C wire hook up on the TACO box

[BrokenOldBottle]

I have an existing thermostat running on two wires with batteries and I want to replace it with a nest thermostat that requires a C wire. I do have a third wire available to me I just need to know where to hook it up on the TACO Box

[laurentbourg · Accepted solution]

There is a C connection hub, typically located on the left hand side of your Taco, it might also say 24V. What is the exact model of your Taco control module?

[Thekideo]

👍

[laurentbourg]

Should work for you but I have seen cases where that was not enough, if the error you get on the Nest impacts its ability to charge the battery, you can add an additional transformer on the furnace side, like this one https://www.amazon.com/Transformer-Thermostats-Compatible-Honeywell-Thermostat/dp/B07MT5VWQN/ref=gp_...

You would then plug the hot to your R and have the C be the common of this transformer instead of the C of the current transformer. 

[Thekideo]

I also purchased a nest power connector. But all the instructions I see for these are for newer furnaces that have circuit boards. Wasn't sure if I'm better off just using this for each one? Thanks again for all your help 👍👍

[laurentbourg]

You can try the Nest power connector since you got one. On the C, R, W side of the Nest power connector, plug a wire from your current transformer into the C. Plug wires to wherever the R and W of your current thermostat wires are going to. On the R,W side of the Nest power connector, plug the R and W thermostat wire coming from the thermostat. If the Nest power connector does not work for some reason, just get a simple 24VAC 300ma or 500ma transformer like I sent you and connect as previously mentioned. 

[Thekideo]

👍 thanks again

[laurentbourg]

Btw considering you have another zone, I think you are better of using the additional transformer approach, instead of the Nest power connector. You will just need one transformer and it will be a cleaner implementation. 

Alternatively, and that’s probably what I would do if it were my boiler, is to swap the current transformer with another one, it’s probably old and not up to spec any more. Big box store or your local hvac store should easily carry one, something like https://www.homedepot.com/p/Honeywell-Home-24-Volt-Transformer-AT72D/202264717#overlay

[Thekideo]

Yeah I was actually thinking about that last night the nest power connect is kind of big bulky and not eye friendly. Yeah it's definitely weird I double checked a lot of stuff last night. When I measured between r and C I had 26 volts. Last night was the only time I got the wiring configuring to say it was correct. When you go in the screen it either lights up orange or red if it's wrong. R and W were all blue. I forgot to look last night I remember in the app it would also tell you C wire was needed I don't remember seeing that. It just kept saying no power on the screen and n260. I'm definitely going to upgrade the transformer like you said. I believe that transformers like 10 years old if I'm correct. I'm not by my system now but I'm curious to see if I still get that stupid message in the app where it says C wire needed

[laurentbourg]

Hopefully it doesn’t complain any more and you are all set. Yeah transformers sometimes get weak over time, voltage will be good  but amount of current/power will be reduced and that you can’t measure with a multimeter. Quick thing to fix anyway if that ends up being the culprit. 

[Thekideo]

 let's try this again. Replace the transformer but do not have power on the load side? Check the connections have power there? 

[Jmcee10]

I have somewhat of a similar problem.  6 zone taco box with only 2 C terminals for 2 nest stats.  I have another 2 stats/zones and taco says I need another external transformer for those 2.  I know where to tap the 120 off of at the boiler but not sure how to configure the low voltage wires from the stat and control box to make a complete circuit.  

[meegswift]

Hi - I also have a 6 zone taco box. Taco told me I needed to add a separate transformer to have enough power to switch all my zones over to the nest. Do all zones get routed into 1 single transformer?