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01-22-12, 01:09 PM   #1 (permalink)  
Help Installing Nest on Millivolt System Using 24v Transformer

I need help hooking up a new Nest thermostat to an old millivolt floor furnace by using a 24v step-down transformer. The model of the floor furnace is: Kresky Gravity Floor Furnace 46 F,D . My current (old) thermostat says "LUXPRO" on the front panel. On the back of the panel, on the inside, it says:

S.P.S.T
T101141 / PSM30
Heat Only
0.15 to 1.2 Amps
Max 30 Volts A.C.

I am upgrading to a Nest thermostat, and have two wires in my wall: Rh and W.

Before ordering the Nest, I emailed Nest support to check compatibility and attached a photo of my wiring. Their response was:

Congratulations! Based on your picture it does appear that your system is compatible with the Nest Learning Thermostat! It appears that you a conventional heating only system. The red and white wires coming through from the back of the thermostat are what will need to be connected to Nest. The red wire will be connected to the Rh terminal for power and the white wire will connect to the W1 terminal for heating.

Unfortunately neither they nor I realized that I had an ancient millivolt system that was actually incompatible with Nest (I figured it out when I traced the thermostat wiring after a power outage a few days ago). FYI: I have what appears to be two insulated wires, and not a single copper one, it's my understanding that the millivolt power is coming from a thermopile (vs. a thermocouple).[1]

Not yet giving up hope, I called Nest support and asked if there were any workarounds. The support technician told me he'd heard some customers had success with using 24v step down transformers in tandem with their existing millivolt system. With this knowledge in hand, I set out to research. So far, I've found three useful threads about using 24v transformers in this kind of situation:

radiothermostat.com • View topic - Powering / Wiring "C" on a 2-Wire, Milliamp System
radiothermostat.com • View topic - Wiring a two wire boiler system
Powering A 24v/millivolt Thermostat To Control A Gas Stove ? - HVAC - Contractor Talk

All of the above threads suggest that the wiring goes something like this:

W from furnace (common, -0.7VDC ?) --> W on T-Stat
24 VAC power from transformer (+/- ?) --> C on T-Stat

R from furnace (+0.7VDC ?) --> RH on T-Stat
24VAC power from transformer (+/- ?) --> RH on T-Stat

As you can see, I'm still a bit fuzzy...
•Which of the 24v wires (+/-) goes to which terminal (C, Rh)?
• Is it really okay that the 0.7VDC furnace/millivolt Rh wire and one of the 24VAC transformer wires will be on the same Rh connector on the Nest? If I understand correctly, the reason it doesn't matter that I'll have two wires on the Rh connector is because they are part of two different circuits--is that right?

Regardless, I will want to check with a multimeter to make sure the voltage coming into the Nest is within 20.5 and 27.5, since those are the min/max it will accept according to this page: Nest | Heating and cooling systems that are inc...

Anyway, please let me know if I understanding any of this incorrectly.

Last but not least, here are some of the step down transformers I found. Would any of them work?

Functional Devices Inc / Rib TR50VA005 Transformer, Control, Input 120, Output 24
Honeywell AT72D1683 Step-down Transformer 24 Volt
Amazon.com: Honeywell AT150A1007 Transformer, 120/208/240 volt, with universal mount, foot, plate or knockout.: Home Improvement
Amazon.com: Transformer: Home Improvement
AT140A1000 - Honeywell AT140A1000 - Foot Mounted, Plate Mounted, Clamp mounted or Panel Mounted 120 Vac Transformer (40VA)

Thanks in advance! I apologize that this is so long, but I wanted to be as specific as possible.

[1] Are kresky furnace repair parts available I have gravity - JustAnswer