# Combining 2 heating thermostats into one



## beenthere (Oct 11, 2008)

Just need to jumper the wires at the zone panel. No need to run a new wire to the stat location.


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## Bob Sanders (Nov 10, 2013)

ppatoulidi said:


> 1) Does this sound like a good idea or does it make sense to keep two zones (at $250 a pop, i do not really feel like buying two separate NESTs)


If you have opened up the area then no, multi zones are no longer necessary and are a waste of money.



> 2) Does combining the two thermostats simply involve re-routing the "Zone 2 Stat" wire to the "Zone 1 stat" location and then wiring it in parallel with the new thermostat? Am I missing anything?


It depends on the heating system. If it's forced air zoned then there will be a central zoning panel and as BT suggests you can simply jumper the zones.

If it's a boiler with zoned rads then there may be some rewiring needed.


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## hvactech126 (Nov 11, 2010)

The nest is very sensitive to wiring and may not like anything being jumped at the zone panel. As Bob states if it is radiant boiler heat then it may require different wiring. In order to correctly answer your question we need more information about your setup.


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## jimn (Nov 13, 2010)

Zone valves or separate circulator pumps? If it's zone valves, you could try hooking them up in parallel. I have three zones and gulp three Nests. They can be wiring sensitive but if your system is fairly simple (red and white) I would think it should work. As an added note, I have had baseboard radiant heat and three or four different programmable thermostats. The Nests work better with my heating system than anything I have had in the past. The warm up anticipation time is about perfect and the evenness of the temp once warmed up is also more consistent . 

Depending on how your boiler is plumbed, combing two zones to one may not be that difficult. Although if you need to pay some one to do it, the second Nest might be cheaper.


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## ppatoulidi (Sep 24, 2014)

All, thanks for all your replies and my apologies for not getting back sooner. 

My system is natural gas, forced hot water, baseboard heat with three zones -- 2 on the first floor and 1 on the second floor. 

The boiler is a Hydrotherm HC-125C (~28 years old) with three Taco 007-F3 cartridge circulators and three Honeywell RA832A-1066 switching relays.

I have attached pictures of the setup (below). Can i just disconnect the thermostat wires from the Zone 2 switching relay and add a jumper cable to the Zone 1 relay? If so, how exactly would this be wired?


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## beenthere (Oct 11, 2008)

The 2 low voltage terminals on the right side should be end switches. Remove the wires from the zone 1 end switch terminals, and connect the stat wire from the zone 2 TT terminals to the zone 1 end switch terminals.

You can confirm that the terminals on the right are end switches, by using a multimeter to make sure there is no voltage on them with no wires connected, and the thermostat calling for heat.


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## zappa (Nov 25, 2011)

beenthere said:


> The 2 low voltage terminals on the right side should be end switches. Remove the wires from the zone 1 end switch terminals, and connect the stat wire from the zone 2 TT terminals to the zone 1 end switch terminals.
> 
> You can confirm that the terminals on the right are end switches, by using a multimeter to make sure there is no voltage on them with no wires connected, and the thermostat calling for heat.


I'm only seeing circulators BT??


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## beenthere (Oct 11, 2008)

Post 6, has 3 pics of circ relay boxes.


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## zappa (Nov 25, 2011)

beenthere said:


> Post 6, has 3 pics of circ relay boxes.


I'm wondering what you meant by end switches. I'm not seeing any zone valves.

Do the stats run directly to those boxes which turn on the circulators?


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## beenthere (Oct 11, 2008)

zappa said:


> I'm wondering what you meant by end switches. I'm not seeing any zone valves.
> 
> Do the stats run directly to those boxes which turn on the circulators?


Yep. if you look at the boxes. You will see 2 sets of terminals in teh top left. One set is for tstat, the other should be end switch terminals.


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## zappa (Nov 25, 2011)

beenthere said:


> Yep. if you look at the boxes. You will see 2 sets of terminals in teh top left. One set is for tstat, the other should be end switch terminals.


Oh, I see what you're talking about. Looks like they are daisy chained together and telling the boiler that there is a call for heat


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## ppatoulidi (Sep 24, 2014)

Thanks for the replies guys, but I am still a little confused. I want to completely eliminate the zone 2 stat so that all of the heat in both zones 1 and 2 is controlled by the zone 1 stat. Therefore, wouldn't I want to take the zone 2 stat wire out of the equation entirely? 

