Nest Thermostat with two wires, 24v adapter. Not working

[seth]

Hi all, I recently removed the old thermostat from the wall and installed a Nest Thermostat (the one with 6 terminals: Y, C, W on the left; G, R, OB on the right). When I first installed the Nest, I connected the available two wires from the wall to the W and R terminals. Of course, the Nest told me that I'd need a  C wire, otherwise I'd experience issues with wifi and other functionality.

I examined our boiler and saw that we don't have a logic board where a C wire can be attached. As a consequence, a Google Power Connector wouldn't work out as a simple solution. I discovered that others have used 24volt power adapter/transformer and so I purchased one as well (this model). I inserted one wire end into the C terminal and I inserted the other wire end into the R terminal (along with the other wire coming from the wall). 

Ultimately, I can't get the Nest to work. I've factory reset it multiple times. I've tried indicating different wires are present (whether they are or aren't). I've had no luck. In some circumstances, I can get through the setup process, but then the power errors (e.g. e298) pop up incessantly. Or in some circumstances, the Nest and my phone simply can't connect, making it impossible to complete the setup process.

If anyone can help out at all, that would be extremely appreciated. I'm happy to share photos of the wiring and boiler—whatever can be of use to those who can help!

Edit: I've attached some photos below of the current wiring and the wiring of the old thermostat. Thank you in advance!

Current wiring. The 2 white wires belong to the 24v adapter.The wiring of the old thermostat (installed by the previous home owner).

[Houptee]

You cannot connect the R and the Power supply wires together like that. You are putting 2 power transformers together on same terminal of the Nest.

Show pic of the boiler where the wires connect from t-stat there is a way to get a Connection because all transformers have 2 wires coming out of them you need to find the other wire (not R) and that is the common return back to the transformer. 

[seth]

Hi Houptee, thank you for replying. I'm not sure which wires on the boiler connect to the thermostat. I don't believe we have a common terminal on the boiler though. Our boiler is a Lochinvar Knight Wall Mount unit. Here is a photo. If I'm not mistaken, I believe the wires in the bottom left corner of the photo are the ones that run to the thermostat.

[Houptee]

There has to be another terminal strip where the T-stat wires connect that pic only shows the Outdoor sensor wires. Try and trace where the wires from existing T-stat are going to and post a pic.

They might be going to a separate box on the wall that is a relay control to turn on the circulator pump.

[seth]

Thank you for following up on this! I'm sorry to say that I don't know how to locate the thermostat wires at the boiler. I've attached a picture of the entire boiler unit. Are you able to spot where the thermostat wires might be? I can take additional photos if necessary. I tried to locate the wires running along the basement ceiling, but to no luck. 

[Houptee]

Ok I see the thermostat is connecting to the bottom left terminals that say Heat/loop R and Demand W.

They must have spliced that brown cable onto the old cable that goes to the thermostat because it was too short.

So since there is no C terminal you would have to use a multi-meter and figure out what wire coming off that transformer on the upper right that has blue and yellow wires on it is the Common of the transformer.

There are 4 wires on a transformer, 2 input at 120 V AC and then the 2 output will be the 24 V AC that you need for the thermostat.

One of the 24 V wires will be going directly to the R Heat/Loop terminal from the back of that board so that will be the + 24 V.

So when the T-stat closes the switch it puts +24 V from R and applies it to W wire which then travels back to the boiler W terminal and energizes a relay (the load of the circuit) to turn on the boiler then the other side of the relay coil returns back to the other wire of the transformer - negative aka Common making a complete circuit.

If you put a meter set on AC volts and test those Blue and Yellow wires on that transformer you will read 24 V AC (I assume those are the low voltage side so that's why you need to test first).

If those read 24V then put the meter from R terminal on boiler to the Yellow wire if you read 24 V then Yellow is Common, If you read nothing then test R to Blue see if that reads 24 V then the Blue is Common.

Once you figure out the Common you can just tap off the wire and use that as the Common going back to the Nest if you can locate where they spliced the old wires or just run a new 5 conductor thermostat cable to the Nest if there are no spare conductors to use.

[Subro]

so there is no option to tie an external adapter  to this Nest thermostat (with no Rh) connector.