# C-wire



## WronGun (Oct 18, 2013)

Trying to figure out why the C-wire must come from the specific control panel by the controlled furnace... if this is simply power , why can't a separate transformer send 24volts to the t-stat? 


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## Glock23gp (Mar 10, 2014)

Did u have a pic you forgot to attach?

Not sure what you mean

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## joebanana (Dec 21, 2010)

Ummmmm......I give up, why?
http://smartthermostatguide.com/thermostat-c-wire-explained/


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## WronGun (Oct 18, 2013)

C -wire is needed for constant power at wifi thermostats.... I installed a transformer separate from the furnace system just to give the tstat the 24volts that it needs... the HVAC company told me the c-wire must come from the control board and not a separate transformer....

Currently I have 24v to ground on C and they say i need 24v from C to RC

Running from a separate transformer gives you 2 extra leads 



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## WronGun (Oct 18, 2013)

joebanana said:


> Ummmmm......I give up, why?
> http://smartthermostatguide.com/thermostat-c-wire-explained/




I just read this whole article before posting this , it doesn't say that it needs to be derived from a specific point 


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## Glock23gp (Mar 10, 2014)

Power is power right?

If the power source isn't controlling or acting off something why would it matter where it comes from? (In my inexperienced opinion)

Sounds like the HVAC guys have drank the installers manual Kool-aid on requirements. (Like every garbage disposal NEEDS a dedicated 20 amp circuit per the manual...)

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## frenchelectrican (Mar 15, 2007)

Ya should have at least 7 conductor thermosat cable there and the actual connection depending on what type of heating and AC unit is set up espcally with multistage units those will kick up to 10 conductors thermosat cable.


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## nrp3 (Jan 24, 2009)

The R in the thermostat is one leg of the transformer in the furnace/air handler/boiler. C is the other leg. You get constant 24v to power the thermostat from R and C. In a battery operated thermostat or one that doesn't need power to operate, R brings one leg up and depending upon the type of system brings it back to the furnace on w, y, etc. C is always there, just often not needed at the thermostat and stays in the heating equipment.


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## WronGun (Oct 18, 2013)

This is my point when you run a new line from a transformer .... this gives you 2 leads +\-.... which you both need to land somewhere for there to be 24volts......but you only have 1 terminal to hookup to (c)


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## WronGun (Oct 18, 2013)

I found the answer ...the second wire lands on RH 


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## nrp3 (Jan 24, 2009)

In many thermostats, there is Rc and Rh, because sometimes there is one thermostat for two systems, say and boiler for baseboard heating and maybe separate forced air cooling. There is usually a jumper there between Rh and Rc if both heating and cooling are controlled from the thermostat and its one system, ie forced hot air and airconditioning. In your case, I'm not sure which R the C is paired up with if there is two control transformers in two different pieces of equipment.


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## LARMGUY (Aug 22, 2010)

WronGun said:


> C -wire is needed for constant power at wifi thermostats.... I installed a transformer separate from the furnace system just to give the tstat the 24volts that it needs... the HVAC company told me the c-wire must come from the control board and not a separate transformer....
> 
> Currently I have 24v to ground on C and they say i need 24v from C to RC
> 
> ...


Any time I added a separate power supply to control anything I had to tie the grounds together to supply a common for the controls.


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## emtnut (Mar 1, 2015)

WronGun said:


> I found the answer ...the second wire lands on RH
> 
> 
> Sent from my iPhone using Tapatalk


So you have the control transformer from the furnace AND the new transformer you installed both hooked up to RC/RH ???


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## WPNortheast (Jun 4, 2017)

Howdy, in my opinion, it is not a good idea to try and introduce a second transformer into the mix. HVAC controls are expensive and we look stupid/costs money when we burn them up. If I were in your shoes I would try to find a spare conductor in the thermostat cable. Than look for common connection in HVAC unit, wire both ends up, fire it up. If no spare availible but common availible fish in new if possible. If nothing works tell them too bad call HVAC guy. Honestly I just pass on these type of thermostats unless Im working on the system as a sub. Ive found it to be more headaches than they are worth and people dont end up loving them.


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## MechanicalDVR (Dec 29, 2007)

WronGun said:


> C -wire is needed for constant power at wifi thermostats.... I installed a transformer separate from the furnace system just to give the tstat the 24volts that it needs... the HVAC company told me the c-wire must come from the control board and not a separate transformer....
> 
> Currently I have 24v to ground on C and they say i need 24v from C to RC
> 
> ...


