Hi everyone,

I am having a problem with my central AC system and was hoping someone could help me out before I go call a technician out and spend a bunch of money. I've searched this site already and I was able to find a few things to try, but no issues like mine on a similar system.

First I'll explain my system. I have a separate heating and cooling system - my heat/hot water is provided by an Oil furnace in my basement, which uses hot water baseboard heaters to provide heat. The AC system is entirely separate central air system, which consists of an outdoor condenser and a forced-air fan/evaporator in the attic. We just had our furnace replaced this winter, not because it wasn't working, but because it was just old and we wanted something more efficient. The heat works great.

However, the problem is with the AC, we tried to turn it on for the first time this spring a couple days ago, but nothing happens. Neither the attic fan nor the outdoor condenser power on. I just replaced the thermostat because it was old, but it did not change. When I lower the temp on the thermostat, I hear two clicks (which I assume is the relays for the fan and condenser power). Again, two clicks when I turn off the thermostat. The fan will not come on even if I select Fan On instead of Auto.

I have checked the circuit breakers and they are not tripped. My attic fan has 3 low-voltage wires going to it, labeled R (red) G (green) and 3 (white). I assume the condenser probably has a safety that won't let it come on if the fan isn't on, so I haven't looked at that.

The thermostat has 5 wires, Rc, Rh, W, Y, and G. The Rh and W go to the furnace. Rc and G definitely go to the attic fan, and possibly Y also (Y is white, maybe the same wire as 3 at the fan).

I tried jumper between R and G at the fan to see if it would come on, but it did not. I used a multimeter to check and there is already continuity between the R and G terminals and the R and 3 terminals at the fan, even with the thermostat turned off.

I'm not sure what else I could check, but I appreciate help that can be provided.

The first question is how did the old thermostat wire up? Y is your cooling circuit. If you only have one transformer for heat and cool then you need to add a jumper wire from Rc to Rh at the thermostat. If you only have 3 thermostat wires that work your heat and air---R 24volts--W heat--Y cooling.

Well, my thermostat has 5 wires, Rc, Rh, W, Y, and G, and they are connected the same way as the old thermostat. The 3 wires I was talking about above are at the actual fan/evap unit in the attic -- R, G, and 3 (actually labeled with the number 3).