NEST Wiring

Alan (California Radiant) Forbes

I've wired a few NEST thermostats; enough to feel somewhat confidant that I can make the darn things work, but not this one. It's a NEST model G4CVZ, the simple one without all the bells and whistles. I'm wiring it to an older Taco ZVC 406 that doesn't have the "C" terminal available, but I can pick up the other 24 volt leg from where the transformer connects to the board.

Originally, I wired it straight over: C to C, R to R and W to W, but the thermostat said "No Power", so I switched C to R and R to C and now, the thermostat works fine, but when there's a call for heat, nothing lights up on the ZVC.

I'm guessing that since R is the hot wire from the transformer, it needs to go to the "R" terminal on the thermostat in order for the "W" terminal to be energized when there's a call for heat. But how to get around this?

PC7060

does that Taco controller require 24VAC applied to a single point or does it require two contacts to be closed?

GW

Been there, I called Taco, I was told the older metal-box Tacos don’t work. Transformers not strong enough. Need a second transformer and a relay.

GW

Hold the phone, this is a plastic box ZVC? Should be basic.  It quite sure why you’re crossing R to C and viseversa 

Alan (California Radiant) Forbes

@PC7060 Single point

@GW Metal box ZVC. Should be a 40VA transformer, no?

I'm crossing R and C because NEST wants their "C" terminal to be the hot side. I originally wired the C side of the transformer to the C on the NEST, but the thermostat displayed, "No Power", so I reversed them.

GW

Alan I’m scratching my head a little bit, I’ve always seen R go to the R and C go to the C

anyways, I was told that the old metal box units did not have enough power to handle modern Wi-Fi thermostats. We simply add a second transformer and tie and a little RIB relay.

EBEBRATT-Ed

@Alan (California Radiant) Forbes

a separate transformer and a rib relay is what I have done. It just seems like the Nest's need to be isolated or they don't play well

Alan (California Radiant) Forbes

This job has 10 zones. I might as well replace the zone valve controllers.

Actually, my best bet is to remove the "C" wire and do the 1000 ohm resistor trick. That has always worked for me.

STEVEusaPA

Nest Wiring Applications

HomerJSmith

I don't understand what's going on here. So, I feel fully qualified to make a comment.

Alan, are you saying that the Nest isn't charging? That you have only 2 conductors connecting the 405 to the Nest? That the 405 has only two TT connections rather than an R W C connections. That you have only one thermostat rather than five or ten? What is the Effective date of manufacture that is on the label of the 405?

GW, I'm trying to think how a transformer and relay would wire up to a five or ten thermostat situation. I don't feel so hot today, so it's a struggle.

If one needed more current carrying capacity, just adding another transformer would be sufficient, without the relay, if there was a connection method between the 405 transformer and the new transformer and the polarity (phase) of the two transformers were correct.

Alan (California Radiant) Forbes

Hi Homer:
The Nest is charging I believe as there is no more error message. There are 3 conductors connecting the 405 and the Nest. Yes, there are two TT connections on the old style 405 and I was able to connect to the common side of the transformer where the transformer connects to the board and this became my third wire. At this point, I am only working with one thermostat and the owner wants to add more later on if I can make this work. I don't know the date of the 405.

The easiest way to make the thermostat(s) work would be to upgrade the ZVC to one that has a "C" wire, but I'd like to know why this isn't working.

Also, if I did change the ZVC, I'd need to know the power consumption of the Nest thermostats to make sure there was enough power available for both the zone valves and stats.

hot_rod

The 6&6 zone Caleffi relays have 80va across all the connections. Some others split the connections 40va on 3, 40va on the other.
The 5&6 relays have two 40va, the 3&4 give you a spot to add a second 40 va transformer

For high current draw stats it works great

SteveSan

Hi Alan, does your ZVC have a metal or plastic cover? If metal, White= Left “T”, Red = Right “T”, Common = # 1 Terminal for each zone at the zone valve. Please see the attach for wiring. If plastic the R-R, W-W, C-C. make sure the Nest have been pre-charged and if you need a resistor ours doesn't work with the Nest. Nest suggests a 220ohm and up to 5watt resistor between W-C. Please give us a call 401-942-8000 Mon-Fri 8am-5pm EST if you need assistance.

Alan (California Radiant) Forbes

@SteveSan Maybe that's my problem. It's a metal box and I wired "R" on the left and "W" on the right. Thanks!

If you look at the .pdf @STEVEusaPA provided above, it says to use a 1K ohm, 1/2 watt resistor on 2 wire applications and that's what TACO provides with their ZV controls, no? I've been using them with Nest thermostats and they work fine.

I see that you can also use terminal 1 on the zone valve connection for a common.

SteveSan

You are correct that Taco supplies 1k 1/2watt resistors. During testing on t-stats only the Nest doesn't work with our resisteors. We reached out to Nest and they suggested when using Nest t-stats to use 220ohm up to 5watt resistors.

Alan (California Radiant) Forbes

Thanks to everyone who helped me figure this one out. I went back today and re-jiggered the wiring and the thermostat now works.

@SteveSan let me know that on the older, metal ZVC controls, the "R" terminal is on the right and the "W" terminal is on the left. I had them reversed. I also pulled the common side of the transformer from the #1 terminal.

But as far as the correct resistor, I've used 1k ohm resistors for Nest stats and they work fine, but I'll certainly get some 220 ohm resistors to keep on hand.

So, a 220 ohm resistor will allow more current (or voltage) to trickle through to the stat than a 1k ohm resistor?

And does anyone know what the VA draw is for a Nest thermostat? How about an Ecobee?