Intuitively, I would think it would make more sense to simply disconnect the stat wire from the zone 2 TT terminals and just add a short jumper cable from the zone 1 TT terminals to the zone 2 TT terminals leaving the end switch terminals alone. That way when the Zone 1 stat calls for heat, it would trigger both the zone 1 and zone 2 relays. 

Do I have this wrong??


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## beenthere (Oct 11, 2008)

ppatoulidi said:


> Thanks for the replies guys, but I am still a little confused. I want to completely eliminate the zone 2 stat so that all of the heat in both zones 1 and 2 is controlled by the zone 1 stat. Therefore, wouldn't I want to take the zone 2 stat wire out of the equation entirely?
> 
> Intuitively, I would think it would make more sense to simply disconnect the stat wire from the zone 2 TT terminals and just add a short jumper cable from the zone 1 TT terminals to the zone 2 TT terminals leaving the end switch terminals alone. That way when the Zone 1 stat calls for heat, it would trigger both the zone 1 and zone 2 relays.
> 
> Do I have this wrong??


Follow instructions in post 7. And you do completely get rid of the zone 2 thermostat. 

You don't want 2 transformers supply power to the nest for heat.


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## zappa (Nov 25, 2011)

beenthere said:


> Follow instructions in post 7. And you do completely get rid of the zone 2 thermostat.
> 
> You don't want 2 transformers supply power to the nest for heat.


Hi beenthere....like the OP, I also got confused following post 7 but I think I know what you are saying. Does this sound correct??

Remove stat 2 wiring from TT and abandon it. Wire zone 1 XX (end switch) to zone 2 TT so zone 1 will also control zone 2.

As an added bonus, if zone 1 is far left and zone 2 is center, the OP could use the existing short jumper wire that is already run between zone 1 and 2.


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## beenthere (Oct 11, 2008)

Thats it.


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## ppatoulidi (Sep 24, 2014)

Ok so quick update... I rewired everything as you guys said and the system seems to be working just how it should. Thank you guys very much for the help!

The only weird thing now is that the nest that I installed in the kitchen (zone 1) says that no power is detected to the W1 (white) wire and the nest that I installed in the bedroom (zone 3) says that it does not detect any equipment hooked up to the Rh (red) wire. However, both Tstats appear to be controlling the heat correctly regardless of these error messages. Do you think it is just something I can ignore?


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## ppatoulidi (Sep 24, 2014)

So I just answered my own question... The Nest troubleshooting site says I can ignore the error message. It is just because of the unique wiring config but everything is working fine


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## beenthere (Oct 11, 2008)

Yeah, Nest thermostats can be a bit touchy and give off errors. But still work.


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## hvactech126 (Nov 11, 2010)

While it may be working now.... make sure the battery is charging.... If not you may come home to no heat.


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## ppatoulidi (Sep 24, 2014)

hvactech you must have esp... I came downstairs this morning to find that the nest would not wake up and the red LED was blinking. Fortunately it seems like it must have just died a little while ago because the house was not too cold. I was able to recharge using a USB cable, but not sure how long that will last.

Is there something simple that I can change in my wiring to fix this? After some research online it looks like some folks have had to add a third, "common", wire to provide constant power for recharging. I do not have any unused wires in the existing bundle, just red and white, so would need to run a new line from the basement. Is this my only option? If so, exactly where do I connect the "common" wire to the switching relay downstairs?

I have not had any problems with the nest on the second floor, but i think that may be because I also have the separate central AC system hooked up to that tstat, so it has a second power source to draw from. Does that make sense?

I have attached a current/updated wiring schematic for the three switching relays in the basement as well as the wiring at each of the two Nests. (Zone 1 is first floor, Zone 3 is second floor and Zone 2 is the one that was combined with Zone 1)


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## ppatoulidi (Sep 24, 2014)

Just looking at that schematic.... When I reconfigured the wiring, did I create a break in the circuit? Should I have added a jumper between the Zone 1 XX terminals and the Zone 3 XX terminals?

