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Green Wire not connected - fan does not work when thermostat is set to "ON"

Green Wire not connected - fan does not work when thermostat is set to "ON"

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Old 03-11-14, 03:07 PM
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Green Wire not connected - fan does not work when thermostat is set to "ON"

i have a trane xe80 furnace about 13 years old.
air conditioner installed about 8 years ago.
programmable thermostat - also 13 years old
Everything works good
EXCEPT.
when i turn the thermostat to 'ON' from auto nothing happens. (i.e. the fan does not run)

i want to leave the fan on all the time to circulate the air.

at one time the fan "on" worked,

OK, so i looked at the wiring and think i know what is likely wrong.
4 wires go to the thermostat.
Red to R
Green to G
Purple to Y
Black to W

with thermostat on auto i have 24 Vac from red on thermostat to each of the 3 wires, turn thermostat from Auto to on and voltages to green drop to near zero.

checked out furnace wiring.
there are 3 sets of wires as follows
Wire 1 - to thermostat
Red to R, Black to W, Purple to Y, Green - taped off (not used). (well this is the obvious reason why the fan does not turn on, but with the wiring below, i dont know if i can just hook up the green)

Wire 2 - to air conditioner
Black to Y, Red to C

Wire 3 - to exhaust fan
Black to R, Red to G

So how this is set up from my understanding is that the fan kicks in when I turn on the exhaust fan.

However, i want to be able to turn on the blower fan without turning on the exhaust fan. in winter i am loosing heat by doing this, and in summer i am loosing cool air.

my question, can i hook up the green wire (from the thermostat) to the circuit board (G) so i can use the thermostat to run the blower without turning on the exhaust fan. OR will this wiring cause something on the blower or thermostat to short.


i have some electrical skills and understand most principles and know how to use a volt meter and that it is not a good idea to stick a knife into an electrical socket.
 
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Old 03-11-14, 03:21 PM
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Wire 3 - to exhaust fan...... Black to R, Red to G
Ummmmm..... ok


So how this is set up from my understanding is that the fan kicks in when I turn on the exhaust fan.
Why would you want the A/C blower to come on when the exhaust fan comes on ??
 
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Old 03-11-14, 03:35 PM
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i am not certain why the exhaust fan is hooked to the furnace blower/fan, that is how it was wired when the house was built (we are the original owners).
Perhaps it is a way to circulate the air and suck air from the kitchen area and push it outside (when i say kitchen area i am saying the exhaust suction port is on a wall in the kitchen, not a fume hood type vent).
 
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Old 03-11-14, 03:54 PM
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It's completely up to you.... but I would disconnect the exhaust fan control of the blower and reconnect the green wire to the G terminal so that you get the FAN ON mode working properly again.
 
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Old 03-12-14, 09:10 AM
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would i have to disconnect the exhaust fan wires from the HVAC blower.

does anyone know why the exhaust fan would have been hooked up to the blower.

my only thought of why this was done would have been that the exhaust is the highest point in the house and it would have pulled the hottest air out of the house and by having the HVAC blower connected it would circulate the air and pull the cold air from downstairs and push it upstairs thereby cooling the upstairs.
 
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Old 03-12-14, 09:16 AM
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You can leave the exhaust wiring as it is and just add the green wire to the G terminal.

does anyone know why the exhaust fan would have been hooked up to the blower ?
I haven't a clue. I'd rather turn my blower on and off if I thought it was necessary.
 
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Old 03-12-14, 09:29 AM
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I shall give that a try tonight.
 
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