Why would you want to run two separate feeds to a t-stat and add the need to phase two separate lv circuits and take a chance on burning something (like an expensive IC board) up or causing a future problem?

Just pull a new cable from the furnace or air handler you are working with and do the right thing.


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## MechanicalDVR (Dec 29, 2007)

WPNortheast said:


> Howdy, in my opinion, it is not a good idea to try and introduce a second transformer into the mix. HVAC controls are expensive and we look stupid/costs money when we burn them up. If I were in your shoes I would try to find a spare conductor in the thermostat cable. Than look for common connection in HVAC unit, wire both ends up, fire it up. If no spare availible but common availible fish in new if possible. If nothing works tell them too bad call HVAC guy. Honestly I just pass on these type of thermostats unless Im working on the system as a sub. Ive found it to be more headaches than they are worth and people dont end up loving them.


Welcome aboard!

Decent answer you gave but this goes back to something I've found over the years, many electricians know close to nothing when it comes to T-stat wiring.

T-stats are little more than a multi position switch at best, taking one hot leg and making it to one or more outputs.


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## splatz (May 23, 2015)

MechanicalDVR said:


> Decent answer you gave but this goes back to something I've found over the years, many electricians know close to nothing when it comes to T-stat wiring.
> 
> T-stats are little more than a multi position switch at best, taking one hot leg and making it to one or more outputs.


That's why I personally wouldn't mix two power supplies, I don't know for sure what happens inside the thermostat and in the HVAC controls and I don't want to inadvertently wind up frying a board. 

It's like two very tiny separately derived systems, I don't know enough to trust the controls to keep them separate.


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## MechanicalDVR (Dec 29, 2007)

splatz said:


> That's why I personally wouldn't mix two power supplies, I don't know for sure what happens inside the thermostat and in the HVAC controls and I don't want to inadvertently wind up frying a board.
> 
> It's like two very tiny separately derived systems, *I don't know enough to trust the controls to keep them separate.*


The typical circuits they use won't separate the grounded side of the trans they are only designed to separate the switch legs (hots). 

They only use in a T-stat for a common or grounded side of the transformer is to complete a battery charging circuit, clock circuit, or memory chip.


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## WronGun (Oct 18, 2013)

Absolute headache ! I'll never do a t-stat job again ! He has 3 of these f'n things.... nothing I did worked .... one tstat was a piggy back connection with only 2 wires on it , ok so now I have 2 wires to land for power...

And I blew a transformer in the process .... I couldn't tell you why it blew just bad luck because I was very aware of how to tap off for 24v and common. 

Ended up using the G for C on 2 of them for power ... this works if you don't care for manual fan control which he didn't... the piggy back tstat I'm clueless on


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## emtnut (Mar 1, 2015)

WronGun said:


> Absolute headache ! I'll never do a t-stat job again ! He has 3 of these f'n things.... nothing I did worked .... one tstat was a piggy back connection with only 2 wires on it , ok so now I have 2 wires to land for power...
> 
> *And I blew a transformer* in the process .... I couldn't tell you why it blew just bad luck because I was very aware of how to tap off for 24v and common.
> 
> ...


How was it for the transformer :laughing:

Sorry .. .couldn't resist :lol:


If you tell us what the 'piggy back' tstat was hooked up to we may be able to help.

The 2 transformer idea was bad to begin with ... and I hate to say we told you so .... but WE ALL told you so !


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## WronGun (Oct 18, 2013)

I decided to scratch the tformer power and all the power now comes from the control board.... 

It still didn't help I now have 2 extra wires a C wire that is 24 volts when tested with the other wire I ran it with... ( on the board 24V and Com)

I don't have anywhere to land the this other wire..

One of the 3 tstats has only RC and W.... I brought a 24v and a COM wire to it , I can land the 24V on the C but where does the COM go




The original installer was an HVAC who gave up and sold the customer the tstats and refunded them for the install , lol

Granted this guy wasn't to knowledgeable..
I've installed dozens of tstats without issue 

I'm only here because I've done lots of work for this client , but it's turning into a nightmare..


