Temperature difference between external sensor and furnace sensor.

Introduction to Temperature Sensors

Temperature sensors are devices that measure the temperature of their environment and convert this information into a readable format. These sensors are used in a wide variety of applications, from home appliances to industrial processes. In the context of heating systems, temperature sensors play a crucial role in ensuring that the furnace operates efficiently and safely.

Types of Temperature Sensors

There are several types of temperature sensors, each with its own advantages and disadvantages:

1. Thermocouples: These sensors consist of two dissimilar metals that generate an electrical voltage when exposed to temperature changes. They are inexpensive, durable, and can measure a wide range of temperatures.

2. Resistance Temperature Detectors (RTDs): RTDs measure temperature by correlating the resistance of a metal (usually platinum) with temperature changes. They are highly accurate and stable but more expensive than thermocouples.

3. Thermistors: These sensors are made from semiconductor materials that change resistance with temperature. They are highly sensitive and accurate but have a limited temperature range compared to thermocouples and RTDs.

4. Infrared Sensors: Infrared sensors measure the thermal radiation emitted by an object without physical contact. They are useful for measuring the temperature of moving or inaccessible objects but can be affected by the object’s emissivity and the sensor’s field of view.

The Role of Temperature Sensors in Furnaces

In a furnace, temperature sensors are used to monitor and control the temperature of the heat exchanger, the air in the supply ducts, and the room air. The most common types of sensors used in furnaces are thermocouples and thermistors.

Heat Exchanger Temperature Sensor

The heat exchanger temperature sensor, also known as the limit switch, is a safety device that prevents the furnace from overheating. It is typically a bimetallic switch or a thermistor that is mounted on the heat exchanger. If the temperature of the heat exchanger exceeds a predetermined limit (usually around 200°F), the sensor will shut off the furnace to prevent damage.

Supply Air Temperature Sensor

The supply air temperature sensor measures the temperature of the air leaving the furnace and entering the supply ducts. This sensor is usually a thermistor or an RTD and is located in the supply plenum. The furnace control board uses the information from this sensor to adjust the firing rate of the burners and the speed of the blower motor to maintain the desired supply air temperature.

Room Temperature Sensor

The room temperature sensor, also known as the thermostat, measures the temperature of the air in the living space. It is typically a thermistor or an RTD that is mounted on a wall in a central location. The thermostat compares the room temperature to the user’s setpoint and sends a signal to the furnace control board to turn the furnace on or off as needed.

External Temperature Sensors

In addition to the sensors inside the furnace, some heating systems also use external temperature sensors to improve their efficiency and comfort. These sensors are usually mounted outside the building and measure the outdoor air temperature.

Outdoor Reset Control

An outdoor reset control is a feature that adjusts the supply air temperature based on the outdoor air temperature. When the outdoor temperature is cold, the furnace will supply hotter air to compensate for the increased heat loss from the building. As the outdoor temperature rises, the furnace will supply cooler air to prevent overheating and improve efficiency.

Dual Fuel Systems

A dual fuel system is a heating system that uses both a heat pump and a furnace. The heat pump is used as the primary heating source when the outdoor temperature is above a certain threshold (usually around 30-40°F), and the furnace is used as a backup when the outdoor temperature drops below the threshold.

An external temperature sensor is used to determine when to switch between the heat pump and the furnace. This allows the system to take advantage of the high efficiency of the heat pump during mild weather and the reliable heating capacity of the furnace during cold weather.

Temperature Sensor Differences and Their Impact

The temperature difference between the external sensor and the furnace sensor can have a significant impact on the performance and efficiency of the heating system.

Sensor Accuracy

One potential cause of temperature differences is the accuracy of the sensors themselves. If the external sensor is not properly calibrated or is exposed to direct sunlight or other heat sources, it may read higher than the actual outdoor temperature. Similarly, if the furnace sensor is not properly positioned or is affected by drafts or radiant heat, it may read lower than the actual supply air temperature.

