I have made multiple failed attempts to get meaningful guidance from NEST, Google, DuoZone tech support, our local heating contractor (that had installed the DuroZone system), and the NEST professionals with OnTech for use of NEST thermostats in our home. Thus, I wanted to share my solution. The NEST website seemed to indicate their thermostats were compatible though with a bit of a disclaimer. After substantial frustration I have been able to make the system work and am sharing the wiring photos so that others won’t have the same frustrations or accept professionals saying they are just not com
The DuoZone system has a relay control box that controls louvers in each of the ducts in our HVAC system to direct heating or cooling to the zone in the house calling for it. The relay system is controlled by two thermostats, one of which is considered the “master thermostat.” This means it has the control of whether our HVAC system will heat or cool through a switch between the two modes. The second thermostat is considered the “slave thermostat” which calls for heating or cooling to the zone in which it is placed based on the thermostat’s settings but cannot control whether the system is set to heat or cool. Pictures of the DuoZone relay system control box prior to my modifications and wiring diagram from the owner's manual are below. The wiring diagram is for a 3-zone system, while ours is a 2-zone system.
Many sources have said that this system does not have a common “C” wire to power the thermostats which in essence means to have power available to charge the thermostat batteries in a consistent manner. To solve this problem I purchased two 24 volt transformers to power each of the two NEST thermostats running these low voltage wires to the thermostats’ connection plates. (One transformer could power both thermostats if you are able to fish the leads to both thermostats. For me it was easier to just have separate transformers.) For both the “master thermostat” (clear insulation wires) and the “slave thermostat” (black wires) these power wires were connected to the "Rc" and "C" terminals on the NEST connection plate as shown below. These provide low voltage AC power, so there is functionally no polarity for these two wires.
With the thermostats thus powered I was able to get both thermostats to boot up at come to life. However, following the wiring of the master thermostat, a blue wire (visible here not connected to any terminal) was connected to the O terminal of the original thermostat. Following the wiring instructions from NEST this blue wire was initially connected to the NEST O/B terminal. Once so connected, however, the NEST thermostat presumes that it has been connected to a heat pump system, and there is nothing in the settings that allows you to persuade the thermostat that there is no heat pump. In order to get the system to work at all this blue wire could not be connected to the O/B terminal, and thus has been left unconnected. This, however, does not allow the master thermostat to communicate with the DuroZone SMZ-SW to control whether the HVAC system will provide heating or cooling.
To solve this problem, an auxiliary switch was wired into the DuroZone SMZ-SW control box toggling between Cooling and Heating as shown below. For cooling, the switch closes the circuit from the Zone 1 “R” terminal (red lead) to the Zone 1 “O” terminal (white lead), thus applying power to the relay that turns the HVAC system to the cooling mode. With this switch in the off position the “O” terminal is not powered, and the system provides heating when either the master or slave thermostat demand it for their respective zones.
The white wire (circled in black) that runs to the blue wire that was left unconnected in the master thermostat wiring plate shown above, is no also left unconnected in the DuroZone SMZ-SW relay box. This lead had previously been connected to the "O" Zone 1 terminal.
With these modifications for connecting the two NEST thermostats, the thermostats work as expected, and can be controlled though WiFi connection and applications on smart phones. The seasonal changes from heating to cooling and back again, however, needs to be manually controlled with the toggle switch on the DuroZone SMZ-SW relay control box.
Though our DuroZone SMZ-SW is older (installed Fall of 2009) it is certainly a functional system that meets our needs and not one that I would be willing to spend significant money to replace to be able to have smart thermostat control. It would seem that this function could be built into the NEST software so that this seasonal change from heating to cooling could, in fact, be controlled by the thermostat itself. There is a “*” wiring terminal available for some special functions, and this multizone control could be one of those functions.
In any case, I wanted to share my solution to this vexing problem. Please feel free to share this information with whomever might find it beneficial.
Thanks.
Mark