Also, for the TT terminals, does it matter which one gets the red wire and which gets the white? On Zone 1, white is top and red is bottom, but Zone 3 is reversed. I did not reconfigure these, so this is how they were originally wired.


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## Bob Sanders (Nov 10, 2013)

You have no C wire. You need one. If you can't run one then you may want to try Venstar's add-a-wire. Some have had luck with it.

http://www.fixmyownairconditioning.com/new-thermostat.asp

The reviews seem reasonable on amazon
http://www.amazon.com/Venstar-Add-A-Wire-Wire-Adapter/dp/B0013LVDQA


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## Bob Sanders (Nov 10, 2013)

ppatoulidi said:


> hvactech you must have esp... I came downstairs this morning to find that the nest would not wake up and the red LED was blinking. Fortunately it seems like it must have just died a little while ago because the house was not too cold. I was able to recharge using a USB cable, but not sure how long that will last.
> 
> Is there something simple that I can change in my wiring to fix this? After some research online it looks like some folks have had to add a third, "common", wire to provide constant power for recharging. I do not have any unused wires in the existing bundle, just red and white, so would need to run a new line from the basement. Is this my only option? If so, exactly where do I connect the "common" wire to the switching relay downstairs?
> 
> ...


End switches are usually dry contacts with no power to them which would mean in your diagram, you have no power to zone 2. They are simply switches. You need to supply the power to be switched.


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## ppatoulidi (Sep 24, 2014)

So here is a picture of the actual zone 1 and 2 switching relays.... both are getting power from the wires coming in at the bottom.

If i were to add a common wire, where would it hook up?


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## Bob Sanders (Nov 10, 2013)

Here is a scematic of your pump (or zone) relays. As can be seen there is no power connection at all to the X contacts. It's just a switch. It is not connected to any power. In your existing wiring diagram above you have nothing but a switch (with no power) feeding zone 2. Under this condition, zone 2 is not functioning.


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## Bob Sanders (Nov 10, 2013)

Forget it. I'm wrong. I'm looking at this and zone 2 has it's own power source! :thumbup:


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## Bob Sanders (Nov 10, 2013)

The c wire would depend on which transformer you're using to power the heating side. You need the OTHER side of THAT transformer up at the thermostat. Follow the red wire from Rh back and that will be the transformer used. Bring the other side of that transformer up and that will be your C wire


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## ppatoulidi (Sep 24, 2014)

I guess I am not sure what you mean by "other side". Looking at the pic above, The red wire for the Tstat in question is the one in the left box on the bottom left "T" terminal. So to which of those terminals to I hook the C wire?


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## Bob Sanders (Nov 10, 2013)

ppatoulidi said:


> I guess I am not sure what you mean by "other side". Looking at the pic above, The red wire for the Tstat in question is the one in the left box on the bottom left "T" terminal. So to which of those terminals to I hook the C wire?


You will have to open up the relay box to get to the other side of that transformer


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## ppatoulidi (Sep 24, 2014)

So i flipped the board around in the relay box and snapped a pic (see below). Based on your instructions, it sounds like I would need to connect the C wire to the terminal indicated by the arrow, is that correct? 

I did some other research in the meantime and I have found that other people who have installed a Nest with a similar set up as mine have had to add an external relay or plug-in transformer to provide the C wire. These do not appear to be very expensive, so I do not mind doing this if it is the preferred method. But if your way is just as good, it may not be worth it. I am just concerned about accidentally frying the thermostat. Thoughts?


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## hvactech126 (Nov 11, 2010)

The proper way to correct the issue without possibly causing damage to your system would be to use the relay and transformer method.


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## zappa (Nov 25, 2011)

hvactech126 said:


> The proper way to correct the issue without possibly causing damage to your system would be to use the relay and transformer method.


Wouldn't that be duplicating what he already has and just adding extra parts? What is the benefit of another transformer and relay?


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## hvactech126 (Nov 11, 2010)

You can't access both sides of the transformer on the existing relay boxes.


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## zappa (Nov 25, 2011)

The OP already did though, look at the pic. I would solder a short pigtail on, bring it to the front of the box, wirenut the common to it.

Can the transformer handle the extra load of powering the nest?