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## MechanicalDVR (Dec 29, 2007)

WronGun said:


> Absolute headache ! I'll never do a t-stat job again ! He has 3 of these f'n things.... nothing I did worked .... one tstat was a piggy back connection with only 2 wires on it , ok so now I have 2 wires to land for power...
> 
> And I blew a transformer in the process .... I couldn't tell you why it blew just bad luck because I was very aware of how to tap off for 24v and common.
> 
> ...


You do realize that the fan relay gets energized through the T-stat sub base to bring the fan on for a/c when 'Y' is made?

No 'G' wire, no fan for a/c.

The 'G' only operates as manual fan in heat mode.


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## MechanicalDVR (Dec 29, 2007)

WronGun said:


> I decided to scratch the tformer power and all the power now comes from the control board....
> 
> It still didn't help I now have 2 extra wires a C wire that is 24 volts when tested with the other wire I ran it with... ( on the board 24V and Com)
> 
> ...


Really? :whistling2:


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## emtnut (Mar 1, 2015)

When measured from 'ground' or a grounded neutral, the C wire should be 0 volts.

The RC and/or RH should be the hot (24VAC)


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## WronGun (Oct 18, 2013)

emtnut said:


> When measured from 'ground' or a grounded neutral, the C wire should be 0 volts.
> 
> The RC and/or RH should be the hot (24VAC)




RC and RH do test 24v but what does this do for getting 24v on C

I thought the C wire is for constant 24volts to run these tstats 


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## emtnut (Mar 1, 2015)

WronGun said:


> I can land the 24V on the C but where does the COM go


 C *IS* the common


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## WronGun (Oct 18, 2013)

emtnut said:


> C *IS* the common




I hate tstats ... I like the line voltage ones for baseboard heat that's all


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## emtnut (Mar 1, 2015)




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## WronGun (Oct 18, 2013)

So I don't need to land 24V and Com.... just Com 


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## emtnut (Mar 1, 2015)

WronGun said:


> I thought the C wire is for constant 24volts to run these tstats
> 
> Sent from my iPhone using Tapatalk


That's where you're messing up ... All tstats have 24V power... even the old old ones with just 2 wires.

The C wire is not needed, unless the tstat itself needs power, then it needs the RC/RH as hot (which they all have) , and the C wire which is common.

Think of it like a switch leg, without a neutral, ... you can't install a new 'smart' timer. ... **edit** without the neutral **


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## MechanicalDVR (Dec 29, 2007)

emtnut said:


>


That really isn't accurate in that the 'C' shouldn't be the same line as 'R' as that would be a direct short.


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## MechanicalDVR (Dec 29, 2007)

emtnut said:


> That's where you're messing up ... All tstats have 24V power... *even the old old ones with just 2 wires.*
> 
> The C wire is not needed, unless the tstat itself needs power, then it needs the RC/RH as hot (which they all have) , and the C wire which is common.
> 
> Think of it like a switch leg, without a neutral, ... you can't install a new 'smart' timer. ... **edit** without the neutral **


NOT so, a 2 wire T-stat is like a switch with no neutral in the 'box'. There is only 24v between the two wires when the switch is open.


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## MechanicalDVR (Dec 29, 2007)

This is a more accurate schematic with 'c' going to the T-stat:


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## scotch (Oct 17, 2013)

Yes....Honeywell came out with the "standardized" terminals....the old Trane and Lennox had their B , O , X , F , etc ...to confuse many !
The Telco had 2 hvac units on a single stat in the rural trailers....24vac side "phased"....so Y1 was one unit...and Y2 the other standby. Of course I got call from a Refrig guy changing a compressor who couldn't figure out how with the unit power off he still had 240vac ....the 24vac from the other unit was boosting thru' the transformer !


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## MechanicalDVR (Dec 29, 2007)

scotch said:


> Yes....Honeywell came out with the "standardized" terminals....the old Trane and Lennox had their B , O , X , F , etc ...to confuse many !
> The Telco had 2 hvac units on a single stat in the rural trailers....24vac side "phased"....so Y1 was one unit...and Y2 the other standby. Of course I got call from a Refrig guy changing a compressor who couldn't figure out how with the unit power off he still had 240vac ....the 24vac from the other unit was boosting thru' the transformer !


Honeywell still uses 'B', 'O', and 'X' on heat pump stats:


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## WronGun (Oct 18, 2013)

I will learn this and become a Tstat expert 


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## MechanicalDVR (Dec 29, 2007)

WronGun said:


> I will learn this and become a Tstat expert
> 
> 
> Sent from my iPhone using Tapatalk


Just think of 'R' as the hot leg and 'C' as neutral.

