Handbook of air conditioning and refrigeration / Shan K

Thermal Stratification
divided into two groups: highly energy-efficient Model Conservation Standard (MCS) houses and a
control group built to current (regional) practice (CP) or standards. Ducted heating systems in CP
houses had 26 percent more air leakage than unducted systems and used 40 percent more heating
energy. Ducted heating systems in MCS houses had 22 percent more air leakage and used 13 percent
more heating energy than unducted MCS houses.
Gammage et al. (1986) studied the ducted warm air systems of 31 Tennessee houses. They
found that the air infiltration rate was 0.44 ach when the forced-air systems were off and 0.78 ach
when the forced-air systems were on.
Field tests have also shown that leakages are greater for the return duct than for the supply
duct because of the greater importance of the supply air. Also, the return plenum is often not
carefully sealed. In five Florida homes with low-pressure ducted warm air systems with supply
and return plenums, Cummings and Tooley (1989) found that when repairs were made to seal
the return plenums, the infiltration in these five homes dropped from an average of 1.42 ach to
0.31 ach.
When a ducted warm air heating system is operating, supply duct leakage in such nonconditioned
space as the attic or basement raises the space pressure to a positive value and promotes
exfiltration. Return duct leakage extracts the space air, lowers the space pressure to a negative
value, and promotes infiltration. Both types of leakage increase the whole-house infiltration.
Suggested remedies to reduce duct leakage and energy use are as follows:
● Externally seal the duct with tape; seal the ducts internally if possible.
● Seal the return plenum and equipment if there is leakage. Seal the duct, pipes, and cable penetrations
through the structures.
● Avoid locating ducts in unconditioned spaces. Provide insulation for ducts running in unconditioned
spaces.
● For a low-pressure ducted warm air system without return ducts, an adequate door undercut or
door louver should be provided.
A low-pressure ducted warm air heating system with a gas-fired furnace always has a high supply
temperature differential �T s. If �T s exceeds 30°F (16.7°C), or if there is a high ceiling, thermal
stratification may form in the conditioned space. The vertical temperature difference may be greater
than 5°F (2.8°C) and cause discomfort, as mentioned in Sec. 4.8. In addition, a higher temperature
near the ceiling may increase heat transfer through the ceiling, attic, and roof.
A greater supply air volume flow rate, lower �T s, higher downward air jet velocity, and suitable
location for the supply grille are remedial measures that reduce thermal stratification and vertical
temperature differences.
Part-Load Operation and Control
HEATING SYSTEMS, FURNACES, AND BOILERS 8.21
For low-pressure ducted warm air heating systems, a thermostat or a temperature sensor is usually
installed in a representative space to control the gas valve of a furnace operating under an on/off or
two-stage step control mode to maintain the required space temperature. The proportion of on and
off time in each operating on/off cycle varies to meet the varying space heating load. Figure 8.6
shows a typical on/off control for a gas furnace, resulting in a temperature variation for a point
20 ft (6.1 m) downstream of the supply plenum and at the supply register.
When the space temperature increases above the upper limit as the space heating load falls, the
controller shuts off the gas valve so that the heat supply to the space is cut off. When the space temperature
falls below the lower limit, the gas valve will open again and raise the space temperature.