Airflow Balancing
For proper performance the unit must operate at the
specified supply and exhaust flow rates. Unit fan speed(s)
and damper positions are theoretically set at the factory
based on the ductwork static pressures and flow rates
specified with the order and in the performance data.
If conditions change or verification is required, airflow
measurements should be taken using AMCA suggested
methods. This would normally be a velocity traverse mea-
surement or flow measuring station (FMS) installed in the
ducts. Where space is limited in the outdoor air and ex-
haust air, pressure drop readings can be taken across the
enthalpy wheel and read or calculated from the Enthalpy
Wheel Pressure Drop vs. Flow Formula and Curves lo-
cated in
for the wheel diameter and thickness
or efficiency rating. Heat recovery performance is tested in
accordance with AHRI Standard 1060 and is accurate to
/− 5% if there is no dirt buildup in the heat recov-
ery wheel.
It is important to locate the FMS in the ‘warm side’ duc-
twork to minimize the effect of differences in air density,
especially when balancing during extremely cold outside
conditions or to take temperature readings and make the
necessary corrections if installed in the ‘cold side’ duct-
work. Air density variations can affect the FMS by more
than 15%. The FMS should be located downstream from
straight sections of duct and not immediately after fans or
obstructions that will cause turbulent flow.
for optional gas-fired furnace module
air balancing information. The installation is to be adjusted
to achieve the air throughput within the range specified on
the gas-fired furnace module rating plate.
Imbalanced airflow may cause supply air temperatures to
be below freezing. Adequate freeze protection such as
glycol or low limit temperature protection for downstream
coils or re-heat to protect building systems must be field
provided.
Setting Flow Rate
Units supplied with belt driven double width double inlet
fans have an adjustable motor sheave factory set at the
midpoint of travel at rpm for the flow rate and external
static specified. With optional VFD driven motors this fac-
tory setting is at 60 Hz. For 100% outdoor and exhaust,
flow rate should be balanced with motors operating at
high speed and at 60 Hz by adjusting the motor sheave
pitch diameter. The VFD can be used for fine tuning de-
pending on sequence.
With the optional direct driven plenum supply fan the VFD
is used for speed setting and balancing at the required
Hertz.
With the optional internal bypass the airflow rate may be
reduced. Consult the factory for setting the flow rate dur-
ing internal bypass.
IMPORTANT
On initial power up, the unit will perform a system check
and operate at high speed for five seconds.
VCES-VHC-IOM-1F – VHC-36, -42 & -50
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