topic Re: Need help wiring my old Sears 600 furnace to a new nest thermostat in Nest Thermostats

Need help wiring my old Sears 600 furnace to a new nest thermostat

crocolocolus — Fri, 07 Oct 2022 19:32:53 GMT

I have an old Sears 600 wall furnace that I am trying to control with a new Nest Learning Thermostat (3rd Gen). I have run a new 5 wire thermostat cable to the furnace from the nest in order to connect a C wire to the 24 VAC transformer common in the furnace (TH-TR connection). Please reference the included wiring diagrams and images for clarity.

Since it was originally designed as a 2 wire circuit with the thermostat switch placed after the gas valve, I am having trouble with how to wire it correctly. Originally on the furnace the RED wire went to the TH connection, and WHITE wire went to the TH-TR (common) connection... The TR connection is where 24 VAC is supplied from the transformer. If I keep the red wire on TH and WHITE on (TH-TR) and also connect the blue C wire to TH-TR, it seems to cause issues with the gas valve and it buzzes.... I assume because power is being draw through the gas valve in order to power the nest. I can power the nest directly from the TR connection with 24VAC, but I am unclear on how to properly trigger the furnace valve with the nest?

Does anyone know how to make this system work with nest?

Above is the wiring diagram inside the furnace

These are wiring pages from the manual

Gas valve connections and 24 VAC transformer

How furnace was originally connected to 2 wire thermostat

Re: Need help wiring my old Sears 600 furnace to a new nest thermostat

Houptee — Fri, 07 Oct 2022 22:46:13 GMT

Try making white TH to W on nest

TH-TR is power make it red to R

TR is common blue goes to C

This is what I see looking at the wiring diagram you posted.

Re: Need help wiring my old Sears 600 furnace to a new nest thermostat

Patrick_Caezza — Sat, 08 Oct 2022 01:25:06 GMT

I'm going to need to make an educated guess on this one.

I think the Rh terminal should connect to the TH-TR terminal on the furnace

The W1 terminal would go to the TH terminal.

I'm about 90% sure this is correct

You will need an external 24Vac transformer to create a C wire for the Nest. You would connect it between the Rc and C terminals on the Nest.

Re: Need help wiring my old Sears 600 furnace to a new nest thermostat

Houptee — Sat, 08 Oct 2022 02:36:13 GMT

Look at the wiring diagram.

The black wire coming from the 24v transformer is the (-) aka Common, at TR

The Yellow coming from the other side of the 24v transformer is the (+) at center TH-TR

Blue is the C wire goes to TR

Red is the + power comes from TH-TR to Nest R

White goes to the Nest on W and goes to the TH to complete the circuit thru the gas valve coil to open the gas valve.

Re: Need help wiring my old Sears 600 furnace to a new nest thermostat

crocolocolus — Sat, 08 Oct 2022 04:47:43 GMT

Thanks! Thats totally it. The original wiring had red on TH which through me off.

Re: Need help wiring my old Sears 600 furnace to a new nest thermostat

Jeran — Mon, 10 Oct 2022 21:10:30 GMT

Hey there crocolocolus,

I'm just checking up on this thread, do you have any more questions or need any additional help?

Thank you for helping out on this thread, Houptee and Patrick_Caezza!

Best regards,
Jeran

Re: Need help wiring my old Sears 600 furnace to a new nest thermostat

crocolocolus — Tue, 11 Oct 2022 15:12:05 GMT

Thanks yes everything is resolved

Re: Need help wiring my old Sears 600 furnace to a new nest thermostat

Jeran — Wed, 12 Oct 2022 16:23:03 GMT

Hiya crocolocolus,

No problem at all! In that case, I'll go ahead and lock this thread. If you have any more questions in the future, feel free to create a new thread.

Have a good one!

Best regards,
Jeran