In reality it only took 5 min to wire after understanding why my set up was different. Pros: Excellent product, easily affordable. Smart purchase. I did many reviews of many products and kept coming back to ecobee. They provide all you need for an install. If you are on the fence - go for it. My final push was I went on vacation for the first time in many many years. I realized it would be nice to control house temp while I was away. Now I can. Hooray technology. Take images of thermostat connections and furnace control board connections before you remove anything. You should call ecobee support for help but this may help you also. Super small Con: Con is only due to my particular wiring - not the device. I did not have a "C" wire at the thermostat, only White, red, green and yellow so this install is with using the PEK device supplied with thermostat purchase, my colored wiring set up and my pre existing connections. If you have questions - Call ecobee support - they sent me images and instructions(3 of the included images) for my set up. Per the wiring image. I had a separate red and white harness in my furnace going to my AC compressor and had to tie it into the PEK device supplied with thermostat which is used with 4 wire cable with no C wire at the thermostat end. If you have 5 (or more) wires you may have a C: wire or other wires for 2 stage heat or AC. I had a single stage everything. Your wire colors may be different it is more important to know what color wire goes to what connection. You should use the supplied stickers to label the wires. For example my yellow wire could be orange or blue in your set up. Put a Y label on it This based more on where you disconnected the cable from more then color though. If you dis connect a orange wire from Y or Y1 - label it Y or Y/Y1 Original Thermostat connections and wires I had a Red wire going to Rh with a Jumper to Rc (which can be ignored) Green to G Yellow to Y White to W Furnace end controller board Red to R Green to G Yellow wire tied to red of seperate 2 wire cable to my AC compressor (Not to Y on furnace controller board) 2nd white wire of AC comporsser tied to wire nut then a white wire (shown as blue in wiring diagram) to C connection on controller board White to W Connections for ecobee thermostat with No C wire Thermostat end Red to Rc Green to C Yellow to PEK White to W1 PEK is magnetic and sticks to wall inside furnace in between wiring from thermostat and furnace control board Wiring from thermostat (4 wires) to PEK device supplied Red to PEK "R" Green to PEK "G" Yellow to PEK "Y" White to PEK "W" PEK to furnace and AC compressor and furnace control board (5 wires on the other end of PEK device) If you happen to have 2 wires to any one terminal - Only disconnect the wire that went to thermostat. Leave the other wire connected as this may go to another device you have such as a humidifier. Leave any other wires on the furnace control board connected. Again - only remove the wires that went direct to furnace. Red (R) to furnace control board "R" Green (G) to furnace control board "G" Yellow (Y) to AC compressor red (with wire nut). White (W) - label looks Grey) to furnace control board "W" Blue (C) to AC compressor wire nut tying AC compressor white wire - PEK blue (C) wire and my short white colored wire (shown as Blue wire in wiring image) to furnace control board "C" Note: My original wiring connections on the furnace control board had no wire to "Y' since my original yellow wire connected to wire nut with Red AC compressor. With ecobee I had to plug the yellow wire to PEK "Y" and the opposite end of PEK device went back to wire nut with Red AC compressor wire. The wiring image shows white AC compressor white tied with wire nut to PEK Blue wire AND a white wire(Shown in blue on Right wire image) to "C' on furnace control board. - don't let color confuse you it could be any color, where it comes from and where it plugs into is more important.