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## Bob Sanders (Nov 10, 2013)

ppatoulidi said:


> So i flipped the board around in the relay box and snapped a pic (see below). Based on your instructions, it sounds like I would need to connect the C wire to the terminal indicated by the arrow, is that correct?
> 
> I did some other research in the meantime and I have found that other people who have installed a Nest with a similar set up as mine have had to add an external relay or plug-in transformer to provide the C wire. These do not appear to be very expensive, so I do not mind doing this if it is the preferred method. But if your way is just as good, it may not be worth it. I am just concerned about accidentally frying the thermostat. Thoughts?


That would work too, but in your case you would have to add an isolation relay.

Yes. You need to add your C wire to that point.


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## hvactech126 (Nov 11, 2010)

zappa said:


> The OP already did though, look at the pic. I would solder a short pigtail on, bring it to the front of the box, wirenut the common to it.
> 
> Can the transformer handle the extra load of powering the nest?


Why go through all the trouble of soldering and making a mess of everything when it can be accomplished using a plug in transformer and simple relay.


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## zappa (Nov 25, 2011)

hvactech126 said:


> Why go through all the trouble of soldering and making a mess of everything when it can be accomplished using a plug in transformer and simple relay.


I guess it's a relative thing. I solder every day at work so it wouldn't be a big deal to me.


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## Bob Sanders (Nov 10, 2013)

hvactech126 said:


> The proper way to correct the issue without possibly causing damage to your system would be to use the relay and transformer method.


I don't think there is really a "proper" way with this at this point in time. The nest simply was not set up to run a system of this nature right out of the box so a certain amount of jury rigging has to be done.

With either method there is a chance of doing damage so it's a matter of simply choosing which jury rig method is easiest for you.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> Can the transformer handle the extra load of powering the nest?


I doubt the nest drinks that much but that's a good point. It's a pretty small transformer... but if there is not enough amperage there, the contacts simply won't pull in.

If that turns out to be the case then I would be inclined to just get rid of the whole self contained relay box and replace it with a conventional (and separate) transformer and relay.


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## ppatoulidi (Sep 24, 2014)

Do you guys have any recommendations on which product(s) to use? Ideally I would just leave the current switching relay in place and just add an isolation relay to provide the C wire or just use an external plug-in transformer (in which case I think I would hook up to the C and the Rc terminals based on what I have read).


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## zappa (Nov 25, 2011)

I'm confused when you say isolation relay for the "C wire"? If you can solder I would do it like the quote below with no extra parts needed.




zappa said:


> I would solder a short pigtail on, bring it to the front of the box, wirenut the common to it.


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## ppatoulidi (Sep 24, 2014)

I am confused too.... Let me make sure I understand my options:

1) If I do it Zappa's way, I am essentially installing the common wire between the transformer and the relay so that there is always a closed circuit with the Nest. However, when the Nest calls for heat, the relay switches over to close the circuit between Rh and W, the circuit between Rh and C will still be closed as well, correct? So I will essentially have two parallel circuits...will that cause any problems?

2) If I add a secondary isolation relay, that will essentially alternate the circuit between W and C, correct? So when the Nest is calling for heat, it will close the circuit between Rh and W and when it is not calling for heat, it will close the circuit between Rh and C. Is this correct? HVACtech said something about needing a second transformer in this configuration.... why, and where does it hook up?

3) As a third option, I thought I could just use a plug-in 120V/24v transformer (like the one shown below) and run two additional wires to the Nest, hooking them up to Rc and C. I could essentially just plug this in whereever it is convenient to run the wires and would not have to make any changes to the existing wiring.... the Nest would just think I had a secondary system hooked up and draw power from that. Are there any problems with this idea?


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## zappa (Nov 25, 2011)

Let me just quickly comment on option #1 while I'm trying to understand 2 and 3. Option #1 is how everything out there is wired that has a common going to the stat. Common of the transformer to power the stat and it also goes to one side of all contactors/relays in the heating/cooling equipment. In Bob's diagram, the T on the far left is the other power lead to the stat. When the nest calls for heat it shorts the red to the white (other T) and actuates the Honeywell relay.

