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We have a modern furnace with an old thermostat that controls heat and has a wire pair traveling to the furnace's control board. I'd like to wire up a local light switch by the furnace to manually turn on the blower fan to do some extra filtration of the air in the interior of the home.

Here's the current wiring feeding down into the PCB input terminals

furnace wiring clamp terminals

Could such a function be achieved by running a wire from the G terminal through to a light switch, followed by a second wire from the switch to one of the terminal clamps (COM, W, Y, or R) shown above? Which one would I connect to and why, and would this external wireup potentially damage or confuse the thermostat during its normal operation?

My understanding is that the G terminal is dedicated to controlling the blower fan. Also for those modern thermostats that can manage 4 wires what are they doing exactly in terms of these terminals and the closing of certain circuits and what not? Does it essentially connect G to R or Y or something to that effect? Does it become a conditional jumper between these terminals?

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    does the fan always turn on/off shortly after the gas does? If so, it's probably on an internal thermal switch, which is why you can use only 2 wires from the thermostat. Still, i think it should be possible to turn the fan on w/o heating, but not off w/heating, but no guarantees that the terminals are respected by the unit... – dandavis Oct 13 '17 at 6:25
  • I believe the two wires originate from old wiring sent to the previous furnace that did not have all the capabilities of control currently possible today. The wiring travels some distance to the thermostat and maybe one day I'll pull the wire and pull a multiwire cable through in its place to make using a modern thermostat possible. The furnace is capable of more wiring connections, its just that the wiring infrastructure sent to the furnace is old styled. – jxramos Oct 13 '17 at 8:11
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    Connect the switch between R and G, to turn on the blower manually. – d.george Oct 13 '17 at 9:55
  • @dandavis forgot to comment on your w,w/o heating. I intend for this to be an OR switch essentially with the thermostat. So either the thermostat tells the fan to turn on along with the burners as during normal operation, or turn on because of my switch. I don't intend the switch to interfere with the normal thermostat operation and all subsequent actions invoked by it. – jxramos Oct 13 '17 at 18:15
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Connect the switch from R to G

Wiring a standard light switch between R and G will do what you want -- the switch will perform the exact same function as the fan switch on a thermostat in gas mode. Namely, you'll be able to run the fan without heat by turning the light switch on, but the furnace will run the fan on its own when heat is called for, using its internal fan-thermostat.

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You can connect a light switch using G and R. There are 2 power terminals, R and C. G, Y and W are broken connections to R. A thermostat essentially is just an automatic jumper between R and G,W or Y. G is fan, W is heat Y is cooling. If the furnace is easily accessible you could mount a toggle switch on the furnace cabinet. It would seem easier to just run another thermostat wire and get a thermostat with a fan switch than to install another switch on the wall. The furnace has 2 speeds, low for heating and high for cooling. With most furnaces the fan (G) will turn on high speed fan. If the fan is on while heat is being called for it will operate on high speed not low speed. Not really a problem but may be slightly louder than normal.

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