Special tools are needed to do this work hand tools and meters are needed to work on an electrical appliance such as a furnace or the condensing unit. A meter; can be a clamp on type or hand held, two fourteen inch pipe wrenches, battery drill with magnetic drivers, quarter inch and five sixteenth inch, strippers, standard and Phillips screw driver, hammer, dikes, refrigeration manifold set, Allen tool, ratcheting refrigeration wrench, sheet metal snips. The technician should locate the breaker panel and disconnect switch for the furnace and condensing unit. First we will assume your job is to change out an old residential furnace for a new more efficient unit. Once the electrical service panel is located the power to the furnace should be switched off, however if the service tech is unable to locate the correct switch, do not fear for it can be found by a process of elimination. Residential furnaces usually have and should be a ten or fifteen amperage breaker, the condenser or outside cooling unit could be a twenty, thirty or even a forty amperage breaker depending upon what the unit size is the tonnage. If you know the size is a two ton then the breaker is going to be a dual twenty, and so on. It's a simpler matter if the furnace fan is in running condition; switch the fan on switch to on. Then go to the breaker panel and systematically switch each breaker switch off until the fan at the furnace stops the same can be done with the condensing unit before turning off the furnace put the thermostat in the cooling mode. The condensers breaker will be perhaps easier to locate as it will be a dual switch or one which is locked together with a small bar and is rated at twenty to thirty amps. Check for which kind of refrigerant the condenser holds by looking at the nomenclature label usually located on the outside of the condenser. Next go to the furnace with a flat screw driver remove the screws holding the switch cover then with your electrical meter check for power at the switch, (one lead to one side of the switch with the other lead to a ground wire or a metal surface on the furnace. Unless the fan blower motor malfunctioned during your process to locate the proper breaker, the meter should read zero. If indeed the meter shows a zero reading you are ready to remove the switch, if no switch is present then there should be three leads from the breaker box, in a residential unit those wire colors are usually black, white and a single bare copper ground wire or a green one, connected to the furnace with colored plastic caps which may be black, orange, yellow or red, the color really isn't important. Here is another opportunity to check for power, with your meter leads put one in the black wire and the other either in the white or directly on the ground wire, if there is no power unscrews the nuts counter clockwise. Next pull the wires apart from each other and unfasten the connector that holds the wire to the junction box on the furnace, next put the wire nuts back on the black and white wire and for safety using plastic electrical tape, tape the nuts to the wire. Why secure the nuts on the wires instead of leaving them free? The breaker which you turned off could be a branch circuit feeding a light or another receptacle somewhere in the house, so you may need to restore that power. Unless you will be removing the feed wire all the way back to the breaker panel. If you are a smoker put out your pipe, cigar or cigarette the next step could be hazardous to your health please extinguish your smokes!