I purchased a Honeywell CT87N101 to replace the old mercury thermostat. I have a gas furnace w/ A/C. I DO NOT have a heat pump, I have a 5 wire system. Blue, White, Yellow, Red and Green... ALL of which were connected to the OLD thermostat before I decided to replace it except the yellow... I thought this was odd. But the AC worked and very well...

Question 1:Is there maybe a reset switch somewhere on the unit that may have been tripped when I installed the new thermostat?

Question 2: I have wired the new thermostat as follows and I have no A/C operation, I get air blowing through the vents but its not cold, nor is it hot for that matter. The unit is at best 3 yrs old. What else could it be?

You need a volt meter to see if you are getting 240 volts to your outside condensing unit on the high voltage line and to see if you are getting 24 volts on the low voltage side. The problem may all be on the outside. Set some reading and post them back I am sure someone will help you then.

The blower motor is coming on but is the compressor coming on?

I don't understand:

ALL of which were connected to the OLD thermostat before I decided to replace it except the yellow.


Is the wire replaced or the stat?

Is the yellow wire connected to the stat or not?

A wire, usually yellow, is connected to the Y terminal, and along with a white wire, picked up at the furnace, goes out the compressor , as a two wire cable, (red &white), and closes a relay, which must be pulled in to start the compressor motor.

Your new stat may very well use different terminals and may not match the old.