Handbook of air conditioning and refrigeration / Shan K

8.20 CHAPTER EIGHT
Duct Efficiency and System Efficiency for Heating
Duct efficiency for heating � du, h, in percent, can be calculated by dividing the heat energy output
from the supply and return duct system by the heat energy input to it, i.e.,
� 100�
where qho, s, qho, r � output heat energy from supply ducts and return ducts, Btu/h (W)
qhi, s, qhi, r � input heat energy to supply ducts and return ducts, Btu/h (W)
qhl, s, qhl, r � heat loss from supply ducts and return ducts, Btu/h (W)
1 � q qhi, s � qhi, r
hl, s � qhl, r
qhi, s � q � hi, r
and the supply duct efficiency for heating � sd, h, in percent, is
� 100
qhi, s
The system efficiency for heating �sy, h, in percent, can be calculated as
1 � qhl, s
qhi, s
where Q f, h � jacket loss and equipment losses from furnace and released to conditioned space,
Btu/h (W)
Q f, in � total energy input to furnace, including auxiliary energy input, Btu/h (W)
Location of Furnace and Duct Insulation
Duct Leakage
� du, h � 100 q ho, s � q ho, r
� sd, h � 100 q ho, s
h sy, h � 100 q ho, s � Q f, h
Q f, in
� 100 � E t� du, h � Q f, h
Q f, in�
(8.10)
(8.11)
(8.12)
The location of the furnace has a significant effect on the system efficiency of heating. In Jakob et al.
(1986), if the gas furnace is installed in a closet, the supply duct is mounted inside the conditioned
space, and the equipment losses of the furnace become the direct heat gains of the conditioned space,
then � sy, h might be 20 percent higher than for those installations where the furnace and supply ducts
are in the attic or basement.
Jakob et al. (1986) also showed that if ducts had an exterior or interior insulation of R5, that
is, an insulation layer with an R value of 5 h�ft 2 �°F/Btu (0.9 m 2 �°C/W), the duct efficiency increased
from 61 percent (without duct insulation) to about 78 percent. The system efficiency saw
a smaller increase because a portion of the heat loss from the duct without insulation had been
used to heat the attic or basement, which in turn reduced the heat loss from the conditioned
space.
ASHRAE Standard 90.1-1999 mandates the minimum duct seal level as discussed in Sec. 17.8.
Field tests in many houses have showed that actual duct leakage in many low-pressure ducted warm
air heating systems is considerably higher.
Lambert and Robinson (1989) analyzed the duct leakage, whole-house leakage, and heat energy
use of 800 electric-heated houses built since 1980 in the Pacific northwest. Tested houses were