Handbook of air conditioning and refrigeration / Shan K

8.22 CHAPTER EIGHT
FIGURE 8.6 Typical on/off control of a gas furnace and the variation of temperature of supply plenum and supply register.
The time period for an on/off operating cycle is generally between 5 and 15 min. Too short a cycle
may result in unstable operation—a condition known as hunting.
8.6 DUCTED WARM AIR HEATING SYSTEMS
Ducted warm air heating systems are heating systems that are part of the air-handling unit, packaged
units, packaged heat pumps, or fan-coil units that use water heating coils, gas furnaces, electric
heaters, or indoor coils in heat pumps to heat the air. Warm air is then supplied to the conditioned
space through the duct system.
A ducted warm air system has the following characteristics:
● It is often integrated with a cooling system, forming a heating/cooling system.
● Its external pressure loss for a duct system is usually between 0.5 and 2 in. WC (125 and 500 Pa),
except for fan-coil units.
● The heating capacity is controlled by modulating the water flow rate to the water heating coil, by
modulating the gas valve, or by controlling the refrigeration flow in an on/off or step control
mode in the heat pumps.
● Heating systems often have a capacity greater than 100,000 Btu/h (29 kW).
Because the perimeter zones of office buildings need strict control of the vertical temperature
difference in their conditioned space, a smaller supply air temperature differential, such as �T s �