I bought an ecobee3 smart thermostat which requires the C wire. At my furnace there is no circuit board but there was a diagram and I was able to locate the C - common wire, but it seems to be used by my air conditioner which is outside. I was able to run a wire up to my thermostat to connect the C wire. When I have the A/C and the Thermostat on the same C wire the A/C kicks in all the time. I had to unplug the A/C unit from the common wire and just leave the thermostat for now and it works fine for heat.
How do I get around this problem? why can't the common wire be used for both the a/c and the thermostat at the same time?
furnace model: ruud ubhc-17j14sfa