The only time you normally bring a 'C' leg to a t-stat is to power a device within the t-stat itself (clock, charging circuit, memory chip, etc).

As I posted earlier a t-stat is normally more than a single pole mulit position switch.


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## emtnut (Mar 1, 2015)

emtnut said:


> All tstats have 24V power... even the old old ones with just 2 wires.





MechanicalDVR said:


> NOT so, a 2 wire T-stat is like a switch with no neutral in the 'box'. There is only 24v between the two wires when the switch is open.


That's what I was saying ... maybe it wasn't clear. 
Yes, the tstat is just a contact, but the 24V is there ... not the common.

Common only needed if the tstat requires power.


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## emtnut (Mar 1, 2015)

MechanicalDVR said:


> That really isn't accurate in that the 'C' shouldn't be the same line as 'R' as that would be a direct short.


Good eye Mech !

Technically that diagram is a bolt short on the xfmer :laughing:


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## HackWork (Oct 2, 2009)

MechanicalDVR said:


> Just think of 'R' as the hot leg and 'C' as neutral.
> 
> The only time you normally bring a 'C' leg to a t-stat is to power a device within the t-stat itself (clock, charging circuit, memory chip, etc).
> 
> As I posted earlier a t-stat is normally more than a single pole mulit position switch.


Good explanation, clears it up.

Sometimes it gets confusing for non-control people to figure out which side of the tranny to use for the common to bring up to the t-stat when actually looking at the unit and the wiring.


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## MTW (Aug 28, 2013)

MechanicalDVR said:


> Just think of 'R' as the hot leg and 'C' as neutral.
> 
> The only time you normally bring a 'C' leg to a t-stat is to power a device within the t-stat itself (clock, charging circuit, memory chip, etc).
> 
> As I posted earlier a t-stat is normally more than a single pole mulit position switch.


With the advent of wifi stats, the common wire is an essential now. There are some wifi stats that don't require a C wire, but most do and it should now become standard practice to bring the C wire to all stat locations.


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## MechanicalDVR (Dec 29, 2007)

HackWork said:


> Good explanation, clears it up.
> 
> Sometimes it gets confusing for non-control people to figure out which side of the tranny to use for the common to bring up to the t-stat when actually looking at the unit and the wiring.


I've spent most of my career wiring this stuff starting in my first years in the trade when most units were all 120v controls. 

The equipment manufacturers have made it overly complicated and for no good reason, I've never understood why. 

The engineers in companies I've worked for have often had problems integrating controls systems with heat pump system controls for just that reason (lack of any standardization).

I carried books of schematics that Honeywell put out showing how to cross reference their controls to multiple manufacturers for years due to the amount of variation.


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## MechanicalDVR (Dec 29, 2007)

MTW said:


> With the advent of wifi stats, the common wire is an essential now. There are some wifi stats that don't require a C wire, but most do and it should now become standard practice to bring the C wire to all stat locations.


You will often find the ones that don't require a 'C' connection will use the 'W' connection with a bleed resistor to power the internals with in the thermostat and on super cold days where the heat stays on all day the thermostat will lose power to the memory function and lose programming.


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## HackWork (Oct 2, 2009)

MechanicalDVR said:


> You will often find the ones that don't require a 'C' connection will use the 'W' connection with a bleed resistor to power the internals with in the thermostat and on super cold days where the heat stays on all day the thermostat will lose power to the memory function and lose programming.


I'm confused, I would think if the heat was on all day the power would flow and the t-stat would be constantly charging. How does it power the t-stat when the heat is off?


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## MechanicalDVR (Dec 29, 2007)

HackWork said:


> I'm confused, I would think if the heat was on all day the power would flow and the t-stat would be constantly charging. How does it power the t-stat when the heat is off?


There is only 24v power to the charging circuit when the 'R' to 'W' switch is open.

When the heat is on the 'R'-'W' switch is closed and has no 24v.

Manufacturers bank on the fact that the heat will constantly cycle on-off.


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## HackWork (Oct 2, 2009)

MechanicalDVR said:


> There is only 24v power to the charging circuit when the 'R' to 'W' switch is open.
> 
> When the heat is on the 'R'-'W' switch is closed and has no 24v.


