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7MUA-PRC001-EN
Application
Considerations
Outdoor Units
Outdoor make-up air units are the most
common approach to relieving negative
pressure inside commercial and
industrial facilities. All units offer the
advantage of a full support system that
is watertight, provides a plenum for
return air, and has easily accessible
piping and electrical connections.
The Need for Make-Up Air Units
When more air is exhausted from a
building than is supplied by the
mechanical systems, the building is
under a “negative” condition. Air will
leak into the building through cracks,
windows, and doors. When a negative
condition exists:
1
Flues and stacks may experience a
backdraft and cause dangerous
contaminants to remain in the occupied
space. In the case of flues, the products
of combustion may condense and
corrode the equipment.
2
Under negative conditions, the exhaust
system sees a greater static pressure.
The capacity of each fan is reduced and
this results in an inadequate removal of
contaminants.
3
Drafts and cross-currents will increase in
a negative condition, causing an
uncomfortable or unhealthy work
environment.
What Fuel to Use?
The most common fuel for heating
make-up air is natural gas. This is
because 100 percent of the energy goes
into the air stream (92 percent sensible, 8
percent latent). Direct firing eliminates
the need for heat exchangers or
combustion chambers that can corrode
or leak. Natural gas is often the least
expensive fuel and is usually readily
available.
Note: Selected horizontal and vertical
units are available with special coil
options (DX, chilled water, steam or
electric).