US 20090171862 A1
A method for adjusting an economic balance point in a dual fuel HVAC system includes a furnace, a heat pump and a control system. The method comprises providing a control device having a preprogrammed algorithm for controlling the HVAC system, providing a communication path between the control device and at least one component of the HVAC system, and accessing current fuel cost data; determining an economic balance point temperature based on the current fuel cost data; and updating the economic balance point temperature in the preprogrammed algorithm to adjust an actual balance point of the HVAC system.
1. A method for adjusting a balance point temperature in a dual fuel HVAC system having a furnace, a heat pump and a control system, by monitoring operating costs for the furnace and the heat pump, the method comprising:
accessing fuel cost data;
determining a balance point temperature based on the fuel cost data; and
updating the balance point temperature in a preprogrammed algorithm to adjust a balance setpoint of the HVAC system, the balance setpoint being the exterior temperature below which the HVAC system switches from the heat pump to the furnace as a heat source.
2. The method of
sensing an exterior temperature; and
controlling an interior temperature by operating the furnace when the exterior temperature is less than the balance setpoint, and by operating the heat pump when the exterior temperature is greater than or equal to the balance setpoint.
3. The method of
determining at least one predetermined parameter of the heat pump at a plurality of exterior temperatures.
4. The method of
continually determining the at least one predetermined parameter and incorporating the at least one predetermined parameter into an algorithm to calculate an application balance point temperature, below which the heat pump heating capacity is less than the rate of heat loss of structure, in response to a change in the at least one predetermined parameter.
5. The method of
6. The method of
7. The method of
8. The method of
9. The method of
10. The method of
11. The method of
12. An HVAC system comprising:
a furnace, a heat pump, a controller, and a communication path between the controller and at least one component of the HVAC system;
wherein the controller is configured to access a remote database including a current fuel cost data and retrieve the current fuel cost data, determine a balance point temperature based on the current fuel cost data, and update a balance point temperature in a preprogrammed algorithm to adjust a balance setpoint of the HVAC system.
13. The HVAC system of
14. The HVAC system of
15. A method for controlling energy use in a structure, the structure including a dual fuel HVAC system having a furnace, a heat pump and a control system, comprising:
selecting an indoor temperature setpoint, a heat loss for the structure, and a heating capacity of the system
determining an application balance point exterior temperature below which the heat pump heating capacity is less than the rate of heat loss of the structure, based on one or more of the selected indoor temperature, the heat loss for the structure, and the heating capacity of the system;
determining an operating cost of the furnace;
determining an operating cost for the heat pump;
determining a balance point temperature based on a point of intersection of the heat pump operating costs and the furnace operating cost;
comparing the application balance point temperature to a balance point temperature and generating a balance setpoint, the balance setpoint being selected as the greater value of the application balance point temperature and the balance point temperature; and
monitoring system parameters periodically to determine whether a change in the balance setpoint has occurred as a result of changes in system parameters.
16. The method of
determining the electric power consumption of the heat pump,
determining the cost of electricity,
calculating heat pump energy cost for the heat pump;
17. The method of
controlling the system to operate the heat pump based on the exterior temperature equaling or exceeding the balance setpoint, and to operate the furnace based on the exterior temperature being less than the balance setpoint.
18. The method of
determining that a current balance setpoint value differs from a preceding balance setpoint and replacing the balance setpoint with the current balance setpoint.
19. The method of
determining the heat loss based on a design heat loss of a structure at a predetermined outdoor temperature
20. The method of
21. The method of
22. The method of
This application claims priority from and the benefit of U.S. Provisional Application No. 61/017,383 entitled METHOD AND APPARATUS FOR DYNAMIC BALANCE POINT SELECTION, filed Dec. 28, 2007, which is hereby incorporated by reference.
The application generally relates to control systems for heating ventilation and air conditioning (HVAC) systems. The application relates more specifically to a method and apparatus to dynamically determine a balance point of an HVAC system having a heat pump and a fossil fuel furnace.
