Dedicated Thermostat "C" Wire Necessity

[Butrbal]

If there is not a 24VAC "C" wire coming from the heating/AC system , but there is already +24VAC (27VAC in my case) coming to the thermostat via the red Rc wire, why cant you jump the Rc over to the C terminal to power the thermostat?

Or can you without any problems?

 

[Jackfre]

You need the common with "power stealing" stats. Some stats do not require a common. Braeburn is one of them. I haven't dealt with this in several years, so my info is dated. Proceed with caution in other words

 

[Mike007]

You need the common with "power stealing" stats. Some stats do not require a common. Braeburn is one of them. I haven't dealt with this in several years, so my info is dated. Proceed with caution in other words

I think you mean "Don't need".

 

[Butrbal]

It often will work that way but it basically depends on how much current the thermostat takes to run compared to how much current it takes to switch on the relay or whatever controlling your system. It's really a poor way to have it set up but unfortunately a lot of houses were wired that way a while back when simple thermostats like the mercury switch ones were common. If your house is newer construction, the contractors who built it did a sub-par job there.

It doesn't hurt to jumper it over and try. You may simply find that your system stays on if you do that.

Ok, that makes sense. I had not taken into consideration the current draw of the thermostat and weather there was enough leftover current to tell the unit to start/stop.

In my situation I have two separate (simple) thermostats (one for heat, one for cooling) and I'm looking to replace them with a single WiFi unit that I can control when away from home. I see what your saying about current draw. I'll try jumping the C to Rc to power the thermostat first, if it doesn't work (or the unit stays running) I can always try powering the thermostat from the Rh. If neither work properly I guess ill have to run a dedicated 24VAC wire to the thermostat.

 

[Trey T]

What brand model do you have? Nest?

If you have a wire to control the fan, you can switch it to Common wire. Of course you have to switch it at the furnace/blower/transformer/logic board.

 

[Butrbal]

What brand model do you have? Nest?

If you have a wire to control the fan, you can switch it to Common wire. Of course you have to switch it at the furnace/blower/transformer/logic board.

I haven't purchased the thermostat yet, I'm just getting everything in order first (knowledge wise).
I would rather not use the fan wire (green in my case) as the common. I like the idea of being able to independently turn on the fan to get air movement throughout the house.

 

[ctfjr]

If there is not a 24VAC "C" wire coming from the heating/AC system , but there is already +24VAC (27VAC in my case) coming to the thermostat via the red Rc wire, why cant you jump the Rc over to the C terminal to power the thermostat?

Or can you without any problems?

From what you wrote I understand it to mean you do NOT have the 'C' wire coming up to the thermostat. So, you only have one half of the 24 volt transformer side.

The other side is 'C' and it goes through your heating relay coil to the 'W' terminal. Likewise it also goes through the cooling contactor coil ('Y') and the fan relay coil ('G'). If you put a typical voltmeter between any of these three connections (Y, G, or W) and the 'R' terminal you will measure ~24 volts - only because the meter has a very high internal resistance and each of the coils resistance is relatively very low.

This is how 'power stealing' thermostats work - they allow a small current to run the thermostat, not enough to allow the relay coil to pull in the relay when the circuit is not 'calling'. If your thermostat truly needs 24 volts for power you will either have to run an additional wire or use a 'wire eliminator' device that is available (I know Honeywell has one)

 

[Butrbal]

From what you wrote I understand it to mean you do NOT have the 'C' wire coming up to the thermostat. So, you only have one half of the 24 volt transformer side.

The other side is 'C' and it goes through your heating relay coil to the 'W' terminal. Likewise it also goes through the cooling contactor coil ('Y') and the fan relay coil ('G'). If you put a typical voltmeter between any of these three connections (Y, G, or W) and the 'R' terminal you will measure ~24 volts - only because the meter has a very high internal resistance and each of the coils resistance is relatively very low.

This is how 'power stealing' thermostats work - they allow a small current to run the thermostat, not enough to allow the relay coil to pull in the relay when the circuit is not 'calling'. If your thermostat truly needs 24 volts for power you will either have to run an additional wire or use a 'wire eliminator' device that is available (I know Honeywell has one)

ctfjr,
See my other thread about using a single thermostat to replace my two existing thermostats. That will give you a better idea of my existing wiring, and what im trying to accomplish. http://www.garagejournal.com/forum/showthread.php?t=281999
Let me know what you think.

 

[Rockhead261]

You've really got to love some of the "advice" on this forum...

You can not jump Rc to C.

Let me say that again....

You can NOT jump Rc to C.

Rc is the power side of the transformer. C is the common. Jumping Rc to C will either:

(A) Do nothing,
(B) Cause erratic operation, or
(C) Cook your $250 Nest thermostat.

If you need C (and you will for just about any Internet enabled stat) you must run a dedicated wire to carry it.

