Control for monitoring flame integrity in a heating appliance

A method for controlling operation of a fuel-fired heating appliance having a burner, a fuel flow control for controlling a fuel flow to the burner, and an combustion air blower for supplying combustion air to the burner, the method comprising:

• sensing a flame at the burner and outputting a time-varying flame current signal including an ionization signal from the flame;
• sampling the flame current signal to obtain a time record of the flame current signal;
• using a Fourier transformation, transforming the time record into a frequency spectrum of frequency components including frequency components of the ionization signal, the frequency spectrum having a spectrum shape defined by various frequency components of the flame current signal;
• comparing frequency spectrum shapes to determine whether the flame current signal changes from presenting a frequency spectrum shape indicative of flame stability and normal combustion to presenting a frequency spectrum shape indicative of flame instability and less than desirable combustion caused by an inadequate air flow rate relative to the fuel flow rate;
• and
• if a flame instability condition is detected, generating a signal for adjusting a speed of the combustion air blower to adjust air flow relative to fuel flow rate or for adjusting the fuel flow control to adjust fuel flow rate to the burner monitoring the flame current signal to detect flame instability by measuring the flame current signal waveform at a given data sampling rate;
• transforming the time record, using a Fast Fourier Transform algorithm, into the frequency spectrum;
• and analyzing the frequency spectrum to determine whether the spectrum exhibits ionization signal frequency components indicative of flame stability and normal combustion, or whether the frequency spectrum exhibits frequency spikes indicative of flame instability;
• generating a signal for reducing speed of the combustion air blower to reduce air flow rate of combustion air to the burner;
• reducing the speed of the combustion air blower until a frequency spectrum indicative of flame instability is detected;
• generating a signal for incrementally increasing the speed of the combustion air blower;
• and incrementally increasing the speed of the combustion air blower until a frequency spectrum indicative of normal combustion is detected.