HVAC Flame Sensing Circuit to Prevent Leakage Current False Positives
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Solution Overview
Problem
Existing HVAC systems face issues with accurately detecting the presence of a gas flame due to leakage current in inverters or capacitors, leading to false indications of flame presence, which can result in safety hazards and inefficient operation.
Innovation Solution
A method and system for flame detection in HVAC systems using a controller to determine the output states of comparators and a tank circuit, which includes a flame detect circuit, a first comparator, a second comparator, and a tank circuit, to accurately transmit notifications of flame presence or absence, thereby reducing false positives and ensuring safe and efficient operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If flame rectification using capacitors and inverters is used to detect flame presence, then the system can provide flame detection indication, but leakage current in inverters or capacitors causes false indications of flame presence
Solution Approach 1:
The patent divides the flame detection function into multiple independent sensing paths: a primary flame detection circuit using flame rectification, a secondary tank circuit for oscillation detection, and multiple comparators for threshold evaluation. Each path independently evaluates flame presence criteria, and only when multiple paths confirm flame presence does the system register a valid flame detection, thereby reducing false positives from leakage current in any single component.
Solution Approach 2:
The patent introduces an intermediary tank circuit that couples to the flame probe through a capacitor and generates oscillations only when a real flame is present. This tank circuit acts as a mediator that translates the complex electrical characteristics of flame rectification into a simpler oscillation detection signal that can be reliably evaluated by comparators, filtering out false signals from component leakage.
2Ease of operation
If conventional flame rectification circuitry is used, then the system can detect flame presence, but it may result in unnecessary furnace shutdowns or hazardous unburned fuel accumulation
Solution Approach 1:
The patent performs preliminary verification of flame presence by evaluating multiple independent criteria before triggering furnace shutdown or fuel cutoff actions. The system first checks the primary flame detection circuit, then verifies with the tank circuit oscillation detection, and only after confirming both indicate flame presence does it maintain furnace operation. This preliminary multi-criteria verification prevents premature shutdowns while ensuring safety.
Solution Approach 2:
The patent implements continuous feedback monitoring where the controller repeatedly evaluates flame presence conditions and adjusts furnace operation accordingly. The system continuously monitors the outputs of multiple comparators and flame detection circuits, providing real-time feedback that prevents hazardous conditions from developing while avoiding unnecessary shutdowns when flame presence is genuinely detected.
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
The solution effectively reduces the risk of erroneously sensing a flame, preventing unnecessary furnace shutdowns and hazardous unburned fuel accumulation, while ensuring accurate flame detection for safe and efficient operation.
Implementation Method 1
Flame rectification to sense presence or absence of a flame is conventional in gas furnace controls technology. When a flame is present, the second capacitor discharges to ground through the flame which acts as a poor diode connected in series with a resistor.
Data Source
AI summary
A method of determining presence of a flame in a furnace of a heating, ventilation, and air conditioning (HVAC) system. The method comprises determining, using a controller, whether a processor signal (G) is active, responsive to a determination that the processor signal (G) is active, determining, using the controller prior to assertion of a flame-test input control signal, an output state of a first comparator, responsive to a determination that the output state of the first comparator is high, determining, using the controller prior to assertion of the flame-test input control signal, an output state of a second comparator, and responsive to a determination that the output state of the second comparator is low, transmitting, using the controller, a notification that a flame is present.