Abstract: A gas-fired heater includes a gas supply line, an air supply line, a valve coupled to the gas supply line and a controller coupled to the valve. The controller is configured to control the valve to control a gas-air mixture. The gas-fired heater also includes a burner configured to receive the gas-air mixture and burn the gas-air mixture, and a heat exchanger coupled to the burner. The heat exchanger is configured to perform a heat exchange process and output heated air. The gas-fired heater further includes a combustion fan located downstream of the heat exchanger, the combustion fan being configured to pull or draw flue gases through the heat exchanger.