Handbook of air conditioning and refrigeration / Shan K

8.4 CHAPTER EIGHT
when the warm air furnace is a part of an air-handling unit or packaged unit in which a supply
fan is always provided), and to extract warm space air for heating again. A centrifugal fan with
forward-curved blades and a double inlet intake is usually used.
Filter. A disposable filter, which is discussed in Chap. 17, is used to remove dust from the recirculating
air. It is often located upstream of the fan.
Venting Arrangements. In a natural-vent warm air furnace, a draft hood is employed to connect
the flue gas exit at the top of the heat exchanger section to a vent pipe or chimney. A relief air opening
is also used to guarantee that the pressure at the flue gas exit is always atmospheric. A draft
hood diverts the backdraft from the chimney, bypassing the burner without affecting the combustion
operation. A direct-vent warm air furnace does not have a draft hood. If the vent pipe or chimney is
blocked, a control system shuts down the warm air furnace. A power vent including a fan-assisted
combustion furnace uses a fan to force or induce the combustion products or flue gas flowing
through the heat exchanger and air vent.
Casing. The outer casing is usually made of heavy-gauge steel with removable access panels.
Furnace Operation. Gas is generally brought from the main to the pressure regulator. The regulator
reduces the gas pressure to about 3.5 in. WG (870 Pa). Gas then flows through a gas valve controlled
by a room thermostat. When a solenoid gas valve opens, the gas flows to the burners and
mixes with the necessary outside primary air for combustion. The primary air–gas mixture then
FIGURE 8.1 Upflow gas-fired furnaces. (a) Natural vent; (b) fan-assisted combustion.