Adding common wire to two-wire system

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Old 12-18-15, 04:21 PM
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Adding common wire to two-wire system

Hi guys -- I have a fairly new boiler with a GeniSys 7505 burner controller connected directly to a Nest thermostat. The controller has only two wire terminals. This is causing problems with the nest, which "pulses" the wire to steal power. So what I want to do is add a C wire. What's the best way to do this? I already have extra wires running from the thermostat, but can use more if necessary. I'm just not sure what I should attach it to.

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Old 12-18-15, 05:01 PM
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The nest cannot be run directly off that control. It uses an electronic switching section instead of a relay which means there is not enough current flow thru the R/W wiring to charge the nest.

You can purchase a 24vac transformer and power the nest directly.
Is this a boiler ? Is there another 24vac transformer there ?

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The red and white go to the controller.
The two blues are an alternate source or 24vac.
That means you'd need four wires to the stat.
 
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Old 12-18-15, 05:12 PM
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How many "extra" wires do you have? I try to avoid any questions on Nest thermostats but as I recall they can use a separate power supply. If you have two "extra" wires they could be used with a stand-alone (or plug-in) 24 volt AC transformer to supply the needed power to the thermostat.

You might ask the question in the thermostats forum.
 
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Old 12-18-15, 05:49 PM
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Thanks guys! The Nest does operate the control properly -- calls for heat are answered, etc., and the 7505 has 24vac power. The problem is that the Nest will occasionally "pulse" the control, presumably to steal more power, causing the boiler burner to start and stop. When I ran thermostat wire to the boiler last year I ran 5-wire cable, so I have 3 extras. You figure I can use two of those to wire in a separate 24v transformer and pick up the power for the nest that way? I'm looking at a Honeywell AT72 D 1006 at the store right now. Seems like it'll work, right?
 
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Old 12-18-15, 06:03 PM
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You only need 10va (watts) so that 40va one you spec'ed is fine.
 
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Old 12-18-15, 06:31 PM
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Great! How about the one that's already powering my doorbell? Can I just stack the two new wires on the 24vac terminals? Here's a picture:

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Old 12-18-15, 06:38 PM
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Yes.... you can use the outside two terminals for 24v.
 
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Old 12-18-15, 07:36 PM
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This worked great! Cost me $0. This after two straight days of the tech from the company that installed my boiler fumbling around in the basement without a clue. Much appreciated, man, you're a hero.
 
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Old 12-18-15, 08:02 PM
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You got that wired up quick. Glad to help.
 
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Old 12-19-15, 06:40 AM
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Might be a good idea to clean up that connection at that transformer,to much bare copper showing.
 
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