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Home Energy Magazine Online September/October 1993
Mobile Homes:
Small Zones,
Big Problems
by Larry Kinney
Larry Kinney is president of Synertech Systems Corporation in Syracuse, New York.
In mobile homes, the furnace is typically located in a tiny room in the
middle of the structure. A distribution fan at the top of the furnace moves
return air downward through a counter-flow heat exchanger to a plenum that
connects to a single long supply trunk. This trunk runs the length of the home
between the belly board and floor (see Figure 1). A series of supply ducts
connected to short vertical risers brings conditioned air from the trunk to the
rooms of the mobile home. In most mobile homes, the home itself is the
return air system, the "formal" part being only a hole in a furnace room door
usually covered with a dusty grille.
For such a simple system, it would seem that little could go wrong. Yet closed
interior doors in mobile homes routinely cause pressure imbalances, driving the
rooms whose doors are closed positive, and the space that includes the
furnace room negative. Worse, the frequent combination of leaks in
mobile home ducts and belly boards results not only in low heating and cooling
system efficiency, but in uncontrolled air leakage. This wastes energy and can
affect indoor air quality, raise moisture levels, cause structural
deterioration--or all three.
In blower door testing of mobile homes, we often find that ducts are the
greatest source of leakage. So in addition to having devastating effects on
furnace efficiency, ducts account for substantial amounts of air infiltration,
even during the off cycle. Closer inspection of the insides of mobile home
ducts (using a mirror at 45 degrees and a strong flashlight) has netted sundry
trash ranging from empty beer cans to an old Raggedy Ann doll. My all-time
favorite was a system with no grille over the duct in the master bedroom
because, as the homeowner explained, that was the entry way for her cat!
In many cases, mobile home duct problems can be treated from inside the
dwelling. These include removing trash, sealing the ends of the trunk, and
sealing the interface between the risers and the mobile home floor. The latter
is the most frequent problem with duct leakage and is straightforward to treat.
Weatherization technicians in Indiana use thin plastic angle stock, 1 in. wide
on each face, to form a kind of collar to connect the duct riser to the mobile
home floor. Mastic completes the job.
The interface between the boot and the plenum is another place where leaks
occur, but this job requires working beneath the structure. Finally,
double-wide mobile homes frequently suffer from poorly assembled and sealed
flex ducts that connect one half of the house to its twin. Usually this is a
pair of supply ducts about 12 in. in diameter that connect the trunk in one
half of the mobile home to the trunk in the other. Making sure they are
unobstructed, well-sealed, and insulated can make a substantial difference in
energy consumption, comfort, and the life of the home.
Figure 1. Mobile home duct system.
Related Articles
"Checking Out HUD's Proposed Mobile Home Performance Standards" (Judkoff)
"Moisture and Mobile Home Weatherization" (Tsongas)
"Beauty and the Beast Upstairs" (Legg)
"Discovering Ducts: An Introduction"
"Duct Fixing in America" (Penn)
"Infiltration: Just ACH50 Divided by 20?" (Meier)
"Leak Detectors: Experts Explain the Techniques" (Proctor, Blasnik, Davis, Downey, Modera, Nelson, and Tooley)
"New Group Hunts Bad Ducts" (Obst)
"The New Monster in the Basement" (Treidler)
"Selecting an Infrared Imaging System" (Snell)
"Sizing Up Skylights" (Warner)
"Telecommuting: An Alternative Route to Work" (Quaid)
"User-Friendly Pressure Diagnostics" (Fitzgerald, Nevitt, and Blasnik)
"Ductionary"
"Duke Power's Success" (Vigil)
"Guidelines for Designing and Installing Tight Duct Systems" (Stum)
"Integrated Heating and Ventilation: Double Duty for Ducts" (Jackson)
"Managing Large-Scale Duct Programs" (Downey)
"One Size Fits All: A Thermal Distribution Efficiency Standard" (Modera)
"Stories from the Buffer Zone" (Kinney and Stiles)
"Two Favorite Test Methods, By the Book" (Modera)
"Will Duct Repairs Reduce Cooling Load?" (Parker, Cummings, and Meier)
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