Nest thermostat C wire question


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Old 11-15-18, 11:28 AM
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Nest thermostat C wire question

I recently installed a Nest thermostat using just the R and W wires. However I notice that the thermostat keeps turning my boiler on to charge. After some research I found out that this is because there is no C wire to allow the thermostat to charge without turning the heater on. I've attached some pictures of the diagram for my boiler circuit board. Would anyone be able to tell me if it's possible to setup a C wire to this?

http://imgur.com/a/nGD6tVE
 
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Old 11-15-18, 01:09 PM
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Welcome to the forums.

Very doubtful that will support the nest directly. I see a very small transformer in there which will not allow it to charge properly. I'm going to need the model number for that boiler.

There is always a way to make it work but it's not cut and dried. You'd need a 24vAC plug in wall transformer and a control relay to activate the boiler. I'll know better when I look up that boiler.
 
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Old 11-15-18, 03:34 PM
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Thanks for the response! The boiler is Baxi Luna 310 Fi. By the way, I wanted to ask. The output on the transformer says 15V, is that an issue?
 
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Old 11-15-18, 03:49 PM
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Thanks for the model number. It confirmed my suspicion. That boiler runs on a lower voltage and it's DC.

The easiest way to connect a nest to it is to use what's called a fan center. It's a combination 24vAc transformer and a relay. It can mount right on an electrical box that you can mount to the wall. You will need three wires from the fan center to the nest although it may run on two wires. I like using a positive C connection. The fan center is shown in the link below. It looks more complicated than it is to wire.

Fan center
 
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Old 11-15-18, 04:03 PM
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I guess I better put the old thermostat back on for now.
 
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Old 11-15-18, 05:45 PM
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Basically you need a thermostat that runs on batteries. Many of the starter line Honeywell thermostats are like that. When a thermostat runs on batteries it just causes a contact closure to the boiler. It doesn't rely on the boiler power.
 
 

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