Furnace with modulating firing rate adaptation

1. A method of operating a combustion appliance that has a burner and three or more different firing rates including a minimum firing rate, a maximum firing rate and at least one intermediate firing rate between the minimum firing rate and the maximum firing rate, the combustion appliance further includes a variable speed combustion blower, wherein each of the three or more firing rates is associated with a different corresponding combustion blower speed, the method comprising:

determining that a flame is lost during operation of the combustion appliance;

determining the firing rate at which the flame was lost;

after it is determined that the flame was lost, initiate a calibration cycle, wherein the calibration cycle comprises:

changing the combustion blower speed of the variable speed combustion blower until a first predetermined flow rate of combustion air is detected;

determining a first combustion blower speed that corresponds to when the first predetermined flow rate of combustion air is detected;

changing the combustion blower speed of the variable speed combustion blower until a second predetermined flow rate of combustion air is detected;

determining a second combustion blower speed that corresponds to when the second predetermined flow rate of combustion air is detected; and

re-calibrating the different corresponding combustion blower speeds for each of a plurality of the three or more firing rates based on the first determined combustion blower speed and the second determined combustion blower speed; and

maintaining a firing rate for subsequent operation of the combustion appliance that is above the firing rate at which the flame was lost, at least until the calibration cycle is completed.

2. The method of claim 1 , wherein re-calibrating the different corresponding combustion blower speeds for the plurality of the three or more firing rates includes interpolating between the first determined combustion blower speed and the second determined combustion blower speed.

3. The method of claim 1 , wherein the first predetermined flow rate of combustion air corresponds to a predetermined minimum flow rate of combustion air for the burner, and the second predetermined flow rate of combustion air corresponds to a predetermined maximum flow rate of combustion air for the burner.

4. The method of claim 3 , wherein the first combustion blower speed corresponds to the minimum firing rate, the second combustion blower speed corresponds to the maximum firing rate, and wherein re-calibrating includes interpolating between the first combustion blower speed and the second combustion blower speed to find an intermediate combustion blower speed for each of the at least one intermediate firing rate.

5. The method of claim 1 , wherein re-calibrating the different corresponding combustion blower speeds for the plurality of the three or more firing rates includes extrapolating from the first determined combustion blower speed and the second determined combustion blower speed.

6. The method of claim 1 , wherein re-calibrating the different corresponding combustion blower speeds for the plurality of the three or more firing rates is based on a linear relationship between firing rate and combustion blower speed.

7. The method of claim 1 , wherein re-calibrating the different corresponding combustion blower speeds for the plurality of the three or more firing rates is based on a non-linear relationship between firing rate and combustion blower speed.

8. A method of calibrating a combustion appliance that has a burner and three or more different firing rates including a minimum firing rate, a maximum firing rate and at least one intermediate firing rate between the minimum firing rate and the maximum firing rate, the combustion appliance further having a variable speed combustion blower, wherein each of the three or more firing rates is associated with a different corresponding combustion blower speed, the method comprising:

receiving a current call for heat to initiate a current HVAC cycle;

setting the combustion appliance to a first firing rate, wherein the first firing rate is above the minimum firing rate;

igniting the burner of the combustion appliance;

once ignited, modulating the firing rate from the first firing rate down towards the minimum firing rate;

determining if flame is lost as the firing rate is modulated down towards the minimum firing rate or after the firing rate has been modulated down to the minimum firing rate, and wherein if flame is lost:

changing the combustion blower speed of the variable speed combustion blower until a first predetermined flow rate of combustion air is detected;

determining a first combustion blower speed that corresponds to when the first predetermined flow rate of combustion air is detected;

changing the combustion blower speed of the variable speed combustion blower until a second predetermined flow rate of combustion air is detected;

determining a second combustion blower speed that corresponds to when the second predetermined flow rate of combustion air is detected; and

re-calibrating the different corresponding combustion blower speeds for each of a plurality of the three or more firing rates based on the first determined combustion blower speed and the second determined combustion blower speed.

