Abstract:
A method for controlling heater ventilation fan operation increases fan speed from low to high after a short delay after turn-on, and continues fan operation for a period of time based on duration of operation, after turn-off. The higher fan speed improves heat transfer and efficiency while the heating system is operating. Continuing fan operation after turn-off maximizes recovery of additional heat from the heat exchanger. Known methods do not provide sufficient air flow to efficiently transfer heat from the heat exchanger to the air, and leave high temperature air (i.e., 110 to 200° F.) in the heat exchanger after turn-off.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to electric or gas furnaces and electric heat pump (heating) systems in heating mode. 
     Heating consumption accounts for 41% of total residential energy use in the United States as reported by the US Energy Information Agency Residential Energy Consumption Survey in 2005. Known central heaters are controlled by a thermostat which turns on a heater ventilation fan after a brief delay following turning on a heat source, and turns off the heater ventilation fan after variable temperature based delay or a fixed time delay following turning off the heat source. Unfortunately, maintaining a low heater ventilation fan speed often results in increased heat soak within the central heating unit and the portion of the heat generated by the heat source is lost to the environment increases the longer the central heating element is on at the low heater ventilation fan speed. Further, the amount of heat soak increases as the central heating unit is operated for longer periods of time leaving significantly higher temperature air (i.e., 110 to 200° F.) in the heat exchanger after the heater ventilation fan is turned off and a portion of this heat is also lost to the environment after the heat source and the heater ventilation fan are tuned off. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention addresses the above and other needs by providing a method for efficient heater ventilation fan control. The method includes switching the heater ventilation fan from low speed to high speed after a brief period P 1  following starting, and continuing heater ventilation fan operation for a variable period of time P 2  after the heat source has stopped. The period P 1  is preferably about four minutes, and the period P 2  is determined by the duration of heating and is generally between two and four minutes. Operating the heater ventilation fan at high speed improves heat transfer and efficiency while the heating system is operating, increases warm air movement to the space, satisfies the thermostat set point temperature in less time, reduces heating system operation, and reduces energy use compared to conventional fan controllers. Continuing heater ventilation fan operation after turn-off maximizes recovery of additional heat from the heat exchanger to increase heat delivered to conditioned space, improve overall efficiency, extend the off cycle time, and save energy. 
     In accordance with one aspect of the invention, there are provided methods for optimizing furnace and heat pump heater ventilation fan operation to improve energy efficiency and save energy by increasing fan speed from low to high speed after the heating system is operated for sufficient time to provide useful heating (i.e., supply air temperatures above 100 to 110° F.). High speed fan operation improves heat transfer and efficiency in the heat exchanger and reaches a thermostat setting sooner to reduce furnace operation or heat pump compressor operation. The efficient fan controller continues fan operation after the heating system has stopped operating to recover additional heat from the heat exchanger to increase heat delivered to the conditioned space, improve overall efficiency, extend the off cycle time, and save energy. 
     In accordance with another aspect of the invention, there are provided methods for optimizing furnace and heat pump heater ventilation fan operation. The length of time of heater operation is saved, and the continued operation of the heater ventilation fan after turn-off is increased for longer periods of operation of the heater based on the saved time of heater operation. 
     In accordance with yet another aspect of the invention, there is provided improved heating efficiency of seven to ten percent above conventional temperature delay and six to eight percent above conventional time delay. For systems with degraded conventional temperature delay sensors, the invention provides improved heating efficiency of seven to 23 percent. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
         FIG. 1  shows a graph of known control and efficient fan control according to the present invention. 
         FIG. 2  is a table of extended fan time delay off P 2  after heating system operation shut-off as a function of the duration of heater system operation P 3 , according to the present invention. 
         FIG. 3  shows the efficient fan control connections to a known heater circuit according to the present invention. 
         FIG. 4  shows a circuit for executing the efficient fan control, according to the present invention. 
         FIG. 5  shows a chart of time periods P 1 , P 3 , and P 2 . 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing one or more preferred embodiments of the invention. The scope of the invention should be determined with reference to the claims. 
     A graph comparing known control  10  and efficient fan control  12  according to the present invention is shown in  FIG. 1 . The efficient fan control  12  optimizes heating system efficiency and reduces electric or gas furnace or heat pump compressor operation by increasing fan speed from low to high four minutes after the heating system is turned on. The efficient fan control  12  further maximizes heat recovery from the heat exchanger after the heating system is turned off with an extended fan delay of two to four minutes, depending on how long the heating system was on during the heating cycle. Conventional time or temperature fan off delay control  10  leave significantly higher temperature air (i.e., 110 to 200° F.) in the heat exchanger which wastes energy. 
     A table  14  of extended fan operation after shut-off period, as a function of the duration of heater system operation, according to the present invention, is shown in  FIG. 2 . For less than four minutes of operation, no significant amount of heat has been stored in the heat exchanger. Between four and eight minutes of operation, varying amounts of heat have been stored in the heat exchanger. For greater than eight minutes of operation, a steady state amount of heat has been stored in the heat exchanger. The fan off delay is matched to the amount of heat stored in the heat exchanger to extract that heat. 
