2-wire thermostat wiring not compatible?????? why?

[JR8]

During my DIY installation on my phone, I am told I need a power connector available for free. But the arrival will be too late for my purposes. Does anyone know what will happen if I proceed without the connector? I have a 2-wire old Honeywell thermostat connected to R and W for a heat-only base board heat boiler system.

[ThermalDave · Accepted solution]

The power is needed to power the thermostat. With two wires the thermostat will not have constant power. You need C for constant power.

[Houptee]

Is it the regular Nest (not the learning model)?

If it is there are 2 AAA batteries that will keep it working until you either

run a new thermostat cable (use 5 conductor if you do) and have a real C wire from the boiler

or buy the power adapter.

But with older boilers they might not have a C terminal to connect to easily.

Can you post a pic of the boiler where your 2 wires connect.

Some boilers use a separate control box that the tstat wires connect and it controls the circulator pumps and fires the burner. This is where you might have an issue if the system is older the control might not have a marked C common terminal.

If you post a pic we might be able to show you where the C can be connected.

[aatienza]

Hey folks,

I'd like to jump in here and check if you've seen Houptee's response. Let us know if you need more help by replying to this thread.
 

Thanks,

Archie

[JR8]

I appreciate the follow up. Honestly, I wasn't expecting so much help. Great community.

[Jhonleanmel]

Hey folks,

JR8, thanks for letting us know. Take your time, I'll be waiting for your update.

Best,
Mel

[JR8]

I can't take a picture until I return from CA on December 30. Miraculously, I received a Govee remote thermometer that allows me to track the temperature in my home while away. Also, the NEST C-wire adapter arrived while I was gone. When I get home, I won't need to DIY anything. Thanks to all for explaining the need for an additional source of power to the thermostat.

[DIPA]

I have 3 Nest “E” thermostats in my home and they are compatibility with a 2 wire Honeywell Thermostat for heat only.  We purchased the newer Nest for our kids for Christmas and Nest has made it extremely complicated to install in a 2 wire heating system which is 1/2 the homes in New England.   Why discontinue “E” when it was simple to install and worked perfectly on that configuration.   Any ideas without rewiring how to get it to work?

[Houptee]

The Nest Learning is supposed to work without a C wire but it is more expensive than the Nest.

You can also get the power adapter as long as you have a C terminal on the boiler to connect the adapter to.

[EdwardT]

Hi folks,

@Houptee, thanks for the help!

@JR8 and DIPA, how's it going with your Nest Thermostat? Still need our help?

Thanks,

Edward

[Jhonleanmel]

Hey there, 

We appreciate your help here, EdwardT.
JR8 and DIPA, I wanted to follow up and see if you are still in need of any help. Please let me know if you are still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.

Thanks,
Mel

[Jhonleanmel]

Hi everyone,

Just one quick final check in here since activity has slowed down. We haven't heard from you in a while so we'll be locking this thread if there is no update after 24 hours. If you have any new questions, updates or just a discussion topic, feel free to start a new thread in the Community.

Thanks,
Mel

[JR8]

If my picture shows up, you will see the brown wire with the blue wire nuts going up to the thermostat. The white wires run the circulating pumps. All wires are sourced by 24-volt power supply.

[JR8]

Boiler with a 2-wire thermostat

[Houptee]

Is the transformer in the Honeywell switching relay control under the wires nuts providing the 24v for the thermostat or is there an external transformer?

If the 2 wires go directly to the thermostat then you will find out there is no C terminal on that control relay.

But if you are handy there is a way to take it apart and access the transformer and tap a wire off the common side of the transformer.

Take the grey cover off the relay and you will see the part number on a decal on the back of the cover.

What is the part number of the Honeywell control?