Boiler buzzing after installing nest learning thermostat without C wire

law852 · December 4

So i have a very old boiler + a 2-wire (Rh + W) thermostat, heat-only system. I recently got the 4th gen Nest learning thermostat, after installing it (without the C wire), there's a loud buzzing sound at gas valve (TH/TR terminal) of the boiler. I presume then there's an undervoltage issue and I need to create a common either via running a new c-wire or the nest power connector.

But what i'm NOT sure of is where to connect the C at the boiler, is it:

a) at the screw with the blue/black wires on the transformer?

b) at the TR terminal?

c) none of the above

Alternatively, can i do a 24VAC wall adapter and connect to Rc & C on the nest thermostat?

I've attached a bunch of photos:

what i think is the transformer

2. all the wires in the boiler - valve is circled, arrow is pointing to the thermostat cable.

(ignore all the wire nuts…this is a heating-only gas boiler, but it's also connected to an electric boiler so taht actually the heating is run by electricity when above a certain outside temperature and gas when below). below i also have photo of the boiler details/model, and wiring diagram for the boiler.

Jamie Hall · December 4

Use a wall wart. You do have a common connection at TR on the gas valve — but playing with gas valves is not my thing. What is a little unusual is that you do NOT have a constant hot connection at the thermostat on either side: it's interrupted by the high limit in this instance.

I can see a wiring work around for this from the boiler transformer — it involves that plug for the powered damper — but the wall wart is so much simpler…

law852 · December 5

Thanks ! So I'll just get a 24 vac adaptor, and connect wires at RC and C?

Big Ed_4 · December 5

Try switching the common to the yellow wire side … :)

EdTheHeaterMan · December 5

Does YOUR boiler have the vent damper? There is an easy work around if you understand wiring diagrams.

Connect the Wire from the R side of the transformer (upper right where BK wire is connected) on the R terminal on the thermostat.
Connect the W on the thermostat to the Y wire on the Limit
Connect the other side of the limit to the other Y wire from the #5 pin from the Molex Plug. This will place the limit between the thermostat and the Molex Plug #5 pin.
Connect the C on the thermostat to the lower right transformer terminal shown in Blue

Screenshot 2024-12-04 at 10.00.48 PM.png

It is just a simple case of moving 2 wires in order to get the R on the transformer to the R on the thermostat.

law852 · December 5

https://forum.heatinghelp.com/discussion/comment/1824748#Comment_1824748

Thanks! My boiler doesn't have the vent damper. based on your diagram then, i'd have to run a new C-wire from the transformer to the thermostat, in addition to the wiring swaps you described? I guess my question then is what's the benefit of this vs a wall wart wired to the Rc/C on the thermostat? For context, the boiler is on one end of my very long 1-storey apartment, and running a wire would have to go past 3 walls (at the minimum) and 5 rooms (a very long long long wire). Whereas there is a wall outlet just on the other side of the wall of where the thermostat is, so i could easily pull the wire through the one wall and connect to the thermostat. Is there any 'electric' benefit of your solution over the wallwart, minus the fact that wallwart is uglier and takes up an outlet? (honest question because i genuinely do not know…) thank yoU!!!

Jamie Hall · December 5

use the wallwart

EdTheHeaterMan · December 5

The advantage is not needing to purchase the Wallwart.

I was under the impression that you already had at least 3 wires. If you don't have the 3 wires needed, then use the Wallwart.

Boiler buzzing after installing nest learning thermostat without C wire

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