For anyone who has moved on from using the Nest Thermostat to control your HVAC system, here is a guide on how it can be repurposed as a dummy or virtual thermostat.
Scenario:
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User has set up virtual zones (rooms or floors) and virtual thermostats with temperature sensors for the zone which feed into some thermostat app or custom logic to determine when the HVAC system should run. These virtual thermostats could also be using Thermostat Scheduler to automate the temperature control of those zones.
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Despite automation, user needs to make manual changes to the virtual thermostat, for example: unusual weather, guests, hosting a party, or a household member has a fever.
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Changes would be made on a personal tablet/phone running a dashboard, going into the virtual device through the Hubitat app, some smart wall device, or an old phone/tablet mounted to the wall.
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User desires a familiar, traditional thermostat interface and has on old Nest Thermostat or can get them inexpensively.
Problem:
Nest requires at least a Rh or Rc wire for power and at least one HVAC function wire before it will display the Nest UI. Nest will only display and "control" functions for which the appropriate wire is connected, e.g. If the W wire is connected, then heat functions will show. When there is already a primary HVAC thermostat, connecting HVAC wires to a dummy Nest thermostat would defeat the purpose. HVAC wires would not be available in remote locations away from the primary thermostat, such as in a bedroom.
Solution:
Trick the Nest Thermostat into thinking it's actually connected to an HVAC system. For more information on how this works and was discovered, refer to this thread.
Implement:
- Supply power to the Nest thermostat: A) if near to the primary thermostat, use pigtails to connect an R wire and C wire (if available). OR B) Using a 24v power supply, connect the +/- wires to Rh and C. My understanding is that it doesn't matter which way the +/- wires are connected. DISCLAIMER: You are responsible for any damage if somehow it does matter!
- Connect short length wires (1-2 inches long and stripped only on one end) into the connector for each function to be controlled. Y for cool, W for heat, G for fan. (insert picture).
- Connect the Nest to WiFi.
- Connect Nest to Nest account.
- Integrate using the Google SDM API hubitat app. All the steps are important. If using an older existing account, ensure that in the Google Project, Pub/Sub is ENABLEd.
- If it doesn't already exist, create a Thermostat Controller for the primary thermostat. Direct control over the primary thermostat is possible, but not advised, and depends on individual use case.
- Use 3rd party apps or rule machine to accept setpoint changes, mode, calls for cool/heat/fan, and other functionality from the Nest and/or other virtual thermostats to combine and feed into the Thermostat Controller.
- Create a Thermostat Scheduler for the Nest thermostat. Optional.
Additional info FYI:
- The Nest automatically jumps Rh and Rc.
- If only using an Rh or Rc wire and the HVAC system is not used very much, the Nest thermostat will need to be charged periodically by pulling the thermostat off the base and connecting a micro-USB charger to the back.
- Other thermostats require a jumper from a power wire WITH a resistor to trick connected functionality. Some users recommend at least 1K Ohm resistor. YMMV
Comment below with additional information, correction, or suggestions.
