My condo apartment current has the HoneyWell T8575D installed and I want to switch to Nest. But I couldn't find which Nest port should the "Fan Common" line go into.
The wiring from the wall is as follows. I did some research on my own, and mapped each wire to the nest configuration. My mappings are labelled red in the picture. It has 4 wires to control 3 fan speeds. By reading online, the nest thermostats 3rd gen can handle it by reconfigure the port * and port Y2 through software.
The current thermostat is Honeywell T8575D. Its circuit diagram is as follows. It appears that wire 6 is connected to the "common" of "Vavle Motor/End switch". My guess is used for turning on the fan automatically when the valve is on.
I took reference from the manual about another HoneyWell Thermostats Model that has the fan "Always ON". It connects "5" and "6" internally in the thermostat, meaning it supplies 24V to line 6 (Fan Common) when the thermostats is turned on.
For the Fan Control Circuit, the manual says it used W6380B. But it has been replaced by ICM6200 recently. (https://www.icmcontrols.com/documents/ss_LIS216.pdf link to the manual). The following is the internals circuit diagram of the fan relay, it corresponds to the W6380B box in the T8575D circuit. It is basically a relay to control "on/off" state of the high voltage (120V) fan via low voltage source (24V)
So my question is about the "Fan Common", which is the #6 Green Wire from the wall. I don't seem to find a corresponding Nest port for it. Is it Ok that I leave it not connected? I think Nest would turn on the fan by supplying 24 Vac to Port "C", which drives the Fan 1 low speed through the relay.