If you're considering Nest be sure to look at the offerings from Ecobee and Honeywell.

As far as power consumption goes, I have never come across a thermostat that requires more power than the system transformer can provide. Keep in mind that thermostats are switches, and switches don't need power. The backlights are LED's and the WiFi module requires so little power you'll need special instruments to measure the consumption.

 

[dfiler2]

I believe Ecobee has a wireless wifi thermostat that would eliminate the need for any wires. They also carry what they call a "Power Extender" that takes care of the missing "C" wire problem.

http://www.globalindustrial.com/p/hvac/controls/thermostats/eb-power-extender-kit-eb-pek-01

http://www.globalindustrial.com/p/h...vSXWRaB6EEr4nT4NzkcdbhdGQT3yMTANUlBoCDzrw_wcB

 

[Rockhead261]

Re: Dedicated Thermostat "C" Wire Necessity

I believe Ecobee has a wireless wifi thermostat that would eliminate the need for any wires. They also carry what they call a "Power Extender" that takes care of the missing "C" wire problem.

http://www.globalindustrial.com/p/hvac/controls/thermostats/eb-power-extender-kit-eb-pek-01

http://www.globalindustrial.com/p/h...vSXWRaB6EEr4nT4NzkcdbhdGQT3yMTANUlBoCDzrw_wcB

I have that exact thermostat. It is not wireless. The power extender is merely a workaround module for use if there is no C wire available. I wouldn't use one and would rather spend the time/money running a wire.

 

[RedDirtRoad]

Rockhead is 1000% correct
DO NOT jump R to C you will fry yout transformer
Look at your thermostat wire closely and see how many conductors you have in the jacket.
If you have an extra conductor its most likely blue in color then attach that to the common of the transformer locared in your furnace/air handler and the other end to C on the T-stat then yout T-stat will operate without needing batteries

 

[tel0004]

I agree with the others that using Rc as a common is a bad idea.

But I have heard many people say that nothing is wrong with using your G wire (fan) instead of a common. Then you jump from the Y-G as well, so turning on the air conditioning will turn on the fan. This would loose the ability to turn on the fan independently, which you said you didn't want to do.

They do make 4 wire to 5 wire adapters. http://www.amazon.com/dp/B0013LVDQA/?tag=atomicindus08-20

I have no experience with them, I just know they exist.

I faced this issue about 6 months ago, but in my case running a common wire was very easy, so I ran a common wire.

 

[Butrbal]

Thanks for the input. Looks like the surest/safest thing to do is run the common. I don't have one now so ill have to pull a 7 conductor thermostat wire set from the unit to the thermostat location.

I am still curious why using the Rc to also power the stat is not viewed as acceptable by some.

As I understand it, there is a transformer in the unit making 24VAC sending it to the stat on the Rc wire. When the stat closes it sends that voltage back to the unit through the white wire that closes a relay and kicks the unit on.

If the stat uses a nearly un-measurable amount of power, how is using such a minimal amount of the Rc current to power the stat a bad idea.

 

[Rockhead261]

Thanks for the input. Looks like the surest/safest thing to do is run the common. I don't have one now so ill have to pull a 7 conductor thermostat wire set from the unit to the thermostat location.

I am still curious why using the Rc to also power the stat is not viewed as acceptable by some.

As I understand it, there is a transformer in the unit making 24VAC sending it to the stat on the Rc wire. When the stat closes it sends that voltage back to the unit through the white wire that closes a relay and kicks the unit on.

If the stat uses a nearly un-measurable amount of power, how is using such a minimal amount of the Rc current to power the stat a bad idea.

It seems there is a slight misinterpretation in terms here.

Readers Digest version: Rc, Rh, R... all the same from a power standpoint, all good for powering the stat. C is the common/ground/neutral, and it is needed to complete the circuit; without it the stat will not "see" power. Power needs somewhere to go, and C completes the circuit. Connecting R (hot) to C (common) is fundamentally incorrect.

Thermostats, in their simpliest forms, are nothing but switches. Switches don't need power. A simple heat thermostat has an "R" terminal and a "W" terminal. R = power, W = heat. When the switch is open (i.e., no call for heat), you can measure 24VAC between R and W. When the switch closes (i.e., call for heat) the circuit closes and there is 0VAC between R and W.

Enter the early programmable thermostat. It had a mechanical clock, and that clock needed power. Conveniently, the clock can "steal" power from the system, but only when there is no call for heat Remember, once the switch closes, there is no longer voltage available at the stat. Manufacturers installed rechargable batteries to carry the clock during on cycles, then the batteries would recharge during off cycles. It worked fine this way for decades, as the venerable Honeywell T-8082 demonstrated.

