Another Nest C Wire Post...

tryingmybest1620

Hey All,

I am going to try to describe my situation in as much detail as possible because it is driving me bananas. I've perused a lot of posts here and there have been several folks with similar situations, but there seems to be some uniqueness to the wiring scheme I've got going on.

GOAL: Common Wire for my three Nest Thermostats

Here goes…

I have a gas boiler with hot water baseboard heat. We have 3 zones, each fed from a taco zone valve. The HVAC is controlled by a Honeywell Aquastat L8148E. The transformer on the Aquastat had started buzzing, so I replaced it a couple months back and upgrade to a 75VA transformer (and it worked!).

I have 3 nest thermostats, one for each zone. There are two wires running to each thermostat (although the wire is actually 18/3 and there is a third wire that is simply not being utilized). I am hoping to use this third wire as a common for my thermostats, which due to some update Google pushed through, are now all acting funky (won't connect to the app, won't connect to wifi).

The current wiring is as follows:

Each Taco zone valve has 3 wires connecting to it:

Terminal 1: Red wire to thermostat
Terminal 2: Red 24 v wire
Terminal 3: Green wire

The red 24v wire ties into a bundle that connects to all three taco zone valves, as well as the red 24v from a separate wall-mounted HVAC transformer (40VA) AND the red 24v from the TV terminal in the Aquastat.

The green wire ties into a bundle that connects to all three taco zone valves AND is ultimately tied into the T terminal on the aquastat.

The white wires from all three thermostats are bundled together and tied to the white on the wall-mounted HVAC transformer mentioned above.

Based on this setup…where should I connect the common wire? I see a few possible options:

1. Tie in to Terminal 3 on each Zone Valve
2. Tie directly into the common side of the transformer (my upgraded transformer has a yellow wire on the common side that I have going into T2 on the Aqustat board
3. I have read here about some folks running their Common to Z/W on the aquastat board.
   

I tried mucking around with a Nest power connector but I just could not get it to play nice with the zone valves, so I think this is going to be the better option if I can get it to work. Grateful for any wisdom and feedback ya'll can share.

EDIT: Added pictures

Jamie Hall

Which wires are connected to R and W at the thermostats?

tryingmybest1620

The white wire to the thermostat is bundled in with all of the thermostat white wires. This bundle connects to the wall-mounted HVAC transformer. The Red for each thermostat is directly connected to terminal 1 on each zone valve.

GW

i’m not a nest expert, but that’s gonna be a tough one. The zone valves share C and “end switch” on see middle terminal

hats off if you can make it work. Most guys go to 4 wires- two for power and two for heat operation.
Or, install a zone controller with R W and C and use the three wires

EBEBRATT-Ed

REV_ZV_555-573_Instruction_Sheet_100-3.2A_032119.pdf

here is the wiring diagram for your valves.

You get the common for the thermostat off the transformer the wire that goes to terminal #2 on the zone valve is your common.

The r wire to the thermostat comes from the opposite side of the transformer from the common wire.

W on the thermostat goes to #1 on the valve

The common from #2 on the zone valves and the wire on #3 on the zone valve are the valve end switch (just a contact that opens or closes) get connected to whatever starts the system up.

SteveSan

Please see below when wiring a Nest to our 570 series zone valve.

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

tryingmybest1620

Thanks All - I am going to give this a shot and let you know how it goes.

tryingmybest1620

So in my setup, I don't have anything labeled T/T. What on the Aqustat board would serve this function? I think maybe TV and T?