I have a five wire system. I have a green, red, white, yellow, and blue. This is an electric heater attached to the air handler and a separate air conditioning system. THis system was controlled by a Lennox mercury thermostat. Could you tell me what each color wire does? Example: Yellow controls the outside unit. I am trying to switch this system to a programmable, digital thermostat.
Have tried two Hunter programmable thermostats without success... the air conditioner and heater are running at the same time.
I removed the yellow wire to cut off the air conditioner, the heat is working okay.
Please advise.

I would go to Air Handler and see which each wire controls which contactor/relay, Red is likely hot(power), green can be fan, white is heat, I see yellow and blue for cool, 2 speed. All can be wrong and bad info. Verify all.

Are you sure the AC is running? Perhaps you have a heat pump and thus you see the compressor running outside and mistook that for the AC running. Generally speaking, any 5 wire controller will work with a fairly modern (within the last 15 years or so) system. And the previous post said what I would have said regarding the wire colors. You can always test to assure the red is the supply if you have a volt meter (should read 24 volts when connected to ground). Then, if you cross red and green, the fan should come on.

Hope this helps.