Limited modulation furnace and method for controlling the same

Abstract

A limited modulation furnace and a method for controlling the limited modulation furnace using a floating control algorithm are disclosed. The method includes providing a furnace comprising a modulating furnace stage and at least one fixed heat output furnace stage, each fixed heat output furnace stage having an operational status of either on or off, passing a heating fluid through the furnace, measuring a temperature of the heating fluid exiting the furnace, determining an error based on a difference between the measured temperature and a set-point temperature, the set-point temperature being associated with a thermostat setting, determining whether the error is within a deadband range for the set-point temperature, and modifying at least one of the heat output of the modulating furnace stage or the operational status of at least one of the fixed heat output furnace stages in response to the error being outside the deadband range.

1 . A method for controlling a limited modulation furnace, the method comprising the steps of: 
 providing a furnace comprising a modulating furnace stage and at least one fixed heat output furnace stage, each fixed heat output furnace stage having an operational status of either on or off;    passing a heating fluid through the furnace;    measuring a temperature of the heating fluid exiting the furnace;    determining an error based on a difference between the measured temperature and a set-point temperature, the set-point temperature being associated with a thermostat setting;    determining whether the error is within a deadband range for the set-point temperature; and    modifying at least one of the heat output of the modulating furnace stage or the operational status of at least one of the fixed heat output furnace stages in response to the error being outside the deadband range.    
   
   
       2 . The method of  claim 1 , wherein modifying the heat output of the modulating furnace stage occurs at a rate determined by a floating control algorithm.  
   
   
       3 . The method of  claim 2 , wherein the rate at which the heat output is modified occurs in a linear fashion based on a magnitude of the error.  
   
   
       4 . The method of  claim 2 , wherein the rate at which the heat output is modified occurs in a non-linear fashion based on a magnitude of the error.  
   
   
       5 . The method of  claim 1 , wherein the step of providing a furnace further comprises providing at least two fixed heat output furnace stages having equal maximum output capacities.  
   
   
       6 . The method of  claim 5 , wherein the step of providing a furnace further comprises providing a modulating furnace stage having a maximum output capacity equal to a maximum heat output capacity of a fixed heat output furnace stage.  
   
   
       7 . The method of  claim 2 , wherein the floating control algorithm includes a subroutine including a rate compensation algorithm.  
   
   
       8 . The method of  claim 7 , wherein the rate compensation algorithm for modifying the heat output is used to both increase heat output and decrease heat output.  
   
   
       9 . The method of  claim 1  wherein the modifying of the heat output of the modulating furnace stage is performed by a modulating valve controlled by a processing unit in electronic communication with a temperature sensing unit in contact with the heating fluid.  
   
   
       10 . A method for controlling a limited modulation furnace, the method comprising the steps of: 
 providing a furnace comprising a modulating furnace stage and at least two fixed heat output furnace stages, each fixed heat output furnace stage having an operational status of either on or off;    passing a heating fluid through the furnace;    measuring a temperature of the heating fluid exiting the furnace;    determining an error based on a difference between the measured temperature and a set-point temperature, the set-point temperature being associated with a thermostat setting;    determining whether the error is within a deadband range for the set-point temperature; and    modifying at least one of the heat output of the modulating furnace stage or the operational status of at least one of the fixed heat output furnace stages in response to the error being outside the deadband range, the modifying of the heat output of the modulating furnace stage at a rate determined by a floating control algorithm and wherein the floating control algorithm determines the rate at least in part on a magnitude of the determined error using a rate compensation algorithm.    
   
   
       11 . A limited modulation furnace comprising: 
 a fluid path having an inlet and an outlet for a heating fluid to pass therethrough;    a modulating furnace stage having a variable output capacity when in operation, the modulating furnace stage positioned to heat the heating fluid in the fluid path;    at least one fixed heat output furnace stage, each fixed heat output furnace stage having a single output capacity when in operation, each fixed heat output furnace stage positioned to heat the heating fluid in the fluid path;    a temperature sensor positioned to detect a temperature of the heating fluid in the fluid path;    a controller in electronic communication with the temperature sensor; and    a modulating valve intermediate the modulating furnace stage and a fuel source, wherein the modulating valve is in electronic communication with the controller and wherein, in response to a rate determined by the controller using a floating control algorithm, the modulating valve is configured to modify the volume of fuel passing to the modulating furnace stage thereby modifying the output capacity of the modulating furnace stage.    
   
   
       12 . The limited modulation furnace of  claim 11  comprising at least two fixed heat output furnace stages.  
   
   
       13 . The limited modulation furnace of  claim 12 , wherein the single output capacity of each fixed heat output furnace stage is equal to the single output capacity of every other fixed heat output furnace stage in the limited modulation furnace.  
   
   
       14 . The limited modulation furnace of  claim 13 , wherein the modulating furnace stage has a maximum output capacity equal to the single output capacity of each fixed heat output furnace stage.  
   
   
       15 . The limited modulation furnace of  claim 11 , wherein the limited modulation furnace comprises at least three fixed heat output furnace stages.