c-wire installation

bitterbatter

hello all,
i need to install a c-wire for my nest thermostat, which is not a problem for me... except that i am a little unclear on where to attach the new c wire on the boiler end. i have an older utica boiler, single pipe steam, no control board, just a simple 24v transformer and 2-wire system. there is a junction box where the thermostat, gas relay valve, LWCO and automatic water feed all tie in. so, should i attach the c wire there? or directly to the neutral terminal on the transformer? or is somewhere else better?
thanks,
joe

Bob Bona_4

Don't think you need the C hooked up for it to function but if you do, you need to attach it to the C on the xformer. R and W count.

bitterbatter

yes, i need it. otherwise the nest doesn't charge properly, the heat turns off and a pipe bursts! which just happened two weeks ago... i have the boiler repair bill to prove it!! it has to do with the way the nest (and other self-charging thermostats) share power with the call for heat wire... so the c wire insures that the nest will charge, even when the heat is not running. then, there will be no need to for the nest to try to charge itself and inadvertently turn the heat off. trust me, i've been on the phone with them for hours about this!

Bob Bona_4

Ok then hook it the C to the common side of your 40va on boiler.

bitterbatter

thanks bob, but i'm not sure what you mean... you mean connect it directly to the transformer...? which is 24v, just to be clear.

thanks again.

Bob Bona_4

Yes. You need to verify which term on the x former is to R, and hook your C to the other term.

bitterbatter

got it. i would think i could do that by metering each terminal across to ground, then determine that the one that reads no voltage is the neutral... correct?

Steamhead

On those Uticas, the R (hot) terminal on the transformer usually goes to the low-water cutoff, then the Pressuretrol, the thermostat, then the stack damper, the rollout and blocked-flue safety switches, then the ignition module if it's not a standing pilot, then the gas valve. The returning wire goes to the C terminal, and that's where your Common wire should go.

But I've seen these wired backwards, so trace it out before changing anything.

Note that with this setup, if the LWCO or Pressuretrol trip, it will cut off power to the Nest. Not sure how long a Nest can go without power, but if this is an issue, use an isolation relay at the thermostat connection on the boiler. Then you can have constant power to the Nest from the transformer, and the switched lead from the Nest will operate the relay to start and stop the boiler.

Jamie Hall

One other minor caution -- do not assume that either wire from the transformer is either a neutral (in the electrical sense) or a ground (also in the electrical sense).

bitterbatter

Steamhead - thanks, yes that is pretty much the way it is wired if I remember correctly. I installed the water feeder myself some years ago, but I thin martha what I remember. My only question is, when you say the c wire should be connected dj the c terminal, where is that exactly? I have no board or wire terminal...

Also, the nest only needs power to recharge it's battery, so the momentary interruptions if rhe LWCO or pressurtrol trip will not affect anything. The problem comes in when, after some long speeches during mild weather when the boiler isn't running that much, the nest tries to charge itself when heat is finally called for and the boiler turns on. But since it shares the wire with the boiler that it used to charge, the nest can inadvertently turn the heat off in an attempt to charge. Just a weird quirk of the nest, and some other digital thermostats. Installing a common wire allows the nest to charge constantly, eliminating this problem.

Steamhead

The C terminal is on the transformer.

bitterbatter

Thank you very much, you were a great help!

JUGHNE

I run a common wire whenever possible. I figure it would be pretty embarrassing to tell the homeowner he has no heat because of $5.00 worth of battery in the T-stat gave up the ghost.

After he paid thousands of dollars for a new reliable system.

Steam

I only had the R & W come out of my L8124C. I needed a C wire in order to hook up my EcoBee thermostat. I purchased an Argo IR 882 isolation relay. I used an old controller box to mount it inside of. The 24 V power comes from a transformer I wired in. This goes to power the board on the designated 24V supply terminals. The R & W from the L8124C go to their designated terminals on the Argo, and out come R, W & C which go to the thermostat.

nicktttt

Im trying to do the same thing, create a c-wire for a utica boiler. So connecting a third wire to the neutral on the furnace transformer should do it?

HVACNUT

Not neutral on the primary (120v) side.
The low volt secondary side will have R and C terminals. Connect to C.