Yes, I know I should probably just change the system but the AC works great still and I just want to make it through this winter. If there is a quick fix please let me know.

Both the fan and the A/C work, however, when I switch the unit over to heat there is nothing. The thermostat is one of those mercury filled glass tubes. When on the A/C and I turn it below, the mercury connects the circuit and there is a spark. When I switch it to heat and turn the thermostat higher the mercury shifts but there is nothing. I have gone up on the roof and can hear the electricity in the unit so I know there is power up there.

Also, the house was vacant for about six months and the gas was turned off until we moved in. I have checked the manual line and the line is open. Even if there was not gas, I should still be getting the fan when the heat turns on right? The problem is that it does not even turn on.

The thermostat is not calling for heat. Any ideas?

You need to be sure that there is gas at the unit and the line was bleed of air to be sure. The pilot could not be working because there is no gas available due to air in the lines. etc. Once that problem has been solved the furnace shoud light. If not you have another problem.