So you are able to flow power thru the W heat circuit like you would the filament of a bulb to keep a pilot light switch on, I assume?


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## MTW (Aug 28, 2013)

MechanicalDVR said:


> You will often find the ones that don't require a 'C' connection will use the 'W' connection with a bleed resistor to power the internals with in the thermostat and on super cold days where the heat stays on all day the thermostat will lose power to the memory function and lose programming.


I figured that's how it worked, electrically there is no other way for it to get 24V power. The Emerson Sensi works this way. 

I use Honeywell stats exclusively when I have the option to install them, and they require a C wire for wifi, for non-wifi there is a C option and battery option. But you probably know this from the HVAC field. :thumbsup:


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## MechanicalDVR (Dec 29, 2007)

HackWork said:


> So you are able to flow power thru the W heat circuit like you would the filament of a bulb to keep a pilot light switch on, I assume?


Yup, they use a resistor in the charging circuit that feeds it power but is such a high resistance it doesn't pass enough current to close the heat relay.


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## MTW (Aug 28, 2013)

HackWork said:


> So you are able to flow power thru the W heat circuit like you would the filament of a bulb to keep a pilot light switch on, I assume?


Yes.


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## HackWork (Oct 2, 2009)

So while we are on the topic, what is the easiest way to ensure the correct side of the transformer to connect the C to? 

The opposite side that the red wire is connected to.
The same side that the furnace is connected to.

Yes?

I remember having a problem with this in a super hot attic where the furnace was the last time I had to get a Nest T-stat working and it took me too many trips back and forth to be proud of.


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## MechanicalDVR (Dec 29, 2007)

MTW said:


> I figured that's how it worked, electrically there is no other way for it to get 24V power. The Emerson Sensi works this way.
> 
> I use Honeywell stats exclusively when I have the option to install them, and they require a C wire for wifi, for non-wifi there is a C option and battery option. But you probably know this from the HVAC field. :thumbsup:


I started doing this stuff in the early 60s and have found Honeywell is the most reliable overall through the years, they have out lasted so many competitors.

I've worked for control companies that were reps for KMC, Metasys, Trane, Carrier, and several other 'top' control brands and they all have weak spots that Honeywell has worked out before introducing new things.


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## MTW (Aug 28, 2013)

HackWork said:


> So while we are on the topic, what is the easiest way to ensure the correct side of the transformer to connect the C to?
> 
> The opposite side that the red wire is connected to.
> The same side that the furnace is connected to.
> ...


Your meter is your friend, verify that you have 24 volts after observing what wires are on what side of the transformer.



> I remember having a problem with this in a super hot attic where the furnace was the last time I had to get a Nest T-stat working and it took me too many trips back and forth to be proud of.


The Nest is widely hated among hvac professionals because it's garbage, so it's no surprise you had trouble with it. I've had a lot of trouble with them too.


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## MechanicalDVR (Dec 29, 2007)

HackWork said:


> So while we are on the topic, what is the easiest way to ensure the correct side of the transformer to connect the C to?
> 
> The opposite side that the red wire is connected to.
> The same side that the furnace is connected to.
> ...


Yes the opposite side of 'R', normally on the transformer inside a furnace or air handler the 'C; side has a ground wire going right to the cabinet of the unit.


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## wendon (Sep 27, 2010)

emtnut said:


> How was it for the transformer :laughing:
> 
> Sorry .. .couldn't resist
> 
> ...


The two transformer scenario is a bad idea. It will work ok if you phase them but, if anyone messes with them down the road, they'll probably end up releasing the magic smoke. Just my two bits but "Common" and "Neutral" are two different things.

Sent from my Samsung with a cracked screen protector


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## MechanicalDVR (Dec 29, 2007)

wendon said:


> The two transformer scenario is a bad idea. It will work ok if you phase them but, if anyone messes with them down the road, they'll probably end up releasing the magic smoke. Just my two bits but "Common" and "Neutral" are two different things.
> 
> Sent from my Samsung with a cracked screen protector


When the common side of the 24v side of the transformer is grounded how are common and neutral so different?


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## HackWork (Oct 2, 2009)

MechanicalDVR said:


> When the common side of the 24v side of the transformer is grounded how are common and neutral so different?


For all intents and purposes, as far as this thread, it's a perfect analogy.


