HVAC, fan not ON working in AUTO mode

[ktnwin]

I replaced an old (mercury type) thermostat with a new Honeywell thermostat model RTH2300B.
The wiring is quite straight forward based on the manual: red to R, white to W, yellow to Y and green to G.
There is no C so the 24VAC common is not connected. I have to install 2 AAA batteries per instruction.

Before installing this new thermostat, I perform a bench test as follows:
1) take a piece of cable with 5 wires in it (red, yellow, white, green and cyan)
2) connect 4 wires (as above) to the connector gang. The other end of this cable has 5 wires sticking out of the thermostat
3) connect the 24VAC hot to the red wire, common is NOT connected.

I use a voltmeter, set to 50 Volts AC range. The black wire connects to the 24V common of the transformer.
Test sequence is:
1) set fan to ON, voltage at the G wire shows around 24 volts (i.e. fan is turned ON)
2) set fan to OFF, and set temperature a few degree higher than current temperature: after a few seconds, the W wire shows 24 volts (i.e. heat is turned ON at the furnace). Problem is the G wire shows 0 volts even after 30 seconds the heat goes on.
3) lower the temperature setting to be lower than current temperature, after a few second the W wire shows 0 volts (i.e heater off)
All these tests are done without connecting any wire to the furnace/AC system. So the problem is somewhere in the thermostat. Bottom line: the fan only turns ON if set to ON. It stays off in AUTO mode no matter what.

I move forward to install this thermostat to my HVAC, the results are identical to the bench test.
I get an identical one from local store and repeat these tests. SAME RESULTS, FAN not working in AUTO mode.

I appreciate all advises I could get on this.

 

[ktnwin]

As a second thought, I am not really familiar with how the HVAC fan works. Probably, in fan AUTO mode, the thermostat does not turn ON the fan, instead some controller board in the furnace turn ON the fan when the gas is burning. This is just my wild guess.

 

[ktnwin]

Doing more research, I learned that the issue is in my old furnace system. When FAN is set to AUTO, the thermostat will leave it up to the furnace system to control the fan. This involves, inside the furnace system, temperature sensor, limit switch, etc... that turn ON the fan at certain speed (lower than the speed set by FAN ON). This is beyond my abilities so I will call a HVAC pro to take care of this (along with the aircon that has been broken more than a decade ago).
Maybe it is time to replace this HVAC system.

At mean time, I will connect the green wire to W connector, this way the fan setting is irrelevant, the fan will follow the heat as both white and green wires are connected to W connector inside the thermostat.

 

[ktnwin]

Connecting both white and green wires to the W terminal of the thermostat (also disconnect the yellow wire) helps a lot. Now, my heater system run based on the schedule programmed in the thermostat. One thing I noticed: after the furnace (and the fan) get shut off for a few minutes, the fan comes on for a few minutes then goes off for good. This means the fan control system does something there, not completely dead. When the cold season is over, I will restore the proper wiring and troubleshoot the fan control system (learned quite a lot about this, thanks to the many videos on youtube).