The only question, is the transformer in the Honeywell large enough to power the nest. Do you know how much power the nest needs in watts or amps? I think it's a 50va transformer and the Honeywell relay needs .4 amps of the 50va.


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## Bob Sanders (Nov 10, 2013)

ppatoulidi said:


> I am confused too.... Let me make sure I understand my options:
> 
> 1) So I will essentially have two parallel circuits...will that cause any problems? You will have two INDEPENDENT parallel circuits that will not interfere with each other (one for the nest and one for the relay). No problems from that direction. There MAY be a problem with that (small) transformer. It may not carry the load of the nest AND the relay. You won't know that however unless you try it.
> 
> ...


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## Bob Sanders (Nov 10, 2013)

oops... double post!


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## zappa (Nov 25, 2011)

Jeeze...these nest people are crazy, this is all I could find so far. Can someone do the math?

*Power consumption*


Less than 0.03kWh per month


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## Bob Sanders (Nov 10, 2013)

zappa said:


> Jeeze...these nest people are crazy,


LOL! Yeah... they may be crazy but they're making money hand over fist with this little piece of .... 'flash'


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## zappa (Nov 25, 2011)

Now I found 30-100ma in another forum. That figure came from nest support.

Bob, this just needs to keep the internal battery charged, right?


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## zappa (Nov 25, 2011)

It's going to get colder than Mars here Thursday night Bob. I can't ever remember a minus zero temperature for the DC area.


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## zappa (Nov 25, 2011)

*ppatoulidi*, from what I'm finding, the nest doesn't draw much current at all and the Honeywell transformer should be plenty big enough. If you can wrap and solder one wire right in the middle of that long jumper, that is what I would do to keep it simple.

Do you have a multimeter? Maybe check the voltage when the transformer is unloaded to make sure it's not too high for the nest. You could also do an AC current measurement to see exactly how much current the nest draws.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> Bob, this just needs to keep the internal battery charged, right?


As far as I know the battery is the biggie. The rest of the circuitry is negligible.


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## Bob Sanders (Nov 10, 2013)

zappa said:


> It's going to get colder than Mars here Thursday night Bob. I can't ever remember a minus zero temperature for the DC area.


We've gotten off pretty easy this Winter. I don't think we've actually hit -40 once yet. I'm not complaining!


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## ppatoulidi (Sep 24, 2014)

Ok, I am going to try connecting the C wire like we discussed tonight and I will let you know how i make out. I unfortunately do not own a multimeter, just a standard voltage tester, so will need to pick up one on my way home (it is probably a good thing to own anyways).

As another backup option, I was doing a little more looking this morning and it looks like the Taco SR501 switching relay is a drop-in replacement for the Honeywell RA832A and provides a common terminal. Costs about $60, but looks like a decent option. do you guys agree?

And of course there is always the option of just returning the Nest and going back to my $20 programmable thermostat that was really working just fine.


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## Bob Sanders (Nov 10, 2013)

ppatoulidi said:


> Ok, I am going to try connecting the C wire like we discussed tonight and I will let you know how i make out. I unfortunately do not own a multimeter, just a standard voltage tester, so will need to pick up one on my way home (it is probably a good thing to own anyways).Good. You should get ~24v between Rh and C
> 
> As another backup option, I was doing a little more looking this morning and it looks like the Taco SR501 switching relay is a drop-in replacement for the Honeywell RA832A and provides a common terminal. Costs about $60, but looks like a decent option. do you guys agree?If you don't feel comfortable splicing in a line on your original switch then that would probably be the next best thing.
> 
> And of course there is always the option of just returning the Nest and going back to my $20 programmable thermostat that was really working just fine. Personally speaking I don't like nest. Aside from the self progamming schedule (which I think is a bit silly in itself), it won't save you anymore money than a conventional off the shelf programmable. I have wifi access to my (honeywell) stat and to be honest, it's nice to be able to program it while sitting in front of the computer instead of in front of the stat because I have more screen real estate on the computer and can view the entire 7 day schedule at one time... but other than that the wifi feature (adjusting heat while away from home) I don't use at all


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## zappa (Nov 25, 2011)

Whatever is easiest for you. The Taco SR501 will work well. Its transformer will supply 625 milliamps, the internal relay draws 180ma, so you will have 445ma available to power the nest.