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## RePhase277 (Feb 5, 2008)

If you had two 24 V transformers (properly phased) each with a grounded common, then that common would be the neutral of a 24/48 V three-wire system. It would just be like a neutral. It would literally be a neutral.


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## RePhase277 (Feb 5, 2008)

MTW said:


> With the advent of wifi stats, the common wire is an essential now. There are some wifi stats that don't require a C wire, but most do and it should now become standard practice to bring the C wire to all stat locations.





> With the advent of occupancy sensors, the neutral wire is an essential now. There are some occupancy sensors that don't require a N wire, but most do and it should now become standard practice to bring the N wire to all switch locations.


Apparently, the second quote makes electricians loose their minds....:whistling2:


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## splatz (May 23, 2015)

HackWork said:


> For all intents and purposes, as far as this thread, it's a perfect analogy.





RePhase277 said:


> If you had two 24 V transformers (properly phased) each with a grounded common, then that common would be the neutral of a 24/48 V three-wire system. It would just be like a neutral. It would literally be a neutral.


Makes sense, the windings are just not continuous in the same can like they are with a utility transformer. 

Are the little 24VAC power supplies just windings, or is there more going on inside there? Can you rely on them to be in phase with the source 120 or 240 VAC source?


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## RePhase277 (Feb 5, 2008)

```

```



splatz said:


> Makes sense, the windings are just not continuous in the same can like they are with a utility transformer.
> 
> Are the little 24VAC power supplies just windings, or is there more going on inside there? Can you rely on them to be in phase with the source 120 or 240 VAC source?


All the 24 V transformers I have seen are just two coils of wire on a laminated core, like any transformer... but if they have gone to some kind of magic box 24 V "power supply", I don't know.


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## RePhase277 (Feb 5, 2008)

As far as phasing goes, if you 'scoped the primary and secondary of a single phase transformer, you would find them out of phase by 30 degrees or so due to the hysteresis of the iron core.


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## splatz (May 23, 2015)

RePhase277 said:


> As far as phasing goes, if you 'scoped the primary and secondary of a single phase transformer, you would find them out of phase by 30 degrees or so due to the hysteresis of the iron core.


So would two simple 24vac transformers necessarily be out of phase the same ~30 degrees, or could they be out of phase enough to make trouble?


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## RePhase277 (Feb 5, 2008)

splatz said:


> So would two simple 24vac transformers necessarily be out of phase the same ~30 degrees, or could they be out of phase enough to make trouble?


I doubt the quality control is very high on simple transformers.... but the tolerance of the equipment it's connected is likely wide enough that no one would notice.


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## nrp3 (Jan 24, 2009)

One of my customers asked me to change out one wifi one for another, and I passed on it after I saw it needed a C. His house is the only one I've seen with what I guess is a chiller system, freon to water loop. It has multiple boilers and multiple airhandlers. Knowing what C was and why its needed wasn't the issue, it was which transformer to take it too. He has a crew from one of the local commercial HVAC companies do the service on that system. Told him to have it done by them since they built and maintain it. He didn't like the Nest because it would learn and since wife was always turning it up and down, when she wasn't home it would do it on its own.


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## HackWork (Oct 2, 2009)

You bring up a good point. A lot of houses around here have a natural gas furnace to heat water or steam, and then a completely separate air handler for the central AC. And they both get wired to the same thermostat. So which one would you use for the common on the thermostat?


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## nrp3 (Jan 24, 2009)

I have wired these before in the past, where the jumper was removed, R and W covered the boiler and RC, Y G did the rest. Still unsure how the C would Pair up in this case. I have all kinds of different heating, heat loops in airhandlers, forced hot water baseboard, but this is one transformer, the airhandler, and then it calls for heat.


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## sbrn33 (Mar 15, 2007)

This thread makes me cry a little. such a simple job made complicated. If you do not understand stat wiring you should not act like you do.


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## RePhase277 (Feb 5, 2008)

sbrn33 said:


> This thread makes me cry a little.


You're so hormonal. Low T?


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## MechanicalDVR (Dec 29, 2007)

RePhase277 said:


> Apparently, the second quote makes electricians loose their minds....:whistling2:


Apparently with some guys bringing some extra 14-3 on a job would ruin their budget....