These inaccuracies can lead to the furnace supplying air that is too hot or too cold for the current conditions, resulting in reduced comfort and efficiency.

Sensor Location

Another factor that can contribute to temperature differences is the location of the sensors. The external sensor should be mounted on the north side of the building, away from direct sunlight and other heat sources. If the sensor is mounted on the south side or near a heat source, it may read higher than the actual outdoor temperature.

Similarly, the furnace sensor should be located in the supply plenum, downstream of the heat exchanger and any air conditioning coils. If the sensor is located too close to the heat exchanger or is affected by radiant heat, it may read higher than the actual supply air temperature.

System Response Time

The response time of the heating system can also contribute to temperature differences between the sensors. When the external sensor detects a change in outdoor temperature, it sends a signal to the furnace control board to adjust the supply air temperature. However, there is a delay between the time the signal is sent and the time the furnace responds.

During this delay, the indoor temperature may continue to rise or fall, creating a temporary mismatch between the supply air temperature and the desired room temperature. This can lead to occupant discomfort and reduced system efficiency.

Minimizing Temperature Sensor Differences

To minimize the impact of temperature sensor differences on the performance and efficiency of the heating system, several steps can be taken:

1. Ensure proper sensor calibration: Regularly check and calibrate the external and furnace sensors to ensure they are reading accurately. Follow the manufacturer’s instructions for calibration procedures.

2. Optimal sensor placement: Mount the external sensor on the north side of the building, away from direct sunlight and heat sources. Install the furnace sensor in the supply plenum, downstream of the heat exchanger and any air conditioning coils.

3. Improve system response time: Upgrade to a furnace with a variable-speed blower motor and modulating gas valve. These components can respond more quickly to changes in temperature and help maintain a more consistent supply air temperature.

4. Regular maintenance: Schedule annual maintenance for the heating system, including cleaning and inspecting the sensors, to ensure they are functioning properly and not affected by dirt, debris, or physical damage.

Frequently Asked Questions (FAQ)

1. What is the ideal temperature difference between the external sensor and the furnace sensor?
   Ideally, the temperature difference between the external sensor and the furnace sensor should be minimal. A difference of more than 5-10°F may indicate a problem with sensor accuracy, placement, or system response time.

2. Can I relocate the external temperature sensor myself?
   It is generally not recommended to relocate the external temperature sensor yourself. Improper placement can lead to inaccurate readings and reduced system performance. If you suspect a problem with the sensor location, contact a professional HVAC technician for assistance.

3. How often should I have my heating system sensors checked and calibrated?
   It is recommended to have your heating system sensors checked and calibrated annually as part of a regular maintenance program. This will ensure that the sensors are reading accurately and that the system is operating at peak efficiency.

4. What are the signs that my temperature sensors may be reading incorrectly?
   Signs that your temperature sensors may be reading incorrectly include:

5. Rooms that are consistently too hot or too cold
6. Furnace short-cycling (turning on and off frequently)

7. Unusually high energy bills
   If you notice any of these signs, contact a professional HVAC technician for a system evaluation.

8. Can temperature sensor differences affect my energy bills?
   Yes, temperature sensor differences can lead to reduced system efficiency and higher energy bills. Inaccurate sensors can cause the furnace to run longer than necessary or to supply air that is too hot or too cold for the current conditions. By minimizing sensor differences and maintaining proper system operation, you can help keep your energy bills under control.

Conclusion

Temperature sensors play a critical role in ensuring that your furnace operates efficiently and safely. The temperature difference between the external sensor and the furnace sensor can have a significant impact on the performance and efficiency of your heating system.

By understanding the types of sensors used in furnaces, their roles, and the factors that can contribute to temperature differences, you can take steps to minimize these differences and maintain optimal system operation. Regular calibration, proper sensor placement, improved system response time, and annual maintenance are all key to achieving this goal.

If you suspect a problem with your temperature sensors or your heating system in general, don’t hesitate to contact a professional HVAC technician for assistance. With proper care and attention, your furnace can provide reliable, efficient, and comfortable heating for years to come.