I have been having this problem for a while and have just not run blower without thermostat calling for heat or AC, system seems to run fine in heat or cool mode, but if I turn on FAN by itself the blower does not run and I see the condenser fan (outside) switch on. Could there be a crossed wire somewhere? It is an oil furnace with W, G, R, and Y wires to thermostat.

Thanks for any suggestions!

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    What make and model are the thermostat and furnace? Is the compressor outside engaging when the fan is called for? Can you post photos of the wiring connections? – ThreePhaseEel Dec 26 '17 at 19:34

You system may require that the call for cooling may also be a call for fan operation using the green lead. If this is the case then you don't have the option of independent fan operation at the thermostat. And the green lead should be disconnected from the thermostat. You need an isolation relay to be installed to get independent operation at the thermostat. You may have a faulty thermostat. You may also have a short between the yellow lead and the green lead; more likely miss-wired.

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