Final answer:
Footings of masonry fireplaces must extend below the frost line to avoid damage from ground movement caused by freezing and thawing cycles. Efficient devices like wood stoves should be used to prevent heat loss.
Step-by-step explanation:
Footings for masonry fireplaces must extend below the frost line. This is to ensure that they are not subject to ground movement caused by the freezing and thawing cycles that can occur in the soil above the frost line. These movements could potentially damage the structural integrity of the fireplace. In addition, it is important to use other efficient devices, such as a wood stove, to minimize heat loss through the chimney.