Troubleshooting Your Air Conditioning Thermostat
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Troubleshooting Your Air Conditioning Thermostat

How to troubleshoot problems with your air conditioning thermostat

For most people the only contact you have with your air conditioner is the wall mounted thermostat, and maybe where the filters go. Unfortunately many of us ignore our heating, ventilation, and air conditioning (HVAC) system until there is a problem and the first place we go to look is the thermostat. Chances are that the thermostat is not even an issue but thinks the thermostat is not functioning properly. It is not uncommon for a person to call an HVAC contractor and tell the secretary that they think the thermostat is broken because their air conditioner or heating system does not work while sometimes true there is usually something wrong with the air conditioning and heating equipment that needs to be repaired. It is a good idea to call an HVAC service technician to look at the problem especially if you want to replace the thermostat, but there are a few things you can check with your thermostat before you call for a repair.

Common Thermostat Troubleshooting

• Your thermostat, sometimes referred to as a “t-stat” is nothing more than a switch that turns the air conditioning system on or off. Old mercury-style t-stats used a glass bulbs filled with mercury to complete a low-voltage connection when a bi-metal spring rotated due to the room temperature raising or lowering. Today’s digital thermostats replace this analog system with circuitry to monitor the temperature and complete the connection. These newer thermostats usually require an outside power source from a battery to operate. Some still use a 24 volt signal to power the t-stat and complete the connection. The thermostat gets the 24 volts from the air conditioning and heating equipment by way of a small transformer which takes the high voltage supplied to the unit and lowers it to 24 volts AC.

• You should check the circuit breaker for the air handler to make sure it is not tripped. Also check the power switch, typically a red switch cover, which should be located near the air handling unit. It is not uncommon for people to mistake these switches for a light switch and turn them off. They unknowingly just turned the power off for their air conditioning and heating equipment, including their thermostat.

• If you have a digital thermostat the power display or display light may not function if you lose power. As stated earlier, some air conditioning and heating thermostats use batteries to power the back light so you should check the power switch and breaker to ensure the power before deciding your thermostat is faulty.

• The back display light for the digital thermostat does not function but the air conditioning and heating system works fine. Check the batteries as some digital thermostats require 9 volt or AA batteries for the back light to function on the thermostat. Digital thermostats can that use batteries for the back light to function or use the power from the air conditioning and heating equipment. If you don’t know which kind of air conditioning and heating thermostat you have then open the thermostat up and look inside. If you don’t see any batteries or a battery compartment inside the thermostat then your thermostat uses power from the equipment an is usually an older model. These types of programmable t-stats are obsolete since every time you lose power you will lose the program inside.

• If your programmable thermostat is always losing its program, then you probably have an obsolete thermostat. Programming thermostat can be a tedious exercise for many people to figure out so a problem like this can be frustrating especially if you live in an area where there are constant power failures. Replacing it with a new thermostat with a battery back-up can save you the trouble and frustration of reprogramming the thermostat over and over again when power failures occur.

• If you have tried the troubleshooting techniques listed above and there are still problems with your thermostat, you may have to go a little deeper.

1. Check the thermostat location. This is very important as a thermostat installed on a wall that has high heat gain or heat loss will accurately read the average temperature of your home and cycle the unit off and on. Check to make sure the t-stat is installed on an interior wall and not where direct sunlight can hit the thermostat. A thermostat located near an outside door or window will be affected every time the door or window is opened or closed. A thermostat should be located close to a return air grille so that it measures the returning air temperature which comes from the living areas. Thermostats located near heat sources like hot or cold water pipes, radiant heaters, fireplaces, electrical devices which produce heat such as computers, televisions, or audio equipment will never read the accurate room temperature.

2. Check for holes behind the thermostat. All thermostats have a hole behind them where the wires come into the thermostat from the air handling unit. Check this and if you find a big hole behind the thermostat stuff some insulation in this hole and cover it with a piece of tape. This will prevent drafts from affecting the thermostat.

3. Check the heating anticipator. The heat anticipator is on mechanical (non-digital) thermostats and needs to be set according to the amp draw on the heating control circuit. The heat anticipator offers a small amount of energy savings and prevents temperature overshooting as it shuts off the main burners prior to reaching the temperature setpoint. On forced air units the fan will continue to run and dissipate the heat which remains in the furnace or heat. Digital and programmable thermostats have built in heating and cooling anticipators which automatically set themselves with no manual adjustments. The mechanical thermostat needs a manual adjustment and you need a tool called an amp meter to determine the proper setting. The cooling anticipator in the mechanical thermostat requires no manual adjustment. You may need to call a technician to properly adjust this is you notice your home is always warmer than the temperature setpoint in the winter.

Heat Anticipator

4. If you have a mechanical thermostat with a mercury bulb switching mechanism inside it this thermostat needs to be level. If the thermostat is not level the mercury switch will not activate at the correct temperature depending on whether it is tilted to the left or right.

Some other thermostat problems may actually be due to the terminal block or voltage supply inside the unit and will require special tools and experience to remedy these. It is always a good idea to call an HVAC professional when you have problems with your air conditioning and heating equipment that are not routine.

Additional Resources

Honeywell Thermostats


Additional resources:

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