Single zone (of 3) not activating, Taco SR504

Chartkov

New issue on a Taco SR504 that has worked for 3 winters (homeowner), was hoping someone could weigh in on: Single zone not activating on demand from t-stat. Zones 1 and 3 work fine, Zone 2 does not.

T-stat (Nest) seems fine, already verified voltage kicks in when it goes into heating demand, same as the other two. But the red zone LED doesn't kick on.

Zone switches on when I manually jumper the R & W, but refuses otherwise. Have already reseated both wires and re-secured the little terminal screws multiple times to no avail. Went so far as to swap the Zone 2 R & W wires over to Zone 1 terminals, and they activate Zone 1 just fine, but then fail to do so when moved back to Zone 2 terminals.

This is hardly my area of expertise; am I missing something obvious, or do I just have a bum part of some sort? Thanks so much!

Chartkov

Relay pic, if it's helpful.

EBEBRATT-Ed

There are always issues with Nest's. There are other current posts about this very topic

Nest's work much better when they are hooked up 3 wire. Do you have any spare wire run up to your stats?

Chartkov

Well, the Nest unit involved is already running off a separate 24V Common (distinct from the Taco relay), and it's never been an issue with the same t-stat for the past few years. When it was *first* installed, yeah, the Nest was super finicky without having its own 24V Common, but it had been great since that got resolved.

Zman

If it connecting to a common from another source, it is probably a coincidence that it works.
You have an extra conductor and a "C" on the Taco. Why not wire it correctly.

Chartkov

It was connecting to Common on the AC unit that t-stat runs, simply b/c that was the way it was setup originally. But for the sake of experimentation, I just swapped it over to Common through the Taco relay and... no change.

Alan (California Radiant) Forbes

Jumper the "R" and "W" terminals on the SR504 and see if that turns on the pump. You can use a short piece of wire and touch them to the two screws. If the pump turns on, there's a problem with the thermostat wire or with the Nest. If it doesn't turn on, the SR504 is suspect.

Moving on, jumper or tie together the wires at the "Rh" and "W" terminals on the Nest. If the pump turns on, there's a problem with the Nest. If it doesn't turn on, there's a problem with the thermostat wire between the Nest and the SR504.

Chartkov

That was a good set of recommendations... ended up pinning it down to the Nest (the backplate specifically). Thanks!