Yet another Nest install nightmare!

mattafact

Hello friends!

I recently decided to take the plunge, sell a pint of blood, and purchase a Nest Learning Thermostat.

My system is set up as follows...
- AC controlled by Unico system in attic (Rc-Y-G)
- Heat controlled by Taco ZVC404-4 in basement (Rh-W)

The issue is the Nest seems to struggling to pull power from my Taco. (Error on my Nest reads = N72 : No power to the Rh wire). When the heat zone is not active, the orange T-Stat call light flickers and is not solid. When the zone is active, is stays solid and eventually the valve opens.

I did not orignally have a c-wire installed, so I ran one from my Unico unit in the attic to the Nest, thinking that this would solve my problem, but the flickering orange light on my Taco persists.

Does the c-wire need to be wired to the Taco panel instead of the Unico? Or is this an issue with my Rh wire?

Pictures of both of my valve panels are attached.



Any words of wisdom would be much appreciated!

Dave H_2

I ran the c-wire from ZVC 404 and haven't had a problem in 4 years.
For what its worth, I also have a Lyric on the same ZVC controller also with the c-wire from the controller.
However, for my AC with a Lux Geo, the c was from the air handler.

Dave H.

mstrbill

Add a relay at the heat zone panel and power it from the Nest AC side of the wiring . Must use one power source - at Nest use the AC -blue splice behind it , wire to common nest base and red of heat wire - , white goes to w - nest base - then powers the relay at heater.

mattafact

Thanks Dave H. I ran the common down to my Taco unit and the same problem persists....no power to the Rh. So I hooked the Nest directly up to the Taco (just W-Rh) and it worked fine! I think there is an issue when the AC wires (Y-G-Rc-C) and the heater wires (Rh-W) are all hooked up together. They don't play nice.

And thank you mstrbill for your input. Unfortunately I'm not sure I follow about how to add a relay at the heat zone panel. Any chance you could explain in detail?

thank you!

ratio

Here is a link to a schematic showing a Nest controlling two separate mini split heads via relays. Your application would be simpler, basically just the one relay marked "W1", the AC would connect to the Nest directly.

mstrbill

Here goes ... add a RIBUC1 to the Taco zone box - yellow - orange to R W of your zone - white/yellow and white/blue of rib to r w that used to be there from stat . Cap spare rib wires . Now at Nest twist together Blue (C) from AC with red (old heat Rh ) and add a 3rd blue color wire splice with tiny wirenut - tuck behind nest base - connect blue wire to C -nest base . Then, old white W back to W - nest base , reconnect Y, G , Rc . Try heat and AC . If Heat does not turn on , try Rc to Rh wire switch , if AC or heat does not then base is no good , order new base from nest. keep wire configuration as told - powered from the AC trans only . Go to settings - equipment - pro setup - turn all yellow lights green by selecting your new configuring , and all will be fine .

mattafact

That is a HUGE help mstrbill! Thank you! One thing I'd like clarification on please. The spare RIb wires....can I cap them all together, or do they need to be individually capped?

Thanks again!

Big Ed_4

individally

mstrbill

yes individually

mattafact

Worked like a charm! Thank you mstrbill!

jsoldo

Same exact problem! Gonna try the RIBUC1 solution mstrbill suggested. Just want to clarify a few things...

(using mattafact's taco image in original post as reference)

FURNACE SIDE:
1) I assume my first step is remove the R and W wires from the appropriate thermostat zone on the taco board. Yes?
2) Now I connect those two lose wires (R W) to the RIB's Yellow and Orange. Correct?
3) Then I plug in the RIB's white/yellow and white/blue to the R W slots on the Taco. The spots the existing wire used to be plugged into. Correct?
4) Cap spare rib wires individually.

NEST SIDE:
5) Twist together Blue (C) from AC with red (old heat Rh) and add a 3rd blue color wire splice with tiny wirenut, tuck behind nest base
6) Connect blue wire to C -nest base .
7) Old white W back to W - nest base
8) Reconnect Y, G , Rc .
9) Try heat and AC

The NEST side makes sense, but I'm a little confused on FURNACE side. Do I have every correct?

