Yet another NEST question

EBEBRATT-Ed

I haven't been keeping up with these things. I would rather not use them. But my nephew called tonight and has had a Nest for 2 or 3 years and has given him some issues but now the heat won't come on so he calls me because his oil dealer told him the two wire thermostat wire are bad. He has a steam boiler with a cad cell control and T & T on the cad cell goes to the nest.....2 wires that's it. He sent me pictures so I could see it. I told him he needs the common wire.

So my plan tomorrow is to pull 3 wires to the Nest and add a 40 va transformer let the Nest pull in a rib relay and the rib contacts go to T & T on the cad cell.

Am I good to go?????????????

STEVEusaPA

Why not just use a Carlin Kool-kit or Beckett AC Ready kit? Use its transformer and you’ll have your common.
https://www.indoorcomfortmarketing.com/its-not-so-common

Zman

A nest on a steam system? You must really like this nephew
I think you plan will work

EBEBRATT-Ed

@Zman

He just wants me to bail him out, I have never seen the job he put or had it put in 3 years ago. If it was mine it would be in the trash can

HVACNUT

Another option is a Taco SR501 or similar. 
Less intrusive than a RIB, transformer and 1900 box.

ChrisJ

I wouldn't expect a Nest to have an issue with 2 wires unless the battery won't stay charged and it should tell you if that's an issue.

If the battery is staying charged, I'd expect a common to have no effect.

HVACNUT

ChrisJ said:

I wouldn't expect a Nest to have an issue with 2 wires unless the battery won't stay charged and it should tell you if that's an issue. If the battery is staying charged, I'd expect a common to have no effect.

Normally yes, when R (24v) powers the thermostat. On an oil primary however, 24v stays inside the control and TT through the thermostat is the Common side of the circuit. When the thermostat contacts are open it will trickle charge I guess through the primary relay coil but I wouldn't rely on it.
Small va transformers and low volt amp ratings too. 

ChrisJ

HVACNUT said:

ChrisJ said:

I wouldn't expect a Nest to have an issue with 2 wires unless the battery won't stay charged and it should tell you if that's an issue.

If the battery is staying charged, I'd expect a common to have no effect.

Normally yes, when R (24v) powers the thermostat. On an oil primary however, 24v stays inside the control and TT through the thermostat is the Common side of the circuit. When the thermostat contacts are open it will trickle charge I guess through the primary relay coil but I wouldn't rely on it.
Small va transformers and low volt amp ratings too. 

Either way, that would still result in a dead, or very low thermostat battery, no? Not a fully charged thermostat that won't fire the burner?

EBEBRATT-Ed

It's fixed anyhow. A 40 va transformer and a rib relay and now works fine.

Hot from the trans through a fuse to Rh on the Nest
W1 on the Nest to the RIB coil
Common on the trans to the coil of the RIB & to C on the Nest

Yellow & Orange NO contacts on RIB goes to T & T on cad cell control

Someone posted on another post that NEST'S take a lot of power that you can only put 2 NESTS on a 40 va transformer.

I don't see how the little transformer in the cad cell control could even run 1 NEST especially when hooked up with no common (no common available) and power stealing.

EBEBRATT-Ed

@HVACNUT

Not sure an SR501 would have a big enough transformer to drive the nest.

I ran a new 3 wire to the NEST. The 4" box on the boiler with the service switch had an open KO. I pulled the cover off and there was a dead electrocuted mouse inside!!!!!

The box only had two wires in and 2 wires out so I had room to put The Rib on that box and a 40va KO mounted transformer

hot_rod

hats off to the folks at RIB, Functional Products they have an assortment of problem solving relays at affordable prices.