes, the red and white need swapped.
Can confirm that C is on the left side of the resistor since on the schematic it shows R and C coming from the transformer. R goes through the fuse but C connects to all of the resistors (zig-zag lines) and in the picture you can see the solder trace connecting the large and small resistors is on the left.

Twisting a C wire around the left side of the large resistor should be fine if you crimp it with pliers but would be better to add a dab of solder.

 

[Plumber69]

wifi thermostat is stupid unless your dog is picky about heat

 

[Reach4]

wifi thermostat is stupid unless your dog is picky about heat

It can be handy to change the schedule on a thermostat that can change the setting at different times of the day. Its easier to do stuff like that at the computer.

If you go on a trip, it can be handy to adjust things remotely. Make it cool while you are away, but let it be comfortable when you walk in the door. It can let you read the temperature remotely too.

 

[Jadnashua]

An alternative to a resistor would be to go directly to the transformer. Removing the board would allow you to trace the leads. On the low-voltage side, you'd get zero volts measuring from one side of the secondary to T, and 24vac when measuring between T and the common (or other side of the secondary).

TO be on the safe side, you'd want to know the VA for the new thermostat and the relays and load the board is trying to power. The transformer is probably large enough to handle a thermostat, but it is a load, and you don't want to overload it. It looks like the transformer is rated at 15VA. What is the rating for the thermostat you wish to use?

 

[Theodore]

Exactly: the kids keep manually overriding the program after we leave for work, and then they're out the whole day at school. Ideal for us to log in and dial it down from work.

 

[Theodore]

Specs say load of transformer is 3.5 VA, so probably not a problem, is it??

Also, just want to say many thanks for all the awesome, verifiable, accurate direction.

 

[Stuff]

Specs say load is .18 amps per thermostat. .18*24*2=8.64VA Add to 3.5 and you get 12.14 which is still less than the 15va the transformer is listed at.

 

[JerryR]

wifi thermostat is stupid unless your dog is picky about heat

Well maybe the dog isn’t picky but our 45 year old Amazon Parrot is picky about temperature.

I’ve had WiFi thermostats on each of our last 5 homes. Right now we are at our vacation home. I adjusted that thermostat via my iPhone when we were 1 hour from the vacation home. Now I’m in bed and adjusted the temp down 1 degree from my bed.

We can monitor them and humidity at all 3 zones at our main home from the bed of my vacation home.

 

[Theodore]

Hello. I'm resurrecting this old thread on-purpose: after following instructions above and creating/installing a common wire, I never noticed (until now) that the house thermostat *loses power* every time there's a call for heat from the indirect water heater zone, which is a priority zone. I always thought it was weird that on very cold days and when someone's taking a shower, it short-cycled the boiler from time to time. And the reason is now obvious: after a call for hot water, it would take about 15-20 seconds for the house thermostat to power up and initialize, and THEN fire up the boiler to continue to heat the house. Just wondering if this is an ok situation or should I have wired this differently? or should I maybe get a different panel controller?? Advice appreciated.
Regards,
Theodore

 

[Reach4]

I am pretty sure you wired it to the wrong place.

Edit: you still have the option to add a separate transformer.

 

[Fitter30]

They don't show a neutral on the diagram and the transformer is solder in. Try common to chassis ground with a meter

Hi!
Digital mutli-meter readings with respect to resistor, terminals, and ground, are per the markup photo below.

Does this help confirm whether I have the red & white wires wrong?

I'm a little wary of how to connect to the left side of the blue resistor without physically damaging it. How would you typically do this? Just loop a wire in there and twist a little? Solder it? How can you be sure I should connect to the left (and not the right) side of the resistor?
Thanks for all advice.

View attachment 49983

Just because it shows 0 vac doesn't mean its common. T to T isn't a dry contact T1 is 24vac.

 

[Fitter30]

Hi!
Digital mutli-meter readings with respect to resistor, terminals, and ground, are per the markup photo below.

Does this help confirm whether I have the red & white wires wrong?

I'm a little wary of how to connect to the left side of the blue resistor without physically damaging it. How would you typically do this? Just loop a wire in there and twist a little? Solder it? How can you be sure I should connect to the left (and not the right) side of the resistor?
Thanks for all advice.

View attachment 49983

Normally when a contact is open voltage is what ever the control voltage is a closed contact is 0. Thru a triac voltage might read differently but doesn't have the power to energize a relay. Red and white might try swapping them but shouldn't make a difference.

