Method and Apparatus for Optimizing Hvac Cycling

This application claims the benefit of U.S. Application No. 63/575,142, filed on Apr. 5, 2024, the disclosure of which is incorporated herein by reference in its entirety.

Typical heating, ventilation, and air conditioning (HVAC) systems used in typical North American (NA) residential and light commercial applications have operating stages. When the thermostat calls for a heating stage or a cooling stage, the relevant components of the HVAC unit are turned on at full capacity until the thermostat determines that the stage demand has been met. The thermostat then turns off the stage until the next time there is a call for a heating or a cooling stage.

As described in U.S. Pat. No. 7,264,175 (the disclosure of which is incorporated herein), cycling the stage on and off at the right rate is important to maximize system efficiency, equipment life and user comfort. Original equipment manufacturers (OEMs) of HVAC systems will typically design and test the systems to optimize these parameters and their published efficiency rating and warranties will be based on optimizing the cycle rate of the thermostat. Thermostats will operate on a cycle rate that is based on the thermostat design and may be dependent on system variables such as the size of the house or size of the HVAC equipment. Therefore, such cycle rates of a thermostat may not be optimized for all installations.

The following describes a system that uses an adaptive method to automatically optimize the cycle rate of a thermostat for all installations. Generally, the method determines an operating characteristic of an HVAC component and adjusts a range of a differential temperature setting based on the operating characteristic.

The following describes a system that uses an adaptive method to automatically optimize the cycle rate for all installations. In particular, the method steps described hereinafter may be implemented within a thermostat having a processing device and a memory which stores instructions executable by the processing device.

The time a heating stage or a cooling stage of an HVAC system is on versus the total time from starting the stage to the next time the stage is started is referred to as the duty cycle. The duty cycle is typically expressed as a percentage. For example, if a stage is on for 5 minutes, then off for 5 minutes before the start of the next stage, the total time for a stage cycle is 10 minutes. In this example, the duty cycle is 50% (5 min time on during cycle/10 min time of cycle), the stage will be cycling at 6 Cycles Per Hour (CPH) (60 (minutes/hour)/10 (minutes/cycle), and the heating or cooling load of the conditioned space is 50% of the capacity of the HVAC system.

Turning to,shows the typical curve for a thermostat cycling a stage from 0% duty cycle to 100% duty cycle. In this example, the cycle rate will reach a maximum of 3 CPH. The CPH is typically a parabolic curve going from 0% duty cycle to 100% duty cycle with the maximum cycle rate at about 50% duty cycle. Meanwhile,is a chart with a typical OEM stage maximum CPH rate for various types of equipment.

Starting and stopping an air conditioning stage which utilizes a compressor frequently reduces the life of the compressor and, since the system takes time to get to temperature equilibrium, reduces the cyclic efficiency. Cycling the compressor faster, on the other hand, can reduce temperature swings and increase user comfort. That is why compressors will typically be tested at a certain cycle rate to optimize efficiency, product life, and comfort. Furnaces, however, can be typically cycled faster to optimize overall performance. Electric heat can optimize performance at a higher cycle rate.

Recognizing these problems, the subject system dynamically adjusts the cycle rate for a stage to optimize the performance of the HVAC system. As seen in, the cycle rate near 50% is relatively close to a flat curve. The subject system thus seeks to calculate an operating cycle rate near a duty cycle of 50% and to adjust the turn on and turn off (hereinafter referred to as on/off) points in the algorithm until the maximum cycle rate is in the desired range.

For example, if a thermostat is controlling a heat pump compressor stage, and the desired maximum cycle rate is about 3 CPH (+/−0.5 CPH) in the cooling mode, the thermostat may use a differential temperature of +/−0.5° C. in the cooling mode to control the on/off points. In other words, the thermostat turns the compressor on when the sensed temperature reaches 0.5° C. above the setpoint temperature and turns the temperature off when the sensed temperature reaches 0.5° C. below the setpoint temperature. If the setpoint temperature is 26° C., the thermostat will turn the compressor stage on at 26.5° C. and turn it off at 25.5° C.

In this example, a duty cycle range of between 40% to 60% is used for CPH adjustment. As shown in, the setpoint on/off points may be adjusted when the duty cycle is inside of the range of 40% to 60% and the CPH is outside of the range of 3+/−0.5 CPH. If the on time is 6 minutes and the off time is 12 minutes, the duty cycle is 33% (6/(6+12)) which is outside of the target duty cycle range. However, if the on-time is 7 minutes and the off time is 8 minutes, the duty cycle is 46.7% (7/(7+8)) and the CPH is 4 (60 minutes/hour/(15 minutes/cycle)). Therefore, an adjustment could be made to increase the differential temperature to slow the cycle rate. This could be in very small increments to avoid overshoot and keep the maximum CPH in the desired range. If the differential temperature were increased to +/−0.6° C., the maximum CPH may increase to 3.8 CPH. An additional adjustment of the differential temperature to +/−0.7° C. may decrease the maximum CPH to 3.6 CPH. This process can continue until the maximum CPH is in the desired range of 3 +/−0.5 CPH in the cooling mode.

While various concepts have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those concepts could be developed in light of the overall teachings of the disclosure. For example, several variations may be used for the algorithm such as: changing the duty cycle range for making the adjustment; using an averaging or exponential smoothing algorithm over multiple readings for making the adjustment to the differential temperature; changing the maximum CPH tolerance; increasing the adjustment to the differential temperature if the maximum CPH is further from the desired maximum CPH, using a different adjustment for the cooling stage vs. the heating stage; using a different type of heating stage such as a compressor vs. a furnace; and using a different maximum CPH or adjustment for each compressor or furnace stage of multiple stage systems. Further, while described in the context of functional modules and illustrated using block diagram format, it is to be understood that, unless otherwise stated to the contrary, one or more of the described functions and/or features may be integrated in a single physical device and/or a software module, or one or more functions and/or features may be implemented in separate physical devices or software modules. It will also be appreciated that a detailed discussion of the actual implementation of each module is not necessary for an enabling understanding of the invention. Rather, the actual implementation of such modules would be well within the routine skill of an engineer, given the disclosure herein of the attributes, functionality, and inter-relationship of the various functional modules in the system. Therefore, a person skilled in the art, applying ordinary skill, will be able to practice the invention set forth in the claims without undue experimentation. It will be additionally appreciated that the particular concepts disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any equivalents thereof.