Help needed with wiring Nest Learning Thermostat

[Aeromass]

Hi! Looking to get some advice on how to wire our Nest gen3 Thermostat to our system. Current Main thermostat wires are as follows, separated by commas. G, Y, W, B, O, RC (with a jumper to RH). It’s an oil fired hot air system, and an A/C unit. From 2000, themostat we current have is a LUX 1500. Thanks!  @CoolingWizard 

[CoolingWizard]

Since you have an Heat Pump and a Oil Burner forced air heating system, you have what is called a Duel Fuel system. The wire attached to B is your Common wire. The Rc wire is the Power 24VAC power in,

Old Theromstat Rc goes to Nest RC. The G goes to Nest G, the Y goes to Nest Y1, the W goes to Nest W1, the O goes to Nest O/B.  The B goes to Nest C, 

You need to decide when to switch from Heat Pump Heat to Oil burner heat. This includes the choice to never use heat pump heat. Typically you would need to calculate the cost of using the heat pump versus the cost to use the Oil burner.  Since both systems require the forced air fan, the cost is to convert the oil use equivalent energy. This is to say convert to Watts of energy. The colder it gets outside, the less efficient the heat pump becomes. There is a point at witch the oil burner heat is less expensive.  The Achilles heel of the heat pump is that I’m very cold nights the coils outside start to ice up. When this happens, the outdoor unit, temporary switch is a unit into a air conditioning mode to take the heat from inside the house outside in order to defrost the coils.

The AC Cooling Wizard

[Aeromass]

Thanks for your reply, this is super helpful. When you mention a heat pump, are you speaking to the a/c (for cooling?) we do not have a heat pump, only the hot air furnace and the a/c. 

[CoolingWizard]

@Aeromass , the wire in your old thermostat has a wire attached to O terminal. The O terminal is defined as the Heat Pump Reversing valve control. Do me a favor, go to your outside unit and get me the Model Number.  I can look it up in my manuals and verify its capabilities. If you indeed have only an Air Conditioner Condenser outside, the wire attached to O will not be required by the Nest.  

AC Cooling Wizard

[Aeromass]

@CoolingWizard  Thanks for your fast reply! Our A/C unit is a Tempstar 10, Model Number NAC048AKA1. See pics for rference.AC ModelWiring Config - LUX Thermostat

[CoolingWizard]

Ok, you indeed do not have a Heat Pump. So the real question is, Why is there a blue conductor on the O terminal?

We need to go to the indoor air handler and see where the blue wire is attaching to.  Do you feel comfortable enough to open up the air handler cabinet. The air handler cam I eat will have multiple front panels. The one where the power and thermostat cable enters the cabinet is the cover we will what to take off. If you can take a picture of the thermostat cable wires and where they connect. 

AC Cooling Wizard

[Aeromass]

Sure thing, this mess is hanging off the side of our furnace :). We're on a two zone system. Themostats are on the main floor and second floor. 

View 1View 2

[CoolingWizard]

@Aeromass , as you can see, the Light Brown wire is not connected to anything.  So we can ignore that wire. Your light Blue wire is attached to the C terminal.  The Red wire is on the R terminal. The Yellow wire is on the Y terminal. And the Green wire is on the G terminal. The White wire goes to the relay above. 

Your NEST Thermostat would be wired with the Red wire on R or Rc.  The Green wire to G, the Yellow wire to Y or Y1, the White wire goes to W or W1.  The Blue wire to C. The Brown wire does not need to be connected to anything. 

AC Cooling Wizard

[Aeromass]

Thanks for your help thus far - I got it wired up as you stated, double-checked that the wires are seated. I'm getting the "Please remove/reattach" message and sweeping blue line on the Nest Thermostat. Also, The dual zone box is making a clicking sound. Any ideas on this one? 🙂 @CoolingWizard 

[CoolingWizard]

@Aeromass ,  Normally the HVAC equipment is wired to the Zone controller and the thermostat is wired to the Zone Controller.  Normally, a zone controller needs its own dedicated power transformer. The wires on the top left are the equipment connections. This is the R,C,Y,G,W of the HVAC equipment.  On the lower left is where your primary thermostat is connected.  It is also where the Zone 1 damper motor is connected. Zone 2 thermostat is connected to upper right side.  If your HVAC contractor did not install a separate transformer for the zone controller, you could be experiencing a power shortage. The nest draws only 200mW of power maximum. That is it much, but if the zone damper motor is drawing to much power and that could be causing the relays to toggle.  
This can only be diagnosed by measuring the voltage and the current at the zone controller when the thermostat is calling for heat or cooling. 
Now, here is the problem with most older zone controllers.  They require the Zone 1 master thermostat to set the system mode.  That is heating or cooling mode.  It does this by connecting the R to the B wire for heating. It connects. The R to Onto set the cooling mode.  The Nest thermostat does. It operate this way. When we run into this situation, we have to install an ON-off-ON switch. This switch has O on one end and B at the other end.  R is in the middle.  This is the system mode switch.  Basically most zone controllers are not intelligent systems. 

AC Cooling Wizard 

[Patrick_Caezza]

Jumping in on this issue.

The DuroZone Model SMZ-2SW Zone Control System you have installed is not compatible with any thermostat that doesn't have separate 'O' and 'B' terminals, including all of the Nest thermostats.

There is a modification that can be done that allows a Nest thermostat to be used but it isn't a simple change in wiring.

I have attached a link to the manual for you controller and a link to the Instructions on how to modify the wiring so you can use a Nest Thermostat. You will want to use the top right diagram for the Nest

SMZ-SW Control Panel Manual 

SMZ Thermostat Wiring