The balance point of an HVAC system determines whether the heat pump or fossil fuel furnace is to be used for heating. Static balance point settings can be primarily based on exterior temperature readings, and on the existing utility rates at the time of the HVAC system installation.
Heat pumps may be installed with indoor air handlers having electric resistance heating elements as the auxiliary or supplemental heating source. However, the rising cost of electricity is causing more HVAC systems to be installed with a heat pump as the primary heating source and a fossil fuel furnace as the auxiliary heating source.
In existing air handler/electric heater installations, the evaporator coil is located in the airflow path before the electric resistance heating elements. However, in a fossil fuel furnace installation, the evaporator coil is located in the airflow path after the furnace heating section. Therefore, the furnace is not permitted to produce heat while the heat pump is also providing heat, because the heat produced by the furnace heating section would be transferred into the refrigerant through the indoor coil, causing the refrigerant pressure to increase. Adding more heat to the refrigerant may cause the refrigerant pressure to exceed a high-pressure limit of the system.
Current methods employ a balance point setting that is static. By static, what is meant is that once the balance point is calculated by the installer and applied to the system, the balance point is not updated during the life of the HVAC system, or the balance point is updated infrequently such as during service or maintenance calls. However, the cost of fossil fuel and electricity changes continuously. Therefore, the balance point setting does not necessarily reflect an optimized balance between fuel sources. Previous heat pump/fossil fuel HVAC systems include some means for setting the balance point for the system. This setting can be made with an accessory kit or fossil fuel kit, which includes an exterior thermostat. The control system electromechanical devices or electronic control board of the heat pump may also include means, such as a shunt jumper or DIP switch, for setting the balance point. Indoor room thermostats are now available with a balance point setting. In all of these methods for setting the balance point, a control device determines the exterior ambient temperature, compares the exterior ambient temperature with the balance point setting, and determines whether to operate the heat pump or the furnace. Other methods use room thermostats to control the switching between heat pump and furnace without monitoring exterior temperature.
One embodiment relates to a method for adjusting a balance point temperature in a dual fuel HVAC system having a furnace, a heat pump and a control system, by monitoring operating costs for the furnace and the heat pump. The method includes accessing fuel cost data; determining a balance point temperature based on the fuel cost data; and updating the balance point temperature in a preprogrammed algorithm to adjust a balance setpoint of the HVAC system, the balance setpoint being the exterior temperature below which the HVAC system switches from the heat pump to the furnace as a heat source.
Another embodiment relates to an HVAC system comprising a furnace, a heat pump, a controller, and a communication path between the controller and at least one of the furnace and the heat pump. The controller is configured to access a remote database including a current fuel cost data and retrieve the current fuel cost data, determine a balance point temperature based on the current fuel cost data, and update a balance point temperature in a preprogrammed algorithm to adjust a balance setpoint of the HVAC system.
Another embodiment relates to a method for controlling energy use in a structure, the structure including a dual fuel HVAC system having a furnace, a heat pump and a control system. The method includes selecting an indoor temperature setpoint, a heat loss for the structure, and a heating capacity of the system; determining an application balance point exterior temperature below which the heat pump heating capacity is less than the rate of heat loss of structure, based on one or more of the selected indoor temperature, the heat loss for the structure, and the heating capacity of the system; determining an operating cost of the furnace; determining an operating cost for the heat pump; determining a balance point temperature based on a point of intersection of the heat pump operating costs and the furnace operating cost; comparing the application balance point temperature to a balance point temperature and generating a balance setpoint, the balance setpoint being selected as the greater value of the application balance point temperature and the balance point temperature; and monitoring system parameters periodically to determine whether a change in the balance setpoint has occurred as a result of changes in system parameters.
Embodiments disclosed herein provide an ability to determine a balance point that is based on current utility rates and to determine an optimal balance point based on a user preferred input and multiple sensor data, and to control a source of heat from one of a fossil fuel furnace and a heat pump.
