If you have a heat-only system connected to your Sensi thermostat, an easy option for adding a common, or c-wire, is to install a 24 VAC wall plug-in transformer. Symptom I managed to resolve my issues by basically running the Nest off the AC transformer and using an isolation relay between the Nest and the Furnace. ... 5.0 out of 5 stars Easy way to make Nest Thermostat work with a millivolt system! Please note the wires from the wall plug-in transformer … Wire it up according to the schematic and it works perfectly. If the existing thermostat cable has a spare wire, connect A2 to C terminal to provide the Nest Thermostat with power. You could do things with an isolation relay(s) if you wanted, and keep things safer. I did this with a 24VAC relay. Though the article addresses adding a transformer and operating the gas valve through the contacts of an isolation relay there are digital thermostats that can be setup for milivolt systems just with the thermostat battery. However....if you have no other choice, the only way I see is to use an isolation relay and operate the 24 volt coil of the isolation relay from your Nest Thermostat. Wires from the low voltage thermostat simply run to and turn on or off the switching relay. The output side of the relay is turning GPIO pin 18 on and off. You might need as many as three of them if you needed to control heat/air-conditioning/fan. Here is my wiring diagram. That way, I can send power to the thermostat without needing separate Rh and Rc (in fact, since there is no AC, without needing Rc at all). You replace the line voltage thermostat with the switching relay, then add low voltage wire from a transformer to power the Nest or other low-voltage thermostat. See drawing SYS-08-031. Connect red wire to A1. A little while back, I wrote about my first year using a Nest thermostat and since then several people have asked me for details on how I wired up my whole-house humidifier. ... As a result another external transformer will be needed to power the thermostats and an isolation relay per zone to connect to the R & W terminals of each zone. I used a PAM-1 relay. I hesitated for a long time to post much detail, since it'd be really easy to completely fubar your HVAC, humidifier and Nest all in one fell swoop. Transformers with 150-400mA output are the most widely available online and easily provide sufficient power for the thermostat. Pam-1 white / 0V > C wire from Nest thermostat Read more. In case it is not clear, the pam-1 wiring is: Pam-1 red / 24 > W1 wire from Nest thermostat. A better choice would be to replace the 555 with a TACO Zone Sentry zone valve and save yourself the aggravation. The lockout temperatures are only enforced when the Nest Thermostat is connected to Wi-Fi so it can track outdoor temperatures. By Cody S. on October 6, 2019 This little relay is cheap but … If Wi-Fi is not enabled, Auxiliary heat will come on automatically when it takes longer than expected to reach the current target temperature: Nest T200577 Thermostat Troubleshooting. Helpful. As I envision it, when the thermostat calls for heat, it will connect Rh to W which will connect the trigger side of the relay, which will close the circuit on the heater wires that used to go to Rh … The thermostat thinks its turning a furnace on and off, but really it's just triggering the relay. The safest way to do this is to run a new cable from the furnace that would bring its common wire to the thermostat. 4) Provide a common power wire. For input, I wanted to mirror the call signal from the Nest thermostat within the Raspberry Pi. Thermostat Installation & Wiring Diagrams; Taco SR503-4 Three Zone Switching Relay Question: I have a Taco SR503-4 Three Zone Switching Relay. 7 people found this helpful. Report abuse. According to the instructions, it's the "isolation relay" part. Relays and transformers are commonly found on HVAC service vehicles. the Nest thermostat along with isolation relay.