Gravity Warm-Air Furnaces
Gravity warm-air furnaces rely upon the fact that warm air is lighter than cold air. As a result, the warmer air rises through the ducts or pipes in the structure, gives off its heat to the rooms, and descends to the furnace as it becomes cooler and heavier. It is then reheated and rises once again to the rooms. A continuous circulation path is thus established through the heating and cooling of the air. Sometimes a fan is added to increase the rate of flow, but the primary emphasis is still the effect of gravity on the differing weights of air.
Depending upon the design, a gravity warm-air furnace will fall into one of the following categories:
1. A gravity warm-air furnace without a fan
2. A gravity warm-air furnace with an integral fan
3. A gravity warm-air furnace with a booster fan
Each of these three categories represents a different type of warm-air furnace used in central heating systems.
Any gravity warm-air furnace not equipped with a fan relies entirely upon gravity for air circulation. The flow rate is very slow, and extreme care must be taken in the design and placement of the ducts of pipes. Sometimes an integral fan is added to reduce the internal resistance to airflow and thereby speed up air circulation. A booster fan provides the same function but is designed not to interfere with air circulation when it is not use.
The round-cased, gravity warm-air furnace illustrated in Figure 10-5 is a coal-fired unit that can be converted to gas or oil; see Chapter 16, “Boiler and Furnace Conversions.” Depending upon the model, these furnaces are capable of developing up to 108,266 Btu at register and up to 144,319 Btu at bonnet.
Floor, wall, pipeless furnaces, and some unit (space) heaters also operate on the principle of the gravity warm-air furnace. They are distinguished by the fact that the warm air is discharged directly into the room (or rooms) without the use of ducts or pipes.