Referring next to
Compressor 62 compresses a refrigerant vapor and delivers the refrigerant vapor to condenser 64 through a discharge line 60, and the reversing valve 150 if operated as a heat pump. Compressor 62 can be a rotary compressor, screw compressor, reciprocating compressor, centrifugal compressor, swing link compressor, scroll compressor, turbine compressor, or any other suitable type of compressor. Refrigerant vapor is delivered by compressor 62 to condenser 64 and enters into a heat exchange relationship with a fluid, for example, air or water, and undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid. Condensed liquid refrigerant from condenser 64 flows through an expansion device 65 to evaporator 66.
Condensed liquid refrigerant delivered to evaporator 66 enters into a heat exchange relationship with a fluid, for example, air or water, and undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the fluid. Vapor refrigerant exits evaporator 66 and returns to compressor 62 by a suction line to complete the cycle, and reversing valve 150 if operated as a heat pump.
Compressor 62 of system 20, whether operated as a heat pump or as an air conditioner, is driven by a motor 70. Motor 70 can be powered by a variable speed drive (VSD) or can be powered directly from an AC or DC power source. A VSD, if used, receives AC power having a particular fixed line voltage and fixed line frequency from AC power source and provides power to the motor 70. The motor 70 used in system 20 can be any suitable type of motor that can be powered by a VSD or directly from an AC or DC power source, for example, a switched reluctance (SR) motor, an induction motor, an electronically commutated permanent magnet motor (ECM).
For an HVAC system incorporating a heat pump and a fossil fuel furnace a determination is first made regarding the conditions that can cause the heat pump to operate, and the conditions that can cause the furnace to operate. The determination is made through a predetermined balance point setting. The balance point is defined by an exterior air temperature below which only the fossil fuel furnace can operate, and above which only the heat pump can operate. To determine the applicable balance point, a heat loss calculation is made of the residence or structure 44.
The application balance point as defined herein refers to the exterior temperature below which the heat pump heating capacity is less than the rate of heat loss of structure 44. When the exterior temperature is below the application balance point, structure 44 loses heat faster than system 10 can generate heat in heat pump mode, and the interior air temperature of structure 44 will decrease even when system 10 is operating at its full capacity. Traditionally, the heat loss of structure 44 is assumed to be linear and inversely proportional to the exterior temperature. A graphic representation of the heat loss (see
System 20 heating capacity and energy consumption at various exterior temperatures may be determined during testing and development of a particular heat pump model or system. System 20 heating capacity decreases as the ambient temperature decreases. The system heating capacity curve is presumed to be linear and is determined based on the heating capacity of system 20 at two standard testing temperatures, 17° F. and 47° F. (see
At exterior temperatures above the application balance point, system 20 can produce more heat than is needed to maintain the interior temperature of structure 44. At exterior temperatures below the application balance point, system 20 cannot produce sufficient heat to maintain the temperature of structure 44 at a desired temperature setpoint, and the temperature within structure 44 will fall or be maintained below the desired setpoint, even though system 20 is operating at or near full capacity.
Referring next to
An economic balance point 201 (
At step 142, system 10 determines the electric power consumption of the heat pump, for example, by a map or profile of the heat pump. At step 144, system 10 determines the cost of electricity, and at step 146 calculates the heat output per unit of electricity, for example, MBTUH per KW. Next, at step 148, the system calculates heat pump energy cost, or operating cost, for the particular heat pump used, at multiple exterior temperatures.
Next, at step 140 system 10 determines economic balance point 201 as the intersection of heat pump operating costs plotted in step 148 and furnace operating cost plotted in step 138.
