A gas-fired air conditioning furnace has a cavity burner configured to combust an air-fuel mixture at least partially within an interior space of the cavity burner. A method of operating a gas-fired furnace by flowing an air-fuel mixture into a cavity burner through a perforated wall of the cavity burner, combusting at least a portion of the air-fuel mixture within an interior space of the cavity burner, and flowing at least partially combusted air-fuel mixture into a heat exchanger. A gas-fired air conditioning device has a cavity burner that has a cylindrically shaped body and a cap on a first end of the body, each of the body and the cap being perforated. The device has a cylindrically shaped heat exchanger inlet tube and the cavity burner is at least partially concentrically received within the heat exchanger inlet tube.