     The connection of an efficient fan controller (i.e., fan delay timer)  211  to a heater circuit including a known thermostat  201  is shown in  FIG. 3 . The efficient fan controller  211  may be connected as shown to a heat source control circuitry (e.g., a furnace control)  202 , and optionally, an air conditioning compressor control  203 . Prior to the installation of the present invention, the fan contact terminal  204  connects the thermostat  201  through wire  28  to the fan/blower relay  205 . With the addition of the present invention, the connection between the thermostat  201  and fan relay  205  is opened as shown by broken line  217 . The wire  28  is reconnected from the thermostat terminal  204  to terminal  214  of the controller  211  and terminal  212  of the controller  211  is connected to the fan relay  205 . When actuated, the fan relay  205  connects the system fan/blower  206  to the 24 volt AC transformer  210 . The air conditioning contact terminal  207  of the thermostat  201  is connected to the air conditioning compressor control circuitry  203 . The heater contact terminal  208  of the thermostat  201  is connected to the heat source control circuitry  202 . The hot terminal  209  of the thermostat  201  connects to the hot side of the 24 volt AC transformer  210 . 
     A connection  216  on the controller  211  is shown connected to the terminal  208  of the thermostat  201 . This wiring path is used in a system where the heat fan time is extended. In the preferred embodiment, no additional power connections are required, unlike some prior art, which requires either a dedicated connection to the transformer or another relay. The controller  211  draws power through the fan/blower relay  205 , thus eliminating the need for external power generally required for similar controllers. 
     An example of a controller  211  circuit according to the present invention is shown in  FIG. 4  in block diagram format. A microprocessor  304  is used to control a switch  301 , receive an input, and provide an output to a user interface  305 . The microprocessor  304  receives power from an AC/DC converter  303  and also receives input from a zero crossing detector  302 , the line from the thermostat fan switch  213 , and optionally the input that enables the NC compressor  215 . The microprocessor  304  performs several major functions. In terms of timing, the microprocessor  304  keeps track of seconds and minutes by monitoring the AC line signal. Each positive zero crossing accounts for 1/60th of a second; therefore, sixty positive crossings occur each second. The seconds are then accumulated to keep track of minutes. The negative crossings are also monitored to provide timing for the switch. In the event the switch is a Triode Alternating Current switch (TRIAC), it must be triggered at each positive and negative zero crossing of the AC line. A TRIAC is a gated switching device that will conduct current in either direction. 
     The user interface  305  is an input device to the microprocessor  304  and provides visual outputs to a user which enable programming of the controller  211 . The microprocessor  304  continuously monitors the user interface  305  to determine if there is any change to the current system operation. If a change is requested by the user, the current programming of a switch state is set to neutral (switch is turned off) and the user interface  305  is monitored to determine the user&#39;s requested action. In the preferred embodiment, the microprocessor  304  contains an EEPROM, which allows the microprocessor  304  to store the user&#39;s programming instructions when there is no power applied to the controller  211 . 
     The AC/DC converter  303  is used to condition the input 24 VAC signal from the 24 volt AC transformer  210  into the DC signal necessary to operate the DC devices within the controller  211 . The zero crossing detector  302  is used to condition the 24 volt AC input to a level that will not damage the microprocessor  304 . The microprocessor  304  generates an interrupt in both the positive going and negative going zero crossings and uses this zero crossing timing to keep track of elapsed time and also to determine when to fire the TRIAC, which may be used as the switching device  301 . The switching device  301  could be either a standard relay type device, a reed relay or some other electro-mechanical device. The switching device  301  could also be a solid state device such as a Field Effect Transistor (FET) (a semiconductor device that outputs current in proportion to its input voltage) switch or a TRIAC. The FET uses a small amount of control current to regulate a larger output current. Switching devices generally require minor modifications for use in the controller  211 , and a controller  211  including any suitable switching device is intended to come within the scope of the present invention. While the presently described embodiment of the controller  211  is based on a TRIAC switch, the present invention is not limited to any specific type of switching device. 
     Figure Reference Numbers include:
         Number  201  is the existing household thermostat;   Number  202  is the heating system (i.e., furnace or heat pump) control circuitry;   Number  203  is the air conditioning compressor controller;   Number  204  is the fan contact terminal;   Number  205  is the fan/blower relay;   Number  206  is the ventilation fan/blower;   Number  207  is the air conditioning contact terminal;   Number  208  is the heater contact terminal;   Number  209  is the hot contact terminal;   Number  210  is the system 24 volt alternating current (VAC) transformer;   Number  211  is the external thermostat fan controller;   Number  212  is the fan relay lead;   Number  213  is the transformer hot lead;   Number  214  is the thermostat fan activation switch lead;   Number  215  is the optional lead to thermostat air conditioning compressor terminal;   Number  216  is the optional lead to thermostat heat terminal;   Number  217  is the break in wiring thermostat to fan relay;   Number  301  is the zero crossing detector;   Number  302  is the AC/DC converter;   Number  304  is the microprocessor;   Number  305  is the user interface;   Number  306  is the optional battery;   Number  307 A is the on/off switch in “on” position; and   Number  307 B is the on/off switch in “off” position.       

       FIG. 5  shows a chart of time periods P 1 , P 3 , and P 2 . 
     While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.