At step 130, system 10 compares application balance point 100, determined in step 128, with economic balance point 201, determined in step 140, and generates a balance setpoint, or actual balance point 100, which is the higher value of application balance point 100 and application balance point 100, at step 152
Economic balance point 201 (see
A fossil-fuel furnace burns gas to produce heat. This gas (natural gas or liquid propane) must be purchased by structure owner at some cost. To determine economic balance point 201, the amount of energy that the furnace can deliver (MBTUH) per dollar spent for the gas must be calculated. Furnace efficiencies may be based only on their efficiencies in combustion. Electrical energy consumption of the furnace to power the blower to distribute the air is not taken into account. However, it will be appreciated by those skilled in the art that electrical energy consumption of the furnace may be incorporated in the algorithms described in
Curve 202 represents the energy produced per dollar for a 95% efficient furnace given a gas cost of $1.086281/Therm. One Therm is equal to 100 MBTU.
The actual balance point setting is typically the higher of the application balance point and the economic balance point. This embodiment allows either the heat pump or the furnace to independently maintain the desired temperature of structure 44. However, in another embodiment, the economic balance point may be lower than the application balance point and the actual balance point may be selected to be the economic balance point. In the latter embodiment, that is, where the application balance point is selected to be the economic balance point, when the exterior temperature is above the economic balance point but below the application balance point, the heat pump will try to heat structure 44 but will be unable to offset the heat loss of structure 44. A two-stage room thermostat will then call for auxiliary heating to be energized. Through control logic built into the software or hardware of system 20, which software or hardware may include a room thermostat, a heat pump controller, a fossil fuel kit, etc. the heat pump is de-energized and the furnace energized. The furnace then operates exclusively, until the demand for auxiliary heat from the room thermostat is satisfied. Once the room thermostat is satisfied, the control system will return to heating the room using the heat pump. This cycle will control the temperature at the desired setpoint as long as the exterior temperature remains between the application balance point and the economic balance point, that is, the scenario in which the actual balance point is lower than the application balance point. Structure 44 will remain comfortable, but the HVAC heating system will not cycle off.
The factors that may be taken into account in selecting the application balance point include heat loss of the structure, exterior design temperature, interior comfort condition, and heating capacity of system 20 when operated in heat pump mode. The heat loss of the structure varies with exterior temperature and physical characteristics of the structure, and heating capacity of system 20 varies with exterior temperature and equipment operational mode. Factors that may be taken into account in selecting the economic balance point include fossil fuel cost, electrical energy cost, and energy consumption of system 20 when operated in heat pump mode. The electrical energy consumption of system 20 in heat pump mode varies with exterior temperature and equipment operational mode.
In another example, should the heating efficiency of system 20 heat pump decrease or the cost of electricity increase, curve 200 would move down the temperature scale, since system 20 would require more energy and cost in heat pump mode in order to generate approximately the same quantity of heat. If the furnace conditions remained the same, the economic balance point 402 would increase. Economic balance point 403, located at 30° F., occurs at a higher temperature than application balance point 100, which occurs at 22° F. Therefore, new actual balance point 403 would occur at 30° F.
If the heat pump heating capacity changes through the use of a multi-stage or fully variable capacity system, the application balance point also changes. In a typical two-stage heat pump system the heating capacity of system 20 can be reduced to 67% or 50% when operating in single stage compressor mode compared to full capacity compressor mode. A heat pump with a fully variable speed compressor can vary the heating capacity with a high degree of resolution between the minimum and maximum heating capacity. Although not represented in a figure, the electrical power consumption of the heat pump varies along with the heating capacity and the economic balance point calculation is also affected.
Similarly, heat loss curve 101 may also change. For example, if the user selects an interior comfort setpoint change that is lower than 70° F., for example, 65° F., the structure heat loss curve will change. Or, if the construction or physical characteristics of the structure change, heat loss curve 101 may change. If heat loss curve 101 changes, application balance point 100 will change accordingly.
An exemplary embodiment relates to a method of specifying a balance point by continuously monitoring the operating costs and determining the current economic balance point for the system based on the monitored operating costs. The method also includes continuously monitoring capacity and performance to determine the current application balance point. The method further includes continually comparing the economic and application balance points to determine the optimum actual balance point and then apply the optimum actual balance point to system 20 operation.
