I have a dual energy electric/oil furnace that is wired to a thermostat with 2 wires R and W. I don't have any AC. Inside the furnace there is a switch relay to control the fan but it is not connected to the G terminal inside the furnace. I would like to run a 5 wire cable from my thermostat and the furnace and connect the W R and G wires to use the Auto / On fan switch on the thermostat instead of the switch relay inside the furnace, and eventually the C wire as well. Fan Switch ad Relay

Since the G terminal inside the furnace is not connected to anything, how can I connect it to achieve what I want? Terminals Disconnected terminal G, third from right

Should I bypass the manual switch or an I wire the thermostat in parallel ?

The fan relay enter image description here

  • Can you post photos of the wiring + a wiring diagram for your furnace? – ThreePhaseEel Jun 1 '19 at 3:32
  • Hi and thank you for your prompt response. The pictures that I have on my phone are too large to post here but I will compress them and upload tomorrow. – user1485737 Jun 2 '19 at 1:43

You need to add a relay to the system to independently operate the fan separate from the heat.

The simplest way is to buy a new high voltage 120VAC (20-30 Amp?) heavy duty relay/contactor that switches with a 24VAC control voltage and connect both relays to the high voltage circulation fan. If either relay is on the fan gets power. The existing fan relay will power the fan when you call for heat, and the new fan relay will power the fan when you call for fan only.

If you don't add a relay and still somehow hook it up, your heat will probably turn on every time it calls for fan only.

  • Thanks for the advice. I believe that I have one or two spare relays left so it would only be a matter of connecting them if they have the right voltage. If I would kike to disconnect the manual switch shown in the first picture, I could possibly reuse the same relay, right? – user1485737 Jun 3 '19 at 1:33
  • So I have been trying to figure out what to do next. I already have a 30A relay that is used for the fan. See the drawing added to the main question. I don't mind removing the manual switch, so I think that I should disconnect the switch and connect the black wire from the furnace directly to the relay. This would get the fan always on, right ? I'm not sure if I should run my wire from the thermostat to the G terminal and then to the relay #3 instead of the red wire or to the relay #2 instead of the black wire that is connected to terminals 8 and 12. I am not sure what are these used for. – user1485737 Jun 10 '19 at 0:52

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