HVAC Return Air Estimation Using Discharge Temperature Offset
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Solution Overview
Problem
HVAC systems often lack dedicated return air temperature sensors, making it difficult to detect faults and optimize performance, as direct measurement of return air temperature is not possible due to design constraints, cost, and signal processing limitations.
Innovation Solution
A controller in the HVAC system measures discharge air temperature when the system is not in heating or cooling mode, using this as a proxy for return air temperature, and stores offsets in a lookup table associated with outdoor temperature and blower flow rate, allowing indirect determination of return air temperature without a dedicated sensor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a dedicated return air temperature sensor is installed in the return duct, then return air temperature measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent uses discharge air temperature as an intermediary measurement to indirectly determine return air temperature. A temperature sensor measures the discharge air temperature from the evaporator, and a controller calculates the return air temperature by adding a predetermined offset value. This intermediary approach avoids the need for direct return air temperature sensing while providing sufficient measurement precision for system monitoring and control.
Solution Approach 2:
The patent creates a computational copy of the return air temperature measurement by using the relationship between discharge air temperature and return air temperature. Instead of physically measuring return air temperature, the system copies the measurement information through calculation using the offset method, thereby achieving the same functional purpose without additional sensors.
2Difficulty of detecting and measuring
If a return air temperature sensor is installed in the return duct, then return air temperature detection capability is improved, but manufacturing cost increases
Solution Approach 1:
The patent employs discharge air temperature as an intermediary measurement point that is already available in most HVAC systems. By using this existing measurement and applying a calculated offset, the system achieves return air temperature detection capability without the added manufacturing cost of installing additional sensors in the return duct.
Solution Approach 2:
The system uses existing components (discharge air temperature sensor and controller) to provide the function of return air temperature measurement. The controller performs the calculation using stored offset values, allowing the system to self-determine return air temperature without requiring external or additional measurement devices.
3Device complexity
If discharge air temperature is used as a proxy for return air temperature, then system simplicity is maintained, but measurement precision deteriorates
Solution Approach 1:
The patent adjusts the measurement parameter by introducing a temperature offset that accounts for the thermal difference between discharge air and return air. This parameter modification compensates for the indirect measurement approach, maintaining measurement precision while preserving system simplicity. The offset value can be predetermined through testing or calculated based on system operating conditions.
4Device complexity
If indirect determination method is used, then device complexity is reduced, but measurement reliability may be affected
Solution Approach 1:
The system incorporates feedback mechanisms to monitor and validate the calculated return air temperature. The controller can compare the calculated value with expected ranges based on outdoor temperature, system operating mode, and historical data. This feedback loop ensures measurement reliability while maintaining the simplicity of the indirect determination method.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables proactive detection of system faults and optimization of operating parameters, improving efficiency and performance by allowing the use of indoor temperature measurements to estimate return air temperature, even in systems without a dedicated return air temperature sensor.
Implementation Method 1
a temperature sensor positioned and configured to measure a discharge air temperature of the flow of air provided to the conditioned space
Implementation Method 2
a blower configured to provide a flow of air through at least one air duct and into a conditioned space
Data Source
AI summary
An HVAC system includes a blower configured to provide a flow of air into a conditioned space. A temperature sensor measures a discharge air temperature of the flow of air provided to the conditioned space. A controller of the HVAC system determines that heating or cooling mode operation is not requested. Following this determination, the blower provides the flow of air at a predefined flow rate. The controller determines that the measured discharge air temperature satisfies predefined stability criteria associated with a change in the discharge air temperature during the first period of time being less than a threshold value. If the stability criteria are satisfied, a temperature offset is determined between the discharge air temperature and an indoor temperature. The temperature offset is stored in a lookup table such that the temperature offset is associated with the flow rate of air provided by the blower and the outdoor temperature.