CONVECTION 



109 



the cold. The jacket allows the cold air to enter at the bottom, 

 usually from outdoors, through a duct. The air circulates through- 



FIG. 87. Hot-water Heat. Why is the expansion tank necessary? 



out the room, and tends to equalize the temperature. In the dia- 

 gram the ventilators have been placed on the sides of the room to 



