Many furnace blower motors are designed for a fixed speed during a heating operation, but this does not mean it can not be safely increased.
A furnace blower motor does have to have a certain airflow through the heat exchanger when flame is present in order to prevent the heat exchanger from overheating. The heat exchanger is the medium through which one material heats another. Most manufacturers have pre-set this speed into the motor. This means that it is dangerous to let the airflow come below the fixed speed not above it.
Increasing the furnace blower motor speed will reduce its ability to conserve energy. The faster the motor runs, the more electrical energy it spends. Thus, if you increase the speed of the motor to deal with a drop in temperature in the house, reduce the speed as soon as the house reaches a comfortable feeling in order to conserve energy.
A Gradual Increase
It is important to gradually increase the motor speed as the furnace temperature rises. This makes the house more comfortable because as the temperature rises, so does the air flow in the house itself evening out the temperature between rooms. In addition, gradual increases versus rapid fire increases conserve energy.