I have central heat and air condition. I currently have a mercury thermostat and I would like to convert to a programmable digital thermostat. But when I look at the thermostats that are available at the local stores, I am not for sure if it is the right one that I need. For example, I do not know if my system is single pump, or 2 stage pump, or even a pump at all. Also, there is an “EM. HT.” selection on my mercury thermostat and I do not know what that is used for. All I know is that it is central heat and air and I have a large machine in a closet in the house and I have a large machine outside. If I cut on either the heat or air, the machine in the house comes on. But I was told the machine outside is for AC. Can you guys please help me figure out what the heck kind of system I have so I know how to it works and what to put for it.