A “variable speed furnace blower” adjusts its speed according to the heat setting of the gas burner. The blowers start off on a low speed and continually ramp up until they achieve the optimum speed for the current heat setting. Conversely, they slow down once the desired room temperature has been achieved. This ensures that cold air is never circulated during the heat up and cool down cycles of the furnace while also circulating most of the heat being generated by it during those segments of the heat cycle. These blowers are also sensitive to airflow resistance that can be caused by restrictions such as a dirty air filter, which causes the blower to increase its speed to maintain the desired airflow.