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## MechanicalDVR (Dec 29, 2007)

HackWork said:


> You bring up a good point. A lot of houses around here have a natural gas furnace to heat water or steam, and then a completely separate air handler for the central AC. And they both get wired to the same thermostat. So which one would you use for the common on the thermostat?


You would need to have a schematic of the t-stats internal wiring but in my experience the t-stat normally uses the 'Rh' connection to get power so you'd derive your 'C' from the furnace or boiler transformer that provides heating.


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## emtnut (Mar 1, 2015)

nrp3 said:


> I have wired these before in the past, where the jumper was removed, R and W covered the boiler and RC, Y G did the rest. Still unsure how the C would Pair up in this case. I have all kinds of different heating, heat loops in airhandlers, forced hot water baseboard, but this is one transformer, the airhandler, and then it calls for heat.


If you think of the 'C' wire, as the bonded metal in both systems .... Why would it matter ?


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## MechanicalDVR (Dec 29, 2007)

emtnut said:


> If you think of the 'C' wire, as the bonded metal in both systems .... Why would it matter ?


Depending on how the wiring was done on the two units it could matter.

You just have to see if you get 24v between the 'C' you pick and 'Rh'.


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## emtnut (Mar 1, 2015)

MechanicalDVR said:


> Depending on how the wiring was done on the two units it could matter.
> 
> You just have to see if you get 24v between the 'C' you pick and 'Rh'.


True, if one system was isolated ... that's a guess.

Any system I've seen has the common bonded to the metal and/or neutral, but I haven't work on nearly as many as you have 

If it was MY liability on the line ... I'd be checking the schematics


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## MechanicalDVR (Dec 29, 2007)

emtnut said:


> True, if one system was isolated ... that's a guess.
> 
> Any system I've seen has the common bonded to the metal and/or neutral, but I haven't work on nearly as many as you have
> 
> If it was MY liability on the line ... I'd be checking the schematics


This may sound off the wall but if I had a nickel for everytime I saw romex coming into a furnace junction box with the ground just wrapped around the outer jacket of the cable or just clipped off completely I'd come buy you lunch.

For some odd reason some guys lose all common sense as soon as they go to wire up an HVAC/R appliance. 

Another semi related example is wiring in pvc conduit inside walk in boxes for mullion heaters, lights, defrosts heaters, and evap motors that no ground is pulled with switch legs or power wiring, see that all the time.


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## HackWork (Oct 2, 2009)

These thermostats should just come with a secondary pair of inputs to use a totally separate transformer if necessary.


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## MechanicalDVR (Dec 29, 2007)

HackWork said:


> These thermostats should just come with a secondary pair of inputs to use a totally separate transformer if necessary.


Some of the larger microprocessor types used to come that way but they were fairly large for a thermostat, especially for resi use.

Shrinking the footprint was the norm for a long while. 

Makes me laugh to see the Nest T-stat went right to the footprint of the old Honeywell T-87F.


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## HackWork (Oct 2, 2009)

MechanicalDVR said:


> Some of the larger microprocessor types used to come that way but they were fairly large for a thermostat, especially for resi use.
> 
> Shrinking the footprint was the norm for a long while.
> 
> Makes me laugh to see the Nest T-stat went right to the footprint of the old Honeywell T-87F.


Yeah, we had 3 of those Honeywells in the house I grew up in, each one did a zone of the hot water heat and an AC unit.

The Nest is a pleasure to wire up, I must say.


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## MechanicalDVR (Dec 29, 2007)

HackWork said:


> Yeah, we had 3 of those Honeywells in the house I grew up in, each one did a zone of the hot water heat and an AC unit.
> 
> The Nest is a pleasure to wire up, I must say.


Those old round Honeywells were the 1911 of T-stats they were the standard for the longest time. 

Jimmy Carter threw a wrench in that working machine with his mandatory energy standards. 

That was when all the electronic crap started coming out. 

When Reagan came into office T-87s made a come back.


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## WronGun (Oct 18, 2013)

This job has been the death of me ... I'm going on 10 hrs of "warranty time"... and I'm about to put all his old thermostats in....

AC works , heat doesn't...

The common which is supposed to test 24 volts is giving me only 12 at the third tstat so it isn't even turning on! 

All I've done here is given him 3 common wires and it's turned into an absolute nightmare !!


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## MechanicalDVR (Dec 29, 2007)

WronGun said:


> This job has been the death of me ... I'm going on 10 hrs of "warranty time"... and I'm about to put all his old thermostats in....
> 
> AC works , heat doesn't...
> 
> ...


