The location where the thermostat is mounted can be a problem. Mounting the thermostat on an exterior wall can result in a temperature exchange with the exterior temperature that give an incorrect temperature reading of the interior. The thermostat can read a lower temperature in winter and a higher temperature during summer.
Mounting the thermostat near a door or window can also create a temperature exchange. Whenever the door or window is opened, outside air can throw off the thermostat reading of the internal temperature. Don't locate the thermostat near a supply register either.
Even having the thermostat located near a lamp that uses an incandescent bulb can be enough to affect the thermostat. The heat from the light bulb can be enough to turn off the heat in winter before the desired temperature has been reached.
The best location for the thermostat is on an interior wall in the middle of the building. Try to avoid mounting away from corners where there is too much airflow. With supply registers near the exterior walls and return grills near the interior, the air movement is going to be measured on your interior walls.
A loose connection at the thermostat, furnace, or air handler control board can also be an issue.Without a screw to hold the wire in a nest thermostat, it may appear that there is a connection when there isn't. Any time you push the wires into a nest thermostat, make sure that they can't pull out, push them in as far as they can go for a good connection.
If the thermostat has screw terminals, make sure they are tight. Use a screw driver with large hand with a good grip for enough torque to get a good connection.
The insulation on the wire should be cut back so that enough copper is exposed to make contact with the terminal. Don't remove too much insulation, there should not be enough copper exposed for wires to touch one another. If the insulation has only been partially stripped, there may be intermittent problems with contact.
Thermostat issues can arise when the thermostat is powered by batteries only and not from the c-terminal from the air handler or furnace. The thermostat can be powered through the c-terminal with the 24 volt line from the furnace or air handler control board. Some thermostats don't have a c-terminal and some have an optional c-terminal or battery so one or the other (or both) can be used.
While some thermostats will function with low batteries, others may not operate at all. Even if the thermostat doesn't indicate low batteries, there may still be a problem with the batteries and not enough power will be available to close the electrical contacts to turn the system on.
Bad batteries or corrosion on the battery contact terminals are other reasons that there won't be enough voltage to the display.
It is very important that a mercury thermostat is level because the small mercury balls being offset can cause the thermostat to shut off at the wrong time. Even if the thermostat was level when it was installed, the wall anchors can loosen and cause it to tilt.
Digital thermostats can be out of level and maintain an accurate temperature.
Selecting the improper thermostat or wiring it correctly is another reason there may be issues with the thermostat. A heat pump thermostat has wiring for an emergency heat that thermostats for gas do not. Some thermostats will have the option to select between heat pump and gas but, if not, the correct thermostat needs to be used for the type of unit.
A lot of times, heat pumps are wired incorrectly and homeowners have to push the button over to emergency heat too often if the auxiliary heating strip is not being powered automatically by the thermostat. It only takes one wire being in the wrong spot to have high electric bills or uncomfortable temperatures.
Wires that intermittently touch each other or the ground frame are another reason the thermostat could be having issues. Not running the wires through the rubber inserts that the manufacturer installed can cause the insulation to rub off on the wires and allow the wires to come in contact with the ground frame.
As mentioned above, wires that have the insulation casing stripped back too far can allow the copper from two wires to come in contact with each other. Squishing too many wires into the same area and compressing them can also result in unwanted contact, even if they are well insulated.
If you need to staple a thermostat wire, be careful that it is not too tight. Missing and hitting the wire can cause a short. Make sure the thermostat wire has a little play and come move under the staple.
If it is stripped wrong, the thermostat wire can be nicked or scratched and it won't be able to make a full connection. Avoid circling around the wire to strip the insulation, pull away instead. Also, don't use diagonal pliers that squish down on the wire. It is better to use wire strippers that are made for different gauges.
The same thing applies when splicing thermostat wires and using wire nuts. When the wire nut is being put on, the spinning and twisting can cause part of a damaged wire to come off. Eventually, this broken wire can end up falling out of the wire nut or can be loose enough to cause intermittent problems.
Thermostat wires that run through crawl spaces can also be damaged by rodents. Check for bad connections and corrosion at any wire nut or spade connector.
The thermostat may be set to turn on and off at the wrong time or can be trying to control the wrong system. Rarely, the instructions for programming the thermostat will be on the back of the thermostat itself. Using the model number, reference the installation instructions online.
A hole behind the thermostat, in the attic or in the crawlspace can let cold air in that causes problems for the temperature sensor on the thermostat. The hole can be filled in with rock wool or fire foam with some silicone right behind the thermostat.
Finally, the problem with the thermostat could actually be the thermostat itself or one of the components that the thermostat controls can be damaged. There is an entire sequence that the thermostat controls and it might be necessary to troubleshoot.
The thermostat is connected to the control board and comes off as 24 volts. The control board may not be sending the voltage to the inducer motor or the capacitor could be bad. Maybe the blower motor is bad or the capacitor isn't sending the voltage to the blower motor. One of the relays could have gone bad and the contacts are burnt. When troubleshooting thermostat issues, you may need to test out various components.