C wire

DaveM · January 1

my taco switching relay just has R & W terminals for old style thermostat. can i use the 24v /com terminal on the upper right corner of the relay to connect a C wire for a nest thermostat

SteveSan · January 1

@DaveM Morning Dave and Happy New Year. How many Nest t-stats are you trying to wire in? If no more than 2, you are correct and would need to wire in the common from the Nest to the com side of that 24vac/com terminal block. You should always get 24v between R-C on a Taco relay.

If you have any questions, please give Taco Technical Services a call during normal business hours Mon-Fri 8am-5pm EST 401-942-8000 and just ask for Technical Services

DaveM · January 1

thanks that’s what I thought id be able to do happy and healthy new year to you and your family

SteveSan · January 1

You’re welcome, Dave. You as well.

GW · January 1

hate to be the party pooper- I didn’t think the transformer had enough VA. But far be it from me- if Taco says it can be done, do it up⚡️

SteveSan · January 1

https://forum.heatinghelp.com/discussion/comment/1831239#Comment_1831239

You should be good up to two Nest on any SR control. Three or more will require an external transformer.

GW · January 2

Steve that’s cool- nests seem finicky: most of the time they cooperate

Taco told me a couple years ago the older metal box controls had smaller transformers. I called because I couldn’t get one Ecobee to power up off of a SR-504, metal box. On this job I added my own transformer

Yet I understand they work even when the math says they won’t

Please comment on my research, I really don’t want to be a nay sayer.

Jamie Hall · January 2

Nothing wrong with your math their, @GW . And no I don't know why they sometimes work just fine and sometimes not. Finicky little beasts.

GW · January 2

@Jamie Hall there’s gotta be some mad scientist out there that can explain it

We’ve had a couple jobs where they worked for months and or years, then simply stopped.

SteveSan · January 2

@GW Math seems correct. The older controls had the same 15va transformer.

What might of happened because you said "older control, metal cover". Only when applying a T-stat with a com wire you need to make sure your Red & White wires land on the correct T.

The SR502-503 metal cover, the Red lands on the left T and White lands on the right T.

The SR504 & SR506 metal cover is the opposite, Red lands on right T and White lands on Left T.

Standard two wire t-stats can be wired to either T connection. Hope this helps.

EBEBRATT-Ed · January 2

My nephew installed a Nest and it worked fine for a year just using 2 wire (R & W) hooked to T &T on an oil primary……..Then it quit.

I am surprised it didn't damage the primary, the transformers in those are tiny.

Put a separate transformer and relay in and it's fine now

GW · January 2

Steve OK I would not have thought of that in a million years, the wiring gets flipped whether is a 2 or 3 vs. a 4 or 6 zone. I'll try hard to remember that one (yet if it's a wifi we are gonna add a trans anyway)

GW · January 2

Ed, that's a big No—, now he knows😊

bjohnhy · January 2

I worked on a system recently that had a ~20 year old Taco SR504 with two Ecobee3 lites and one Nest all supplied by the onboard 15Va transformer.

Do you think that will last? Will a new Taco SR support all that without a separate transformer?

Google AI reports the ecobee pulls 0.2 amps and the Nest 0.08 in standby. That all adds up to just under the transformer capacity (0.625 amps, 15Va at 24v?).

SteveSan · January 2

https://forum.heatinghelp.com/discussion/comment/1831485#Comment_1831485

The new Taco control ( -5 ) have a 20va transformer with the option to add another 20va transformer for these Nest and Ecobee power robbing t-stats.

With any metal cover ( older SR controls ) please make sure to wire Red & White wires per below and com to the com side of the 24vac/com terminal.

The SR502-503 metal cover, the Red lands on the left T and White lands on the right T.

The SR504 & SR506 metal cover is the opposite, Red lands on right T and White lands on Left T.

GW · January 2

@bjohnhy it's rolling dice. Knowing what I know now, I wouldn't sign off on it. The little buggers have a mind of their own

C wire

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