Thanks!
Jon

mattafact

jsoldo attached is the wiring diagram that I used on my system. It's crude, but hopefully you get the idea!

thegreek66

I am having a similar issue. Just merged 2 old thermostats that controled my oil boiler and ac system. I added a C from my air handler so I have a Rc, W off the boiler and a Rc, C, Y, G from the AC. When all 6 wires are connected I get the N72 error and the Rh is grayed oit on the diagram. But the boiler works fine and turns on and off as I adjust the temp. Havent teated AC as it is to cold right now. Tried a binch of stuff with Nest support and they are sending me a unit.

I already ordered the RIBU1C from amazon so I can have it ready to go if the new nest behaves the same way, which I am guessing it will.

Seems simple but can slmeone explain what addimg the RIB does in the setup.

Also, wouldn't the voltage be doubled by splicing the C and Rh. I am sure its ok, but just curious.

thegreek66

One more thing. If i disconnect the 4 AC wire from the nest, Rh is no longer being grayed out and no error comes up

Ironman

You should have "RH" connected to the boiler, not "RC".

thegreek66

Sorry typo. Rh and W are from boiler. They are hooked up to and old honeywell zone relay. And c, y, g and Rc from the AC control board

Ironman

Then I don't see where an additional relay would help. There may be something in the installer setup menu that's not set correctly.

However, a Nest thermostat with a hydronic system is a bad choice and it will probably make the system less efficient and certainly less comfortable. Hydronic systems respond slowly and don't like being turned up and down continuously. Use a standard thermostat and leave it at one temp and you'll be better off.

thegreek66

Like I said, I have the same issues as those described earlier in the thread and it seems that adding the relay solves the problem. I have seen this solution in multiple places. I am just curious on how/why it works

ratio

The Nest, and some/many other electronic stats, use SCRs instead of relays to control the equipment. This allows them to do clever things like an automatic Rh/Rc jumper, but they aren't actually relays & they don't always play well with electronic equipment. An interposing relay alleviates this.

I am, as a general rule, opposed to other peoples' cleverness. My cleverness, however, is top notch!

pwaghwani

Hello,

I have a weird issue going on with my heating system. I have a 6 zone Taco valve (zvc406) gas heating system with the steam boiler and baseboard heats/radiators and two-zone AC system across the house.

Recently, I have started to replace the Honeywell thermostats with google nest thermostats.

I have installed two google 3rd gen and two E gen thermostats.

I noticed that for some reason, the taco valve couldn't power on my 3rd gen thermostats but they work fine with E gen thermostats. So I powered the 3rd gen with the AC/handler wires.

My problem is with the two E gen thermostats. When heat is not in use, I am seeing yellow light flashing on my taco valve system for both E gen thermostats zones, however, when the heat is turned on at the req temp, the yellow light becomes solid, then the red light comes on and heat seems to be working through radiators. I have only two wires connected, w1 (heat) and R (power).

Is the flashing yellow light normal or is there anything I should do to fix this?

Please advise! Thanks!

SFO_TACO

Same issue but I don't have an A/C unit. Can anyone help with how to wire up RIBU1C to a ZV404-4?

backshift_dividers

https://forum.heatinghelp.com/discussion/comment/1504613#Comment_1504613

Came across this old thread. Is it also possible to power the relay via the 24V common on the Taco relay? Seems simpler than splicing the C at the thermostat, no?

Jamie Hall

There are ways to fiddle these things. However, much the simplest is a standalone 24 VAC transformer. One lead to R, one to C. Also a connection from R and W to the zone valve.

However… that assumes that the standalone transformer is in phase with the power on the control valve and board. Not guaranteed — and not guaranteed to stay that way.

So… a better solution is that standalone transformer to R and C. One lead to R. Then a wire from W tp one terminal of a relay coil, and a wire back to the C from the other terminal of the relay coil. The normally open contacts of the relay connect to the zone valve control terminals just as the old reliable thermostats did.