Energy control system

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.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates an exemplary embodiment of an HVAC system for a typical residential structure.

FIG. 2 illustrates schematically an exemplary embodiment of a vapor compression system.

FIG. 3 illustrates schematically another exemplary embodiment of a vapor compression system.

FIG. 4 is a graph illustrating design heat loss of an exemplary structure as a function of outdoor temperature.

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