A little more reading and drawing it all out before you did the initial work may have saved you all the problems and burned up parts.


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## WronGun (Oct 18, 2013)

Just doesn't seem difficult ... common right to the taco board COM











up to the Tstat C-terminal /no power thermostat won't turn on ...

This is the piggy back or slave unit I was talking about 












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## WronGun (Oct 18, 2013)

This house has 2 Air handlers , 2 condensers , and a boiler for radiant heat .....


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## MechanicalDVR (Dec 29, 2007)

WronGun said:


> This house has 2 Air handlers , 2 condensers , and a boiler for radiant heat .....
> 
> 
> Sent from my iPhone using Tapatalk


Did you make note of where the original wiring was landed on each t-stat before you removed them?


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## WronGun (Oct 18, 2013)

MechanicalDVR said:


> Did you make note of where the original wiring was landed on each t-stat before you removed them?




Yes the same way they are landed now ...

G to G
W to W
Y to Y and so on 


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## MechanicalDVR (Dec 29, 2007)

WronGun said:


> Yes the same way they are landed now ...
> 
> G to G
> W to W
> ...


Could you have smoked a board?


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## emtnut (Mar 1, 2015)

WronGun said:


> Just doesn't seem difficult ... common right to the taco board COM
> 
> 
> 
> ...


Try moving the R - R & Rc dipswitch to 'r & rc'


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## WronGun (Oct 18, 2013)

MechanicalDVR said:


> Could you have smoked a board?




AC runs , heat doesn't ....

Common to the slave Tstat doesn't bring power , board works fine 


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## mitch65 (Mar 26, 2015)

You could run 2nd xfrmr R to C, remove the jumper from Rc to R and run furnace controls using furnace power. Tstat controls basically dry contacts Rc-II-W, Rc-II-Y etc.


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## WronGun (Oct 18, 2013)

emtnut said:


> Try moving the R - R & Rc dipswitch to 'r & rc'




I have the wire out and testing with my meter .... I should have power for this tstat


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## MechanicalDVR (Dec 29, 2007)

mitch65 said:


> You could run 2nd xfrmr R to C, remove the jumper from Rc to R and run furnace controls using furnace power. Tstat controls basically dry contacts Rc-II-W, Rc-II-Y etc.
> View attachment 106601


There are no 'dry' contacts on a t-stat.


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## mitch65 (Mar 26, 2015)

MechanicalDVR said:


> There are no 'dry' contacts on a t-stat.


In the diagram, If you remove factory installed jumper from R to Rc and Rh, the contacts in the Tstat are basically dry contacts.


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## MechanicalDVR (Dec 29, 2007)

mitch65 said:


> In the diagram, If you remove factory installed jumper from R to Rc and Rh, the contacts in the Tstat are basically dry contacts.


I hear what you are saying but without a schematic of the internal stat wiring that is making a big assumption. Granted each internal relay gets the power to operate those relays from the system power for said relay.


ie: Rh powers the heat relays, Rc powers the cool relays.


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## WronGun (Oct 18, 2013)

Ended up calling my friends HVAC company ..... lessoned learned I don't be getting into Tstats ....

After spending time on the phone with Honeywell .. 

they said we need an isolation relay installed at boiler .....

Mind you 3 older basic Honeywell tstats worked just fine 


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## mitch65 (Mar 26, 2015)

MechanicalDVR said:


> I hear what you are saying but without a schematic of the internal stat wiring that is making a big assumption. Granted each internal relay gets the power to operate those relays from the system power for said relay.
> 
> 
> ie: Rh powers the heat relays, Rc powers the cool relays.


Easy enough to meter out. R powers relay coils, Rc powers switched ccts in Tstats I've seen with that jumper.


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## MechanicalDVR (Dec 29, 2007)

mitch65 said:


> Easy enough to meter out. R powers relay coils, Rc powers switched ccts in Tstats I've seen with that jumper.


If R only powers the relay coils that would work.


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## nrp3 (Jan 24, 2009)

I like the honeywell round, todays version for older folks. No batteries, no programming, a format they are used to. I'm going to try out one of the Honeywell ones designed for the RA2 system I have in my house. The I phone app makes things easier to program. I hate programming the routines on the stats themselves.


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