I've installed a new nest thermostat in our new house, on an old Mars furnace (single zone, heating only system). Since the furnace doesn't have an obvious c-wire terminal and there is no c-wire in my wall (just the R and W that power my previous thermostat), I've installed the nest power connector with an external 24v transformer power supply following the instructions for 6d in the Pro Installation Guide.
The nest is getting power, see's the power connector in its diagnostic, and is not giving me any error messages. However with this setup, when I set the target heat above the current the nest tells me it's heating, but the furnace never switches on. I also get no response if I take the nest plate off and just connect the W and R wire to each other. I've tested with the nest power connector jumper in or out - it doesn't work either way.
If I remove the power connector and then reconnect the W and R wires directly at the thermostat, the furnace fires up. This is leading me to think the problem is with the signal properly passing through the power connector. I've checked all the wire and lever-nuts multiple times, and tried switching the W and R wires between the furnace and power connector but can't get the furnace to turn on under any setup when the power connector is in the loop.
I don't know how else to test or what to do. Any ideas?
Wiring diagram attached. I'm connecting all the wires with lever-nuts (which I labelled as clips in the diagram)
can you get me the MARS Furnace model number please? I want to check the manual on that unit. The Mars furnace should have its own step down transformer. When you used the Red and White to a thermostat, the power comes from the Mars on R, to the thermostat, when the heat is needed, the thermostat sends the power back to the Mars on the W. This will signal the MARS to start the heating cycle.
The external step down transformer COMMON does not reach the MARS. So the R form the MARS needs the COMMON from the MARS.
You simply need to remove the step down transformer you added, then connect the C of the of the Power Connector to the COMMON side of the transformer inside the MARS furnace.
Ken, AC Cooling Wizard
Thanks Ken! The Mars Furnace model number is H80 and here are pics of the wiring diagram inside and the layout. The two blue wires on the right of the value are what run up to the thermostat.
Do you know where the COMMON is on the furnace?
Thank you so much CoolingWizard for the helpful replies.
I wanted to check in and see if you managed to see CoolingWizard's post. Please let me know if you have any questions from here. I would be happy to assist, and make sure you are good to go.
Nick, you will need to acquire a HVAC 40 V amp step down transformer that will except 110 V AC and provide 24 V AC on output. I’ve marked up your furnace schematic to show you how it would go in and how you’re gonna have to connect it.
Ok, here is the problem, that gas valve operates on a 500 to 700 milliamps and defiantly not 24AC power. The only way we can make this work with the Nest is if we have 4 wires going to the thermostat. How is that. Well, on the Nest the Rh and W1 are the gas valve wires that went to the old thermostat. Then, the transformer you purchased would be Rc and C. Bottom line is that the Nest thermostat is not compatible with millivolt control systems.
Ken, The AC Cooling Wizard
Thank you Ken, I didn't realize this furnace was a millivolt since it's freestanding and not built into the wall or the floor.
My nest only has 6 slots (Y, C, W, G, R, *OB) - how would I wire in the 24v power supply?
If you are wanting to install a power supply to your system, I would recommend having a HVAC pro come and take a closer look. They would be able to ensure everything is installed properly.
the system will need to be checked to determine the source line voltage. Then, the proper step-down transformer will need to be selected. The system power turned off and safety locked out, install the transformer. A new thermostat cable needs to be run, I would recommend an 8-conductor cable be installed for the future. This all should be left up to and HVAC technician that is also a Nest Pro accredited.