I have a honeywell hz432 and just installed a few NEST thermostats with a common wire and they seem to run fine. I added another NEST E thermostat, but this one didn't have a common wire, and when it calls for heat I notice that it seems to be stuck in a loop where it's cycling the system in some way. I see that as soon as the system ignites the burners ready to send heat, it immediately goes into purge mode and then the whole sequence repeats. I put the old thermostat back on and it seems to run fine.

Does this mean my Nest E is incompatible with my system without a common wire? I read somehwhere that I can jump the fan wire into the common somehow if I don't need to control the Fan independently. The other choice is to run an extra wire from the thermostat into my basement through the walls which would be quite a challenge.

  • Usually the lack of a common wire causes issues when the Nest is idle, not when it's calling for heat. In order to charge their internal battery, they can sometimes cause your system to run momentarily, this will happen more often depending on the state of the Nest battery. It's best to use a dedicated common wire if possible. Some people have had success using the fan wire, but it depends on how your system is controlled (need to make sure the fan switch still works with a call for heat only). Another option is a Venstar Add-A-Wire. – Matt Jul 22 at 16:28

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