SteveSan

@SteveSan let me know that on the older, metal ZVC controls, the "R" terminal is on the right and the "W" terminal is on the left. I had them reversed. I also pulled the common side of the transformer from the #1 terminal

Only when you are introducing a com wire does the TT matter where to land the r-w. So in your case when trying to connect a com you must make sure on the "older" metal cover ZVc's to wiring Red to right T and white to Left T.

Jamie Hall

On the resistor trick to powering Nests and their kin -- keep in mind that what you are trying to do is run enough current (low enough resistance) to charge the dang things, but not so much (which would require a higher resistance) that the connected device -- zone valve, control panel, even burner control -- thinks the thermostat is calling for heat. I doubt -- despite the wisdom of the Silicon Valley gods -- that there is just one size which fits all the cases...

Solid_Fuel_Man

Every wifi stat I've put in.....and it's been more than a few, I've put in a dedicated 30va transformer and a RIB or PAM for the TT.  I really just like isolating those buggers. 

Other than the newer Taco relays which have the RWC marked out. 

hot_rod

Caleffi has had 3 connections since it came out in 2014, 20va on the 5&6 switching relays. The resistor “fix” in built into the board

-one Thermostat Connections

All Caleffi Z-one controls use the same interface to connect to thermostats. This interface is very flexible and allows many different types of thermostats to be connected to the control including 2, 3 and 4 wire versions. In addition, any type of switching device can be used to signal a heat demand by connecting the R & W (T & T) terminals together. The terminals are dual labeled

to help simplify wiring.

2-Wire Thermostats

There are a few different types of 2 wire thermostats including bi-metallic, power stealing and battery powered. All of these thermostats connect in the same manner to the Caleffi Z-one series of controls. Connect R to R and W to W. See below.

To connect a 4-wire thermostat to the Z-one series controls, connect R to R, W to W and C to C. A jumper in the thermostat is also required between R & Rh as shown below. Similar to a 3-wire thermostat, R & C from the Z-one Control

  Description

R = 24 VAC Supply

W = Heat Call

ER C = 24 VAC Common

hot_rod

I believe they are in version 4 of the Nest. Early versions were problematic on some systems. Latest software 6.1 was Sept 2021

Alan (California Radiant) Forbes

Thanks, hr. Many images you and others post are larger than my screen. Is there an easy way for me to reduce the size and make it fit my display?

hot_rod

Maybe it is just when I post from my I-phone and screen grab?

EBEBRATT-Ed

@Solid_Fuel_Man

rib relay and a transformer and walk away from it, I agree

Erin Holohan Haskell

How's this @Alan (California Radiant) Forbes ? Appearing ok? Thanks!

Solid_Fuel_Man

@EBEBRATT-Ed

I've done many this way, in fact just installed one for a customer Friday. Scorched air too....

I cannot comment if the smart stats are good or not, but customers seem to like them. 

I use the basic Honeywell FocusPro in my own house. Set the CPH to 1, and walk away. 

Alan (California Radiant) Forbes

Thanks @Erin Holohan Haskell!

Please tell me how you did that. My IT guy is on vacation. :{

Erin Holohan Haskell

Glad it worked, @Alan (California Radiant) Forbes. When you share a photo, you can choose to attach it as a file (document icon) or an image (photo icon).



If you choose the file option, then the photo will appear as an attachment to a post. Others will have to click on the attachment and open it on their devices. I suspect this is what you're running into with the images being too large to view.

If you choose the image option (as I did in my previous post), the image will appear in line with text.

Please let me know if anyone has any questions. Thanks!

Alan (California Radiant) Forbes

Fantastic!

If someone attaches a document that is too large to view easily, is there any way to make it smaller?

Erin Holohan Haskell

@Alan (California Radiant) Forbes, you can usually zoom out, but it depends on what device you're using. Send me a message if you'd like help troubleshooting on your specific device.

EBEBRATT-Ed

@Solid_Fuel_Man

My nephew has a steam boiler he installed a Nest 2 wire hook up himself. They were using RC & W but somehow it worked and ran his boiler for almost a year until he started getting error codes or something.

I don't claim to know anything about a Nest and don't wan't to but hanging out here and looking on line seems like it needs the C wire

So I just added a transformer and a rib and he has no problems.

Lucky for me their was a spare wire in the wall. he likes it because he can turn the heat on or off from his phone

hot_rod

The issue we have seen is the relay behaves okay until 18 months or so. That is when the lithium battery runs out and needs a charge. The charge function is too much draw for some transformers. Mainly the 15- 20va commonly used on switching relays.
The zone relays usually have 40-80va and can handle the draw better. So many finicky things with those stats

Solid_Fuel_Man

My standard practice is to pull a new cable if there is only 2 wires at the stat. I carry 4 conductor so that's what I use. 

I use a dedicated 30va "doorbell" transformer and the standard RIBU1C which I'm sure we all have a shelf of in our vans. 

I generally only use 3 conductors for a heat only system R, W, and C. Unless for some odd reason there needs to be fan control. Then I use a separate RIBU1C for that too. I never let any thermostat operate anything but a relay. 

EBEBRATT-Ed

@Solid_Fuel_Man

the other + is the light on the rib tells you if you have a call for heat

Alan (California Radiant) Forbes

Why mess around with wiring separate components (RIB + transformer) when you can go with a transformer relay where all the wiring is done for you..............and it's got lights, too.

Solid_Fuel_Man

Alan (California Radiant) Forbes said:

Why mess around with wiring separate components (RIB + transformer) when you can go with a transformer relay where all the wiring is done for you..............and it's got lights, too.

Cause I'm cheap, and dont like to use an SR501 for everything. I'm an control guy, as well as an installer. 501s just look like the fuel company did it.....

But a 501 does get the job done. I generally find it easier to put a box extension somewhere and hang a small transformer and the RIB on it. Then it's just the 2 conductor to theburner.