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Leaky Supply Ducts
Figure
3 shows perhaps the simplest example of duct leakage.
Here the supply ducts leak, but the return ducts are air tight.
Even though half the duct system is good, two bad things still
happen. First, some of the air that has just been warmed by
the furnace is lost. Second, this air has to be replaced.
If it isn’t, the house would soon be pumped down to a vacuum,
and we know that doesn’t happen. What does happen is that
cold air from the outside is drawn into the house through
cracks and small holes in the outside walls. Usually these
occur around doors and windows. Some houses have more of these
than others, but no house is air tight. So we’ve lost some
of the hottest air in the house (air that just came from the
furnace), and replaced it with the coldest air around (air
from the outside). In other words, a leak-ing supply duct
is an energy loser in two ways: the energy loss that does
not go to the rooms, and the extra energy needed to heat cold
air that leaked into the house.
Leaky Return Ducts
Suppose the supply ducts are tight but the returns
leak, as shown in Figure
4. The return duct is at a low pressure— lower than the
house or the outside —so cold air from the outside is pulled
into this duct. This cold air is heated in the furnace (along
with air that came from the house through the return registers).
The amount of air delivered to the house by the supply registers
is greater than what the return ducts took from the house
(the difference being the cold air that leaked into the return
ducts). To equalize the flows, heated room air leaks out of
the house through the same holes and cracks that, in the previous
example, allowed air to leak in. So cold air is pulled in
and warm air leaks out. In addition to creating energy losses,
leaky return ducts can create health problems (see below).
Zone Pressurization
Ducts can cause air leakage in the house even
if neither the supply nor the return ducts leak themselves.
Figure 5 shows how this can happen. Imagine that a home has
a return register in one room but no supply (the room on the
left in Figure 5),
and a supply register in another room but no return. Now close
the door between these rooms. The room with the supply duct
(the room on the right in Figure
5) will have relatively high pressure. The supply duct
will be trying to blow this room up like a balloon. Similarly,
the room with the return will have relatively low pressure.
So inside air will leak out from the room on the right, and
outside air will leak into the room on the left. This places
an added load on the heat-ing equipment. The situation described
here is some-what simplified to show the basic idea, but variations
of it are common in real homes. Most new homes built today
do not have duct returns in each room. The problem can be
avoided in rooms with no return register and doors that are
often closed by installing an opening covered by a louvered
grill in the door or in the adjoining wall.
Energy Losses When the Fan Is Off
So far, we’ve been talking about what happens
when the central furnace fan is running. But even when it’s
off (which is most of the time) the leaks in ductwork add
to the air leaks in the rest of the house. The cracks in duct-work
typically have an area that is 10 to 20 percent of the leakage
area of the house. Over the course of a heat-ing season, the
energy losses from ducts when the fan is off can be nearly
as great as when the fan is on!
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