Now that mercury switches and mechanical clocks are sitting beside carburetors and rotary telephones in museums, thermostats are moving towards system power to maintain their WiFi and other internal circuitry. In order to provide power to the thermostat during both off and on cycles, you must have the "other" side of the transformer (C) present.

Batteries are still in place but are primarily to provide power for memory during power outages.

Run a 7 conductor bundle and be done with it.

FWIW, my Ecobee runs off 4 wires between the panel and the stat. 2 are DC communication lines and 2 are 24VAC. It also requires complete wiring between the control panel and the system. It's a truly advanced control system, and with it's recent integration with IFTTT, I expect it wil grab the lions share of the market.

 

[RunninOnEmpty]

Yeah, that's a good point, the thermostat would short out its own power source. I was going to include in my previous replies that the stat would see a lower voltage when the heating call was sent, but I forgot to account for the stat output shorting out. Thanks for correcting me on that - I feel a bit silly now. It could still be tried, just adds another potential reason why it might not work. You might find the stat turning off as soon as it tries to turn the heat on. (The previous claims that it's going to break stuff are still paranoid, but that wasn't from Rockhead.)

 

[ctfjr]

It seems there is a slight misinterpretation in terms here.

Readers Digest version: Rc, Rh, R... all the same from a power standpoint, all good for powering the stat. C is the common/ground/neutral, and it is needed to complete the circuit; without it the stat will not "see" power. Power needs somewhere to go, and C completes the circuit. Connecting R (hot) to C (common) is fundamentally incorrect.

Thermostats, in their simpliest forms, are nothing but switches. Switches don't need power. A simple heat thermostat has an "R" terminal and a "W" terminal. R = power, W = heat. When the switch is open (i.e., no call for heat), you can measure 24VAC between R and W. When the switch closes (i.e., call for heat) the circuit closes and there is 0VAC between R and W.

Enter the early programmable thermostat. It had a mechanical clock, and that clock needed power. Conveniently, the clock can "steal" power from the system, but only when there is no call for heat Remember, once the switch closes, there is no longer voltage available at the stat. Manufacturers installed rechargable batteries to carry the clock during on cycles, then the batteries would recharge during off cycles. It worked fine this way for decades, as the venerable Honeywell T-8082 demonstrated.

Now that mercury switches and mechanical clocks are sitting beside carburetors and rotary telephones in museums, thermostats are moving towards system power to maintain their WiFi and other internal circuitry. In order to provide power to the thermostat during both off and on cycles, you must have the "other" side of the transformer (C) present.

Batteries are still in place but are primarily to provide power for memory during power outages.

Run a 7 conductor bundle and be done with it.

FWIW, my Ecobee runs off 4 wires between the panel and the stat. 2 are DC communication lines and 2 are 24VAC. It also requires complete wiring between the control panel and the system. It's a truly advanced control system, and with it's recent integration with IFTTT, I expect it wil grab the lions share of the market.

Best answer so far, almost 100%. The Op has two systems, one heating one cooling (from what I gather from his posts). He wants to replace the two stats with one. Ahhh there is the rub!

He has 2 24 volt sources (one in the heating equipment, one in the cooling equipment). It is not good practice to join the two "R" terminals together. That's why there is usually a removable jumper between Rc and Rh on the thermostat. If you only had 1 24 volt source you would leave the jumper in.

It sounds like he needs 6 wires from the stat to the equipment - Rh and W for the heating and Rc, Y, G and C for the cooling. The jumper should be removed at the stat from Rc and Rh.

Not all thermostats are exactly as described but I would guess far more than 90% are.

 

[RunninOnEmpty]

Oh, combining thermostats. **** - the problem is that I can't read.

Definitely run new wires, 7 or 10 wire... The advice I gave before cannot be used for this as this cannot be done with 2 wires no matter what... There is no way to apply what I said.

Sorry for the waste of time. Great explanation by Rockhead.

 

[Rockhead261]

It is not good practice to join the two "R" terminals together. That's why there is usually a removable jumper between Rc and Rh on the thermostat. If you only had 1 24 volt source you would leave the jumper in.

Absolutely correct. I made my "all the same" remark to drive home the point that power should only be applied to "R" terminals and didn't consider that it could be taken out of context in a multi-system application. Thanks for clarifying the Rc-Rh issue.

 

[LS6 Tommy]

Oi. If there's no C wire from the T-stat to the transformer, jumping Rc to C will not do ANTYHTING. The T-stat probably won't work, either...

Tommy

 

[Butrbal]

C is the common/ground/neutral, and it is needed to complete the circuit; without it the stat will not "see" power. Power needs somewhere to go, and C completes the circuit. Connecting R (hot) to C (common) is fundamentally incorrect.

Now it all makes sense.
I was under the impression that the C wire was a hot wire coming from the unit to power the stat.
When instead it is a common (neutral/ground) wire, hence the "C".