BTW, the reason I mentioned wrapping and soldering the common to the center of the existing jumper is because it's easier to not disturb the soldered connections that are already there.


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## ppatoulidi (Sep 24, 2014)

So actually that brings up another question.... if you look back at that picture, it looks like that jumper IS a piece of soldering wire. So will it be tricky to try and solder in the C wire where you recommend because the jumper itself may melt?


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## zappa (Nov 25, 2011)

No, it's tinned (solder coated) wire. I don't know your soldering experience so I was just recommending the easiest place to attach the wire.


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## ppatoulidi (Sep 24, 2014)

I've done quite a bit of soldering in plumbing applications, but admittedly have not done any electrical. I understand the two are quite different, so will just need to take my time. I have found some videos online that provide some good fundamentals, so confident I should be able to do it.


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## zappa (Nov 25, 2011)

I think you will be fine. Just like copper, the cleaner the better and heat the joint not the solder. Rosin core electronic solder and an iron. Lightly tin the added wire before wrapping it. Also re-flow the area that you will be attaching the wire to get rid of any oxidation.


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## zappa (Nov 25, 2011)

Something else that works really well is to blob a fair amount of solder on the tip so it will contact both wires and heat them up at the same time. If the iron or gun is big enough you will be adding solder to the joint in about 1 or 2 seconds.

#20 or #22 stranded would be a good size wire to use.


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## ppatoulidi (Sep 24, 2014)

Alright the latest wrinkle to this whole mess.... I have soldered a pigtail in the switch box where we discussed and run a new 18/3 wire from the box to my thermostat and connected all the wires. I confirmed with the multimeter that I am getting 24V between the red wire and green (common) wire. However I hooked everything up to the nest and it giives me an error (e29) saying that it does not detect any power to the common wire. I have confirmed and reconfirmed the voltage and that the wires are seated properly in the base. Any ideas???


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## zappa (Nov 25, 2011)

Do you still measure 24 volts while the nest is hooked up? To be sure that the nest isn't loading the circuit down to a lower voltage.


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## ppatoulidi (Sep 24, 2014)

Yes, with the nest hooked up, but not calling for heat, I get the following:

Red-to-White = 25V
Red-to-Com = 25V
White-to-Com = 0V

And when the Nest is calling for heat, I get the following:

Red-to-White = 0V
Red-to-Com = 23V
White-to-Com = 23V

Does this all seem right?


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## beenthere (Oct 11, 2008)

The red wire in the pic is on the top T terminal. if you soldered your common to the wire you have the red arrow pointing. Then you need to put the red wire on the lower T terminal, and the white wire on the upper T terminal.


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## zappa (Nov 25, 2011)

ppatoulidi said:


> Yes, with the nest hooked up, but not calling for heat, I get the following:
> 
> Red-to-White = 25V
> Red-to-Com = 25V
> ...


Those are perfect readings. :thumbsup:

*beenthere*, if you look at his latest pic and drawing he has already moved the red to the lower terminal and the voltage readings reflect that. I'm at a loss as to why you are getting the error *ppatoulidi.*


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## ppatoulidi (Sep 24, 2014)

So was just looking around at some other message boards... it sounds like this could potentially be an issue with the Nest itself. Found a couple cases where people had the same exact issue and ended up getting a new unit and it worked fine. I will try swapping it with my other Nest when i get home tonight to see if it solves the problem and, if so, will see about getting a replacement unit.

If not, then i will likely just return both units and exchange for another brand, maybe honeywell... this experience has been less than ideal.


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## zappa (Nov 25, 2011)

Gotcha....another forum that I visit never has anything good to say about the nest. You might try giving their customer support a call also.


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## ppatoulidi (Sep 24, 2014)

alright, so swapped the upstairs and downstairs units tonight and both seem to be working just fine now. Getting approx. 39 VOC upstairs, 32 VOC downstairs and battery voltage between 3.8 and 3.9V on both units. No idea what the problem was, but I guess I will just leave them be and hope they continue working. 

Thank you all for your help... I really appreciate it. Hopefully this is the last we'll be talking for a while =)


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## zappa (Nov 25, 2011)

Nice! Your welcome, I'm glad you got it working.


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