Nest 3, buzzing noise while calling for heat (HVAC questions)

[ch33zw1z]

Updated 10/18 - new thread title, so people searching may find it easier.

Link to photos: https://goo.gl/photos/NhPFFytrHFL3BTHs8

specs:
Nest 3 t-stat, 1 for each zone
Zone Valves: TACO 571-2
Controller: Honeywell L8124A
24v transformer: Honeywell AT72D 1683

problem: Nest 3.0's make a buzzing noise while calling for heat.

Quick background info:
I have a tankless hot water heater supplying hot water to the house, and also running the baseboard hot water radiators. Two zones, upstairs (zone 1) and downstairs (zone 2)

Last fall, we had some work done on the plumbing for the heating, turns out the old Honeywell mercury t-stat was failing to call for heat.

So I picked up a Nest 3. Loved it so much, I grabbed a second one to replace the somewhat newer digi t-stat is zone 2

I noticed during the winter that t-stat upstairs was making a buzzing noise, but seemed to be working ok.

I finally called Nest support, worked with them, moved the t-stats between locations, and the problem continues. Both seems to be doing it now.

Thoughts:
Nest support is requesting I add a C-wire. I don't mind doing this, just trying to figure out where to tie it into.

My Honeywell Aquastat boiler controller doesn't have an obvious C-wire connector.

Can I add a C-wire from the t-stat to the 24v transformer neutral? This seems to be the most logical step, as I understand the C-wire is added to complete the hot loop on the t-stat so it always has current to maintain battery charge and other bells/whistles.

note: whoever wired the 24v transformer hooked it up backwards, so now the Common terminal is hot, and the load terminal is neutral.

 

[NetWareHead]

note: whoever wired the 24v transformer hooked it up backwards, so now the Common terminal is hot, and the load terminal is neutral.

Your boiler has a set of controls where the thermostat wire originates from. If you open this, you'll find a set of terminals. Correct the wiring situation here.

Usually, you will find the unused wires tucked behind the thermostat in the wall. Look for it here. If not, you'll have to run a new cable.

 

[ch33zw1z]

I don't mind running a new cable, currently there's 18/2 cabling.

The Honeywell aqua stat doesn't have a common terminal, which is really where this thread is coming from.

I've found a couple sites that identity this problem which the same model aquastat as mine (L8124A), and they say this controller has an isolated secondary, and there's possibly a trace in the board that you can solder a c wire to, I haven't actually found that yet.

If attaching a c wire to the 24v neutral will accomplish this goal safely, then that's what I wanna do

 

[ch33zw1z]

Here's a link with the last comment regarding my problem

http://www.doityourself.com/forum/b...4738-common-wire-boiler-aquastat-relay.html#b

More info

http://www.doityourself.com/forum/t...llation-honeywell-aquastat-relay-l8124.html#b

 

[ch33zw1z]

Update, for those who come thru the thread and want to know how it turned out. Had a good plan, wanted a sanity check so ended up just having the HVAC guy stop by. He wad perplexed, but went thru my diagram and confirmed some voltages, then we added the common...

Common wire from Nest to common terminal on 24v transformer.

Per Nest support, there's a voltage in the menu's "Technical Info > Power" that's labeled "lin", and when the common is installed it will read 200ma. Before the common was installed, one Nest read 40ma, the other read 20ma.

Nest now reads "lin 200ma" as expected, and the buzzing appears to be gone.

I replied to Nest support's email chain with some unhappiness that this issue wasnt addressed by their compatibility checklist. Nest is offering to help offset some of the cost for the HVAC visit.