14
BALANCING 45M,N,R PARALLEL FLOW UNITS
A parallel fan terminal is designed to operate with the fan sup-
plying air equal to 40 to 60% of the VAV damper maximum set-
ting. Adjustments to the parallel units fan should be made with
the primary air closed off. Refer to unit capacity tables to ensure
airflow through the electric heater meets the minimum require-
ments before operating heater.
Each control option has specific procedures required for bal-
ancing the unit, but some steps are common to all parallel fan
units, as described below.
To balance parallel fan unit:
Setting Fan Airflow
NOTE: If the unit has electric heat or hot water heat, temporarily
disable these functions before balancing the fan.
If unit has optional electric heat disconnect downstream of
fan motor connections to power, open disconnect.
If unit does not have optional electric heat disconnect, re-
move one electric heat power line connection. Be sure to insulate
loose line from ground wire or other wires.
1. Set the controller to provide heating airflow demand only.
Typically, this is accomplished by setting the thermostat to
the highest possible temperature setting.
NOTE: A minimum of 0.1 in. wg downstream static pressure is
required in the duct to ensure proper heater operation.
2. Determine that the VAV damper is fully closed. This may
require a temporary override of the VAV controller. Do not
adjust minimum and maximum cfm set points at this time.
3. Using a flow hood or duct traverse, determine the delivered
fan airflow (cfm).
NOTE: Both flow hood and duct traverse are subject to
measurement errors. Be sure that all applicable measure-
ment precautions are taken.
4. Compare the required design cfm in heating mode to the
actual delivered airflow. If there is a minimum setting for
the VAV damper in heating mode (as recommended by
ASHRAE [American Society of Heating, Refrigeration,
and Air Conditioning Engineers] Standard 62), this quanti-
ty is included in the total measured airflow.
5. Adjust the fan SCR at unit control box to achieve the de-
sired airflow rate.
Setting of VAV (Primary) Airflow
Adjustment of Set Points
Each parallel fan unit is equipped with a pneumatic or
electronic volume controller which regulates the quantity of
cold primary air entering the terminal and the conditioned
space. If required airflow levels are specified with the job or-
der, the minimum and maximum cfm levels will be set at the
factory. If minimum and maximum levels are not specified, a
default value is used. Other settings of minimum and maxi-
mum primary airflow must be set in the field. Airflow (cfm)
ranges for the primary air damper are shown in Tables 12-14
for 45M,N,R units. The minimum primary airflow (other than
zero) is the minimum flow rate controllable by the unit vol-
ume controller. The primary air damper can be set at zero for
shutoff or at the minimum cfm listed.
Field Adjustment of Minimum and Maximum Airflow Set
Points
Each parallel fan unit is equipped with a four quadrant multi-
point center averaging flow probe which provides an amplified
differential pressure that is proportional to the unit airflow. Out-
put from this probe is used to provide a flow signal to both pneu-
matic and electronic controls. Unit airflow (cfm) can be read
directly from the flow probe on the unit.
1. After the unit airflow from the fan has been set, turn on pri-
mary (VAV) air supply and turn off the fan.
2. To set cfm in the field, connect a gage to the flow probe
and check the differential pressure.
3. If a minimum VAV flow is required in heating mode, adjust
the volume controller until the differential pressure corre-
sponds to the cfm required.
4. Some control sequences allow the fan to start before the
VAV damper reaches minimum setting, for an overlapping
of fan and VAV flow. For these sequences, after controller
min airflow has been adjusted, the total airflow with both
fan and primary airflow should be checked. For sequences
that call for the fan to start as the first stage of heat, the
cooling minimum cfm can be verified at the diffuser.
Setting the minimum control point will typically require
careful adjustment of the thermostat to create a minimum
cooling demand signal.
5. a. Set the controller to provide maximum cooling de-
mand. This is typically accomplished by setting the
thermostat to the lowest possible temperature set-
ting. For most control sequences, this will cause the
fan to shut off.
b. Adjust the volume controller until the differential
pressure (measured through the flow probe as
described above) corresponds to the cfm required.
6. Return all reheat options to normal connections.
7. Cap the ends of the inlet flow sensors.
8. Reset the thermostat to a normal setting.