9. The method of claim 8 , wherein re-calibrating the different corresponding combustion blower speeds for the plurality of the three or more firing rates includes interpolating between the first determined combustion blower speed and the second determined combustion blower speed.

10. The method of claim 8 , wherein the first predetermined flow rate of combustion air corresponds to a predetermined minimum flow rate of combustion air for the burner, and the second predetermined flow rate of combustion air corresponds to a predetermined maximum flow rate of combustion air for the burner.

11. The method of claim 10 , wherein the first combustion blower speed corresponds to the minimum firing rate, the second combustion blower speed corresponds to the maximum firing rate, and wherein re-calibrating includes interpolating between the first combustion blower speed and the second combustion blower speed to find an intermediate combustion blower speed for each of the at least one intermediate firing rate.

12. The method of claim 8 , wherein re-calibrating the different corresponding combustion blower speeds for the plurality of the three or more firing rates includes extrapolating from the first determined combustion blower speed and the second determined combustion blower speed.

13. The method of claim 8 , wherein re-calibrating the different corresponding combustion blower speeds for the plurality of the three or more firing rates is based on a linear relationship between firing rate and combustion blower speed.

14. The method of claim 8 , wherein re-calibrating the different corresponding combustion blower speeds for the plurality of the three or more firing rates is based on a non-linear relationship between firing rate and combustion blower speed.

15. An appliance controller for controlling a combustion appliance that has a burner and three or more different firing rates including a minimum firing rate, a maximum firing rate and at least one intermediate firing rate between the minimum firing rate and the maximum firing rate, the combustion appliance further having a variable speed combustion blower, wherein each of the three or more firing rates is associated with a different corresponding combustion blower speed, the appliance controller comprising:

an input for receiving a call for heat;

a first output for setting the firing rate and combustion blower speed of the combustion appliance;

a second output for commanding an igniter to ignite the burner;

a controller operative coupled to the input and the first and second outputs, the controller configured to receive a current call for heat via the input, and in response, the controller is configured to:

set the combustion appliance to a burner ignition firing rate and combustion blower speed via the first output, wherein the burner ignition firing rate is above the minimum firing rate;

ignite the burner of the combustion appliance by sending a command to the igniter via the second output;

once ignited, modulate the firing rate from the burner ignition firing rate down towards the minimum firing rate;

determine if flame is lost when the firing rate is modulated down towards the minimum firing rate;

if flame was lost, reignite the burner by sending a command to the igniter via the second output, and maintain the firing rate of the combustion appliance above the firing rate at which the flame was lost;

change the combustion blower speed of the variable speed combustion blower until a first predetermined flow rate of combustion air is detected;

determine a first combustion blower speed that corresponds to when the first predetermined flow rate of combustion air is detected;

change the combustion blower speed of the variable speed combustion blower until a second predetermined flow rate of combustion air is detected;

determine a second combustion blower speed that corresponds to when the second predetermined flow rate of combustion air is detected; and

re-calibrate the different corresponding combustion blower speeds for each of a plurality of the three or more firing rates based on the first determined combustion blower speed and the second determined combustion blower speed.

16. The appliance controller of claim 15 , wherein the controller receives an input from one or more sensors of the combustion appliance that provide a measure of the flow rate of combustion air in the combustion appliance.

17. The appliance controller of claim 16 , wherein the controller determine the first combustion blower speed that corresponds to when the first predetermined flow rate of combustion air is detected based, at least in part, on the input from the one or more sensors of the combustion appliance.

18. The appliance controller of claim 16 , wherein the one or more sensors comprise one or more of a pressure switch, a pressure sensor and a flow sensor.

19. The appliance controller of claim 15 , wherein:

the first predetermined flow rate of combustion air corresponds to a predetermined minimum flow rate of combustion air for the burner, and the second predetermined flow rate of combustion air corresponds to a predetermined maximum flow rate of combustion air for the burner;

the first combustion blower speed corresponds to the minimum firing rate, the second combustion blower speed corresponds to the maximum firing rate, and the controller re-calibrates by interpolating between the first combustion blower speed and the second combustion blower speed to find an intermediate combustion blower speed for each of the at least one intermediate firing rate.