HVAC system 10 includes a control device with a preprogrammed algorithm (See, for example,
The present application contemplates methods, systems and program products on any machine-readable media for accomplishing its operations. The embodiments of the present application may be implemented using an existing computer processors, or by a special purpose computer processor for an appropriate system, incorporated for this or another purpose or by a hardwired system.
As noted above, embodiments within the scope of the present application include program products comprising machine-readable media for carrying or having machine-executable instructions or data structures stored thereon. Such machine-readable media can be any available media which can be accessed by a general purpose or special purpose computer or other machine with a processor. By way of example, such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code in the form of machine-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer or other machine with a processor. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or a combination of hardwired or wireless) to a machine, the machine properly views the connection as a machine-readable medium. Thus, any such connection is properly termed a machine-readable medium. Combinations of the above are also included within the scope of machine-readable media. Machine-executable instructions comprise, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing machines to perform a certain function or group of functions.
In many structures 44, Internet access is available through a local personal computer (PC). A wired or wireless network (not shown) may also be available. HVAC system 10 network described above may be incorporated as a part of structure 44 network. Preferably, however, HVAC system 10 will be part of a separate network that may be connected to a second network in structure 44 through a gateway or network coupling device (not shown).
System 10 may apply methods which are well known, to transition to the new actual balance point setting. Such transition methods include waiting until system 10 is not operating, that is, there is no thermostat call for heating or cooling, to make the change; forcing system 10 to “Off” mode to make the change and re-starting system 10; and applying predetermined rules specified by the manufacturer, installer, or structure owner.
System 20 heating capacity in heat pump mode, and energy consumption of system 20 at various exterior temperatures, are determined during testing and development of a specific heat pump. In one embodiment one or more heat pump algorithms for continually determining actual heat pump performance and efficiency, which are known in the art, may be applied or incorporated into the algorithms set forth in steps 84 and 86, to dynamically calculate the application balance point.
In another embodiment, performance data for the heat pump may be stored in the memory of the heat pump controller during production of the heat pump. The stored performance data is used by the heat pump controller in steps 84 and 86 if the dynamic balance point algorithm is implemented in the heat pump control. Alternately, if the algorithm is implemented by another controller device on the network, the heat pump controller is configured to transmit or transfer the performance data to that other control device when the performance data is installed on the HVAC network during installation of the HVAC system.
The respective electrical and gas or fuel oil utility companies determine electricity rates and fossil fuel rates. Current energy cost information may be transferred in a variety of ways to a control device that implements the algorithm. In one embodiment, an HVAC network is in communication with or is a node on structure 44 network having access to the Internet. Therefore, the control device can access the rate information provided by the applicable utility company via the utility company websites. Utility companies currently provide a variety of communication channels to communicate with HVAC devices, for example, thermostats and temperature controls, for purposes such as load shedding and load monitoring. These communication channels can also be used for acquiring the utility rate information. Methods will vary with each utility provider and geographical area, as will be readily appreciated by persons skilled in the art.
For installations in which real time utility rate information is unavailable—for example, because there is no Internet connection associated with the network to which the control device is connected, or the utility companies do not make the rate information available, another embodiment is disclosed. In this embodiment, a user may enter utility rate information manually. Users may obtain rate information through various means, for example, a monthly utility bill, an Internet source, a utility company hotline, etc. User may enter the rate information at predetermined intervals, for example, monthly, or on an ad hoc basis as needed. For example, the user may update the information only when rates change or monthly when they receive their utility bill. The interface with the HVAC network could be any of a variety of methods including through PC software, room thermostat menus, cell phone/PDA software, etc. Network interfaces may include features that prompt the user to input the information at a predetermined or specified interval. Manually entering the utility rate information is less precise compared with automatically updating the rate information from the Internet, the manual entry of rate information provides more accurate information than traditional HVAC installations. Additionally, some users may prefer manual entry of rate information so that they have control over how the system is functioning.
