C wire to boiler no power.

sschoe2

Hi all I need some help. We are having a big cold snap here in Chicago and my Nest is acting up. I have a boiler in the basement with W1 and RH going to it from the Nest and an AC/air handler in the attic with Y1, G and Rc going to it. Due to the cold snap I started geting error messages about power. From what I read it is because without a C wire the nest can only recharge when the boiler is off and it is running constantly during this cold snap. So I went to Home Depot and got some thermostat wire and ran a wire from the C on the thermostat to C curled arround the C screw on the boiler transformer (pictured). Nest reports no power to C. What now.

Jamie Hall

Go back to Home Depot and get a reliable battery powered two wire thermostat and mount that instead. Then when it warms up we can all take the time to figure out how to get the Nest to work.

pecmsg

Jamie Hall said:

Go back to Home Depot and get a reliable battery powered two wire thermostat and mount that instead.

Fixed!😇

sschoe2

Another possibility I am researching. They sell a plug in 24V adapter with 2 thermostat wires. Since I already ran 2 to the basement, I can plug it in, splice it and run it to the termostat.

The only issue is it looks like it is supposed to plug into C and Rc and my system already has a Rc wire going to the AC in the attic. Not sure where to put the other wire.

EBEBRATT-Ed

Maybe you have the W & R wires reversed at the stat?. You will have to get a test meter and check from r & C at the thermostat to see if you have 24 volts

Would need to know more about your wiring and how many transformers you have. There is usually one in the air handler and your pictures show a fan center with a transformer so need more information. There could also be a transformer in the boiler.

sschoe2

I put my multimeter on R and C at the boiler transformer and read 0. So looks like Nest is correct. Somehow it isn't hooked up or something?

109A_5

Hello @sschoe2,
Make and model of your boiler so the wiring diagram can be looked up in the I/O manual.

sschoe2 said:

I put my multimeter on R and C at the boiler transformer and read 0. So looks like Nest is correct. Somehow it isn't hooked up or something?

Was your multi-meter set on AC Volts ? If so check the 120 VAC side of the transformer, you may have other issues, or with messing with it you burned out the transformer.

sschoe2

It says Burnham Model 204NCL-BE12 low pressure boiler.

I ordered the plug in transformer. I just have to figure out where to put the other wire. One goes into the Nest C terminal but the other is suposed to go to Rh or Rc and both are occupied and it is not advised to put 2 wires in 1 terminal.

The Nest is currently working on battery but I can't adjust anything with it as it is not using wifi to conserve charge.

109A_5

Hello @sschoe2,
So the boiler transformer is actually good ?

I don't advise just adding a 2nd transformer at the NEST without a relay.
See this thread kind of near the end, the posting just before wiring diagrams and on.
https://forum.heatinghelp.com/discussion/comment/1782234#Comment_1782234

Often the NEST may charge from the unused AC equipment. Is that equipment dead too ?

HVACNUT

When using 2 power sources at the thermostat, the power for the thermostat will be from Rc and C.
Connect Common from the air handler, not the boiler. 

sschoe2

Running a C wire from the attic may not be feasible. At least in the basement there is a conduit running from the basement ceiling to right behind the thermostat so fishing it was easy. I don't think the attic has that. The AC is a pain to work with plus it is in the freezing attic. I need to try the plug in relay. I read about a dozen posts. Some say attach the second wire to Rc some to Rh and the polarity matters. Can I just check the voltage between W and C with my multimeter to ensure I hook the wires the correct way?

The heat is currently working fine with the Nest using its backup battery. I remove it twice a day for 1 hour to charge it with a miniusb and powerblock. Before this cold snap I set it to always 68F.

109A_5

Hello @sschoe2,
I was under the impression that the NEST would charge from the idle AC system power with 24 VAC across Rc to Y and/or G, without a C wire to the AC system. This may be incorrect.

To me, when using two transformers on the heating side of the NEST they need to be phased correctly so there is not 48 VAC across the NEST when not calling for heat. The NEST may not like 48 Volts across W to C. An isolation relay solves that issue.

sschoe2

From what I have read the Nest without the C wire only charges when the system is idle and since the boiler is going almost constantly in this subzero weather it has not enough time to charge. It started giving me power errors when this cold snap started on Sat/Sunday.

sschoe2

I assume I can check to see if there is 48VAC with my multimeter and if there is just switch the 2 wires from the new transformer adapter.

sschoe2

Well I received and installed the plug in transformer and installed it to Rc and C and the termostat and heat are working great. I tried measuring C and W with my multimeter and read 24 VAC.

109A_5

Hello @sschoe2,
As long as it works OK.