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SYMPTOM SOURCE PROBABLE CAUSE
7. No straight duct at
fan outlet
• Fans, which are normally used in duct systems, are
tested with a length of straight duct at the fan outlet. The
performance will be decreased If there is no straight
duct at the fan outlet. If it is not practical to install a
straight section of duct at the fan outlet, the fan speed
may be increased to overcome this pressure loss.
(Observe fan RPM limits.)
Air flow low
8. Obstructions in high
velocity air stream
• Obstruction near fan outlet.
• Sharp elbows near fan outlet.
• Improperly designed or no turning vanes.
• Projections, dampers or other obstruction in part of
system where air velocity is high.
1. System
• Oversized ductwork.
• Access door open.
• System not balanced. Resistance less than specified.
• Registers or grilles not installed. Dampers set to by-pass
coils.
• Filter(s) not in place.
2. Fan
• Backward inclined impeller installed backwards(HP will
be high)
• Variable motor sheave not adjusted.
• Fan speed too fast.
Air flow high
3. Duct
• Duct is too small
1. Filter
• Filter(s) is dirty
Air in room
not fresh
2. Fresh air
• Fresh air volume is not adequate.
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