Heat loss curve 101 for the structure 44 is entered into the control device when the system is first installed, and updated as needed, for example, when significant changes are made to structure 44 such as major remodeling, or more efficient windows or doors are installed. Heat loss information may be entered into the HVAC system controller in a variety of ways, for example, thermostat menus, service tools, control panel interface, cell phone, and PC software. Since the standard method assumes a straight line representing the heat loss of the structure (see, for example,
System 10 provides a dynamic interior comfort temperature. Instead of using a heat loss curve calculated only at the time of installation using one interior comfort temperature, system 10 can recalculate heat loss curve 101 as the interior comfort temperature changes. System 10 applies the available information based on the input of the user by continuously recalculating linear heat loss curve 101 as a function of the current interior temperature setpoint.
Initially the user enters one interior comfort temperature to establish an initial linear heat loss curve. For instance, the interior comfort temperature might initially be set at 70° F. Generally, however, the interior comfort temperature is not constant in actual operation. The interior comfort temperature is the temperature setpoint of the room thermostat. Therefore, the interior comfort temperature changes based on, for example, a programmable thermostat that changes automatically with the time of day, or if the structure 44 is occupied or unoccupied, or that is manually overridden based on the preference of the user. In one embodiment, the invention provides the ability to automatically recalculate heat loss curve 101 based on changes in the interior comfort temperature. The system also recalculates actual balance point 100, since actual balance point selection is based on heat loss curve 101.
Other, more elaborate methods may be incorporated into the system algorithm to dynamically determine the actual heat loss of the structure by applying data acquired through additional sensors or information derived from the analysis of room temperature, time, or similar parameters.
Additionally, because heat loss curve of structure 44 is, in reality, non-linear, in another embodiment a non-linear curve may be calculated which more accurately represents the actual heat loss of structure 44.
The greater value of the application balance point or economic balance point is typically selected as the controlling parameter of the system. However, in another exemplary embodiment, a balance point may be selected that is below the application balance point or above both the economic and application balance points. When an actual balance point is selected that is below the application balance point, the heat pump and furnace operate alternately while the exterior temperature remains above the selected balance point and below the application balance point. One advantage of selecting a balance point that is below the application balance point is that the long run times maintain the interior temperature more constant, as opposed to the situation where a the application balance point is below the balance point, which causes the interior temperature to drop when the system cuts off, and to spike when the system is turned on again. In some installations users may prefer to operate the furnace at temperatures above the application or economic balance point. Their decision may be based on perceived comfort as described below or simply a matter of personal preference.
The control device may optionally include selection elements to allow the user to specify which method of operation is desired, or to set a specific static setpoint and override the control algorithm.
Additionally, the control device may provide the ability for the user to specify conditional operational parameters. For instance, some individuals do not like feel of cooler air temperatures at the duct registers in a heat pump system. Discharge air temperature of a fossil fuel furnace is typically higher than that of a heat pump. Both types of heat are capable of heating the structure, but the cooler discharge air may be perceived by some users as uncomfortably cold. However, when heat pump heating is cheaper than fossil fuel heating, many users select financial savings over comfort. Given these alternate user preferences, system 20 allows the user to select or program the operating parameters in financial terms. For instance, the user might select heat pump operation below 20° F. only when it will result in a predetermined level of cost saving.
System 10 may optionally permit the user to specify a minimum duct register air temperature, by incorporating in the control system an additional sensor or algorithm that senses or calculates the air temperature of the duct register. The user interface would permit the user to display the economic impact so they could decide the level of comfort versus cost savings they are willing to trade. The user interface and controller are configurable to display the economic impact data based on time intervals of monthly, bi-monthly, seasonally, or annually.
The heat loss of structure 44 and the heat pump heating capacity also change over time. Such changes are gradual when compared with the fluctuations in fuel costs and the other time variable factors. In an alternate embodiment, structural heat loss and the heating capacity of the heat pump are factors that are monitored by the control system through appropriate sensing devices, and are incorporated into the control system/control algorithm to further adjust the application balance point.
While only certain features and embodiments of the invention have been shown and described, many modifications and changes may occur to those skilled in the art (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (for example, temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described (that is, those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the claimed invention). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.