Digital thermostats such as those sold by Honeywell are designed to be easily installed and operated. Not only are they more accurate than older thermostats, but they also no longer use mercury switches, which makes them much safer. Professionals can replace an older thermostat with a Honeywell digital in no time, but you can do it yourself with the right know-how. The most challenging aspect is keeping track of the connecting wires.
Go to the circuit breaker box in your home and shut off the switch leading to your thermostat.
Remove the existing thermostat from its mounting panel and note the placement of the wiring. Each wire will be connected to a terminal screw marked with a letter or combination of letters. These lead to various components of your home's heating and air conditioning system.
Take a piece of masking tape and write down the letter on the top terminal screw. Then wrap the masking tape around the wire connected to it. Repeat the process with every other wire. The precise configuration of wires will vary depending on your thermostat, but as long as you label them properly, you should be able to connect them to your new Honeywell thermostat without a problem.
Once the wires have been marked, loosen the terminal screws with a screwdriver and pull the old thermostat free. The wires could slip back into the hole in the wall from where they emerge, so act with care. (You can secure them to the wall with tape if you need to.)
Unscrew the wall plate of the old thermostat and set it aside.
Fit the mounting plate for the new Honeywell thermostat over the wiring hole such that the wires can be pulled through the plate at the proper point. Secure the plate to the wall using screws or drywall anchors as appropriate. (The precise method will vary by model.)
Connect the wires to the terminal screws in the Honeywell thermostat, matching each wire up to the corresponding marked screw. Some Honeywell thermostats have the screws on the wall mount, while others have them on the thermostat proper. In addition, there may be instances where a given letter code doesn't correspond to the same letter on the Honeywell's terminals. in that case, check the owner's manual. It should tell you which wire goes to which screw.
Fit the thermostat onto the mounting plate and snap it into place. Most Honeywell digital thermostats don't require any tools to locking onto their mountings. Instead, they use tabs or snap cases which can be secured just by pushing down on them.
Turn the circuit breaker back on, restoring power to the thermostat. You may now program it as normal.
Thermostat placement is very important. If your Honeywell digital thermostat is near a vent or heating duct, if it picks up heat from a stove or fireplace, if it gets light from a nearby window, or if it is placed in a "dead zone" near the corner of the room, it will not work as efficiently as it should. Place it in the middle of the wall---preferably one with both sides indoors----and away from any doors, windows, or hating/cooling implements.
Because they contain mercury, older thermostats needs to be taken to a proper waste disposal facility. Your city government should have details on where you can find one in your area. Contact them and follow their instructions to the letter when disposing of your old thermostat. Do not throw it away with the other garbage under any circumstances.