What are some abnormal HVAC thermometer readings

High Readings: Potential Cooling/Heating Issues
When the HVAC thermometer reading is consistently above the set point, it typically indicates that the system is not effectively cooling or dissipating heat. This condition can be caused by a variety of factors, ranging from simple maintenance oversights to complex mechanical failures.
First, insufficient refrigerant is a common culprit. HVAC systems absorb heat from indoor spaces by circulating refrigerant. If refrigerant leaks, circulation is reduced, and the system's cooling capacity is significantly reduced, leading to persistently high temperatures. In addition to abnormal thermometer readings, users may also notice insufficiently cool air coming out of the air conditioner's vents, or even ice forming.
Second, dirty evaporator or condenser coils are a major culprit. These coils perform the critical function of heat exchange. When dust, dirt, or mold accumulate on the coils, they form an insulating layer that significantly hinders heat exchange. The system must run longer to reach the set point, often in vain, resulting in persistently high thermometer readings.
Furthermore, a faulty or inefficient compressor is a serious problem. The compressor is the heart of the refrigeration system, responsible for compressing the refrigerant. If the compressor's internal components wear, the capacitor fails, or it overheats, its efficiency can significantly decrease or even stop working altogether. This directly results in insufficient cooling capacity, which will naturally be reflected in the thermometer reading.
Finally, blockages in the supply or return air ducts can indirectly affect temperature readings. If the return air is blocked, the system cannot effectively draw in warm air for processing. Similarly, blockages in the supply ducts prevent the even distribution of cool air, resulting in localized high temperatures that affect the central thermostat reading.

Low Readings: Potential Overheating/Overcooling Issues
In contrast to high readings, when the HVAC thermometer reading is consistently below the set point, it often indicates that the system is underheating or overcooling.
A common cause is an inaccurately calibrated or improperly positioned sensor. If the temperature sensor is installed near a cold air intake or heat source, its reading may not represent the average temperature of the entire room. For example, if the sensor is installed near a window, cold winter drafts can cause an abnormally low reading, misleadingly suggesting that the system continues to heat.
In refrigeration systems, a faulty expansion valve can cause an excess of refrigerant to enter the evaporator, resulting in overcooling. This can cause the evaporator temperature to drop sharply or even freeze. This can cause the thermometer to read lower than normal.
A faulty heating element or inadequate combustion are common causes of low readings in heating systems. In electric heating systems, if the heating resistor burns out or has poor contact, the system won't produce enough heat. In gas furnaces, a clogged burner nozzle or igniter malfunction can reduce combustion efficiency, resulting in insufficient heat output and a low thermometer reading.
Furthermore, a malfunctioning fan motor can affect temperature readings. If the fan speed is too low or stops completely, hot or cold air can't be effectively drawn in or out, leading to uneven temperature distribution within the room and potentially lower temperatures in areas close to the sensor.

Excessive fluctuations in readings: System control imbalance or circulation issues. A stable HVAC system should maintain a stable temperature with small fluctuations around the setpoint. Frequent and large fluctuations in the thermometer reading are usually a sign of a control system imbalance or air circulation issues.
A malfunctioning thermostat is the first area to investigate. If the circuit board or sensor inside the thermostat is damaged, it will be unable to accurately sense temperature changes and send the correct commands to the system. This can cause the system to frequently cycle on and off, resulting in large temperature fluctuations, commonly known as "short cycling."

Secondly, temperature fluctuations can also be caused by malfunctioning solenoid valves or expansion valves. These valves control the flow of refrigerant or hot water. If they fail to open or close reliably, cooling or heating will be unstable, and the thermometer reading will fluctuate accordingly.

Uneven airflow distribution is another often-overlooked factor. If supply or return air vents are blocked by objects like furniture or curtains, the airflow within the room becomes disrupted. Heat or cooling is unevenly distributed throughout the room, causing some areas to be too hot and others too cold. While the thermostat may display an average reading, the fluctuation will be significantly increased.

Finally, leaks or damage in the ductwork can also be a significant cause. Cooled or heated air can leak from the ducts during transport, resulting in unstable airflow and temperature at its destination. This directly affects the stability and uniformity of the indoor temperature, causing the thermometer reading to fluctuate.