To know when a furnace or AC thermostat is bad you can look to see if the display is working properly and showing the proper indoor temperature.
If the room feels much hotter or colder than what the display on the thermostat say sor what the dial on the thermostat says then the thermostat is not working properly and cannot accurately measure the rooms temperature.
Or if the thermostat is not coming on or changing temperatures and the furnace or AC is not coming on when it should then the thermostat could also be bad.
To calibrate an indoor thermostat you'll use a screw that is located in the center of a curved piece of metal that is inside the thermostat.
Then to calibrate the indoor thermostat you will turn the screw back and forth with a screwdriver until you feel the contacts have opened.
Then you wait for a few seconds and then turn the calibration screws of the thermostat again until the contacts have closed.
A thermostat for a furnace or AC should last around 10 years.
Some furnace and AC thermostats can last 15 to 20 years depending on the make and model.
As the thermostat ages it can become to malfunction due to normal wear and tear and also dust.
The older mercury and non digital thermostats can last decades and the newer digital thermostats most often last at most 10 years.
Thermostats do need to be level if they are the mercury bulb type thermostats or they won't work properly.
If the thermostat is not level then the thermometer in the thermostat will not operate the correct way and make the thermostat almost useless as it won't keep up with the correct temperature of your home.
The non mercury bulb or digital thermostats should be level but they don't have to be perfectly level.
You can replace a thermostat without an electrician.
A thermostat for your furnace or furnace and AC are pretty easy to replace and the are low voltage.
Before replacing the thermostat you should mark the wires so you can tell which wire is which.
The new thermostat should also have a wiring diagram which is pretty easy to follow.
If you're unsure on how to replace the thermostat then a handyman could replace the thermostat or an HVAC company could also replace the thermostat for you.
If your thermostat controls your heat, you will have a white wire.
The Y wire is yellow and connects to your air conditioning compressor.
The G wire is green and connects to the fan.
Rc and Rh: The red wire(s) are the power source for your thermostat.
A typical eight-wire thermostat will have the following wire configuration: red wire to "R" terminal, white wire to "W" terminal, green wire to "G" terminal, yellow wire to "Y" terminal, black wire to "C" terminal, blue wire to "Y2" terminal, brown wire to "W2" terminal, and orange wire to "Aux" terminal.
Red or hot wire for 24V powers the thermostat continuously, linking to the R terminal.
Blue wire or common wire for 24V to complete the circuit linking to the C terminal.
A white wire connecting to the heating unit linking to the W terminal.
A green wire connecting to the fan linking to the G terminal.
While your thermostat may not technically need a C-wire to operate, it gets the power for its screen and Wi-Fi capabilities through “phantom power” or “power stealing.” Basically, this means your thermostat gets a kick of extra power whenever your HVAC system is running.
The most common thermostat cabling is 18/5 (18 gauge, 5 conductor), because 5 conductors typically offer enough functions for a modern thermostat/low voltage device.
Some older thermostats do indeed have three connections: 2 for switching and one (neutral feed)to power a heating resistor to act as an "anticipator".
However if you are using two 2-port motorized valves, then you need the third wire to isolate the valves from each other.
What if I have an O and B wire thermostat?
O, B, O/B: These wires are responsible for switching the changeover valve in a heat pump system.
The O wire reverses the valve from heating to cooling, and the B wire switches the valve from cooling to heating.
Sometimes it might be a single O/B wire instead of two separate wires.
There is no standard for the color of the RH wire, but most units use a red one.
The RC wire, on the other hand, refers to “red cooling”.
It's the same with the RH wire in that it powers the thermostat.
If there are no yellow wires, it is probably because the manufacturer has chosen to color this wire blue instead.
If you know that your system has two stages of cooling, but there is only one yellow wire, it's also possible that the second wire is colored light blue.
If you have two wires, R or RH go into the R terminal and RC into the RC terminal.
If you have more than one wire (for example, you have a wire labeled R and another wire labeled Rc), remove any jumpers between the R and Rc terminals or push the switch to open the RC terminal to insert a wire.