 

[Jadnashua]

Many WiFi enabled thermostats really only work well when you have the common lead connected. A simple thermostat is basically just a switch...when it calls for heat, it connects 24vac to the white wire, which goes to a relay, that has the common on the other side to engage it, causing the heat to come on. There's no complete path for the thermostat to provide power to it...just having 24vac coming in without the common (return) means it doesn't have a complete circuit. Some can parse a little bit of that power to charge an internal battery, but it may not be on often enough to keep them charged properly. There are devices designed to provide a common so the thermostat can charge its internal battery, but that doesn't always work well as I discovered helping a friend out with his...we ended up running a new cable to provide a common. It might have worked with other situations, but not for him as this was a second home, and the system wasn't on that often unless he was there. Much better to have a fixed wire for common. There are other techniques to use a separate transformer to power the thermostat. Simpler thermostats work with replaceable batteries, but not when you want to keep a WiFi connection active.

 

[Stuff]

Yes, with a priority zone the power gets cut to the second zone's terminals when the first is running. The short cycling shouldn't be an issue as the burner is designed to cycle anyway. I don't know about the thermostat itself, though. If you want to fix the issue you could add an isolation relay with some simple wiring.

 

[Fitter30]

https://www.amazon.com/Transformer-Thermostat-Competible-Versions-Honeywell/dp/B07DJ7RHS5/ref=sxin_20_ac_m_br?ac_md=2-1-VklJVlJJQQ%3D%3D-ac_m_br_brs_brs&content-id=amzn1.sym.f62f8258-875d-4a87-9dd9-697696c6a50f%3Aamzn1.sym.f62f8258-875d-4a87-9dd9-697696c6a50f&cv_ct_cx=nest+thermostat+c+wire+adapter&gclid=EAIaIQobChMImObxjIuM_AIVTQqtBh3vzgPOEAAYASAAEgJBwfD_BwE&hvadid=406658286384&hvdev=t&hvlocphy=9023792&hvnetw=g&hvqmt=e&hvrand=2093516695280897019&hvtargid=kwd-849311479620&hydadcr=1722_10954124&keywords=nest+thermostat+c+wire+adapter&pd_rd_i=B07DJ7RHS5&pd_rd_r=e278f517-3dad-4906-b864-149612a124b7&pd_rd_w=bDBwz&pd_rd_wg=yYLCu&pf_rd_p=f62f8258-875d-4a87-9dd9-697696c6a50f&pf_rd_r=QY228XSMW88HV8YQVDBJ&qid=1671672978&sr=1-2-9ebaf11c-ec22-494f-9d9a-a52f73a4267f

 

[Stuff]

https://www.amazon.com/Transformer-Thermostat-Competible-Versions-Honeywell/dp/B07DJ7RHS5/ref=sxin_20_ac_m_br?ac_md=2-1-VklJVlJJQQ%3D%3D-ac_m_br_brs_brs&content-id=amzn1.sym.f62f8258-875d-4a87-9dd9-697696c6a50f%3Aamzn1.sym.f62f8258-875d-4a87-9dd9-697696c6a50f&cv_ct_cx=nest+thermostat+c+wire+adapter&gclid=EAIaIQobChMImObxjIuM_AIVTQqtBh3vzgPOEAAYASAAEgJBwfD_BwE&hvadid=406658286384&hvdev=t&hvlocphy=9023792&hvnetw=g&hvqmt=e&hvrand=2093516695280897019&hvtargid=kwd-849311479620&hydadcr=1722_10954124&keywords=nest+thermostat+c+wire+adapter&pd_rd_i=B07DJ7RHS5&pd_rd_r=e278f517-3dad-4906-b864-149612a124b7&pd_rd_w=bDBwz&pd_rd_wg=yYLCu&pf_rd_p=f62f8258-875d-4a87-9dd9-697696c6a50f&pf_rd_r=QY228XSMW88HV8YQVDBJ&qid=1671672978&sr=1-2-9ebaf11c-ec22-494f-9d9a-a52f73a4267f

OP only has a 3 wire cable run to the thermostat. To add a transformer Rc and Rh would need split at the thermostat. Transformer to Rc and C with zone panel to Rh and W. If Rc and Rh were not split then the priority zone would no longer disable zone 2.