HVAC Thermostat Variable Fan-Off Delay to Reduce Short Cycling
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Solution Overview
Problem
Existing HVAC systems lack a method for a variable fan-off delay based on measured conditioned space temperature, leading to potential short cycling and inefficient energy use, particularly in heating and cooling applications.
Innovation Solution
A thermostat system that compares current conditioned space temperature (CST) values during a fan-off delay period to historical values, adjusting the fan-off delay based on heating or cooling cycle durations, and continues to operate the HVAC fan until specific temperature conditions are met, ensuring optimal thermal comfort and energy efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a fixed fan-off delay is used, then the system is simple to operate, but it causes short cycling and reduces energy efficiency
Solution Approach 1:
The patent implements a variable fan-off delay that dynamically adjusts based on measured conditioned space temperature (CST) changes during the delay period. Instead of using a fixed time delay, the system continuously monitors temperature and compares current CST values to historical values, extending or reducing the fan-off delay accordingly to prevent short cycling and optimize energy recovery.
Solution Approach 2:
The system incorporates feedback by continuously measuring the conditioned space temperature during the fan-off delay period and using this information to determine whether to extend or reduce the delay. The controller compares current temperature measurements to historical data and adjusts the fan operation accordingly, creating a closed-loop control system that optimizes energy efficiency while preventing short cycling.
2Loss of energy
If the fan-off delay is extended to recover heating energy, then energy efficiency improves, but the system complexity increases
Solution Approach 1:
The system performs self-service by automatically measuring conditioned space temperature, comparing it to historical values, and autonomously determining the optimal fan-off delay duration. The controller independently manages the entire process of temperature monitoring, data comparison, and delay adjustment without requiring external intervention or complex user configuration, simplifying operation while achieving energy recovery.
Solution Approach 2:
The patent replaces traditional mechanical timing devices or fixed delay circuits with an electronic control system that uses temperature sensing and computational logic. Instead of relying on predetermined mechanical delays, the system uses electronic temperature measurement and processing to dynamically determine fan-off delay, enabling more precise and adaptive control.
3Loss of energy
If a variable fan-off delay based on temperature measurement is implemented, then energy efficiency and thermal comfort improve, but the device complexity increases
Solution Approach 1:
The thermostat controller performs multiple functions: it controls HVAC system operation, measures conditioned space temperature, stores historical temperature data, compares current and historical values, and determines optimal fan-off delay duration. By integrating these diverse functions into a single universal controller, the system achieves improved energy efficiency without proportionally increasing overall system complexity.
Solution Approach 2:
The system changes the parameter of fan-off delay duration based on measured temperature parameters. Instead of using a fixed time parameter, the controller dynamically adjusts the delay duration according to actual conditioned space temperature measurements and their rate of change, enabling adaptive optimization of energy recovery while maintaining thermal comfort.
Data Source
AI summary
A Fault Detection Diagnostic (FDD) variable differential and variable fan-off delay Heating Ventilating Air Conditioning (HVAC) thermostat control method. The FDD method detects a fan-on setting, reports a FDD fan-on alarm, overrides a fan-on setting, and turns off an HVAC fan. The FDD method detects, reports, and corrects a short-cycle by providing a variable differential based on: a cooling or heating cycle duration, or an off-cycle time. The variable fan-off delay is based on: a Conditioned Space Temperature (CST), comparing a current CST to the CST measured during the fan-off delay, a rate of change of CST with respect to time (dT/dt), dT/dt reaches an inflection point, the CST crosses a fan-off-delay differential, a cooling cycle duration, a heating cycle duration, a cooling temperature split, a heating temperature rise, an outdoor air temperature, a supply air temperature, or the rate of change with respect to time of these HVAC parameters.