SERVICING
37
STATIC PRESSURE DROP
If the total external static pressure and/or static pressure
drop exceeds the maximum or minimum allowable statics,
check for closed dampers, dirty filters, undersized or poorly
laid out duct work.
AIRFLOW CONSIDERATION
Airflow demands are managed differently in a fully commu
-
nicating system than in a non-communicating wired system.
The system operating mode (as determined by the thermo-
stat) determines which unit calculates the system airflow de
-
mand. If the indoor unit is responsible for determining the
airflow demand, it calculates the demand and sends it to the
ECM motor. If the outdoor unit or thermostat is responsible
for determining the demand, it calculates the demand and
transmits the demand along with a fan request to the indoor
unit. The indoor unit then sends the demand to the ECM
motor. The table below lists the various communication sys-
tems, the operating mode, and airflow demand source.
System
System Opera
�
ng
Mode
Air
fl
ow Demand
Source
Cooling
Outdoor Unit
Heat Pump Hea
�
ng
Only
Outdoor Unit
Con
�
nuous Fan
Thermostat
HP + Electric Heat
Strips
Either Outdoof Unit
or Indoor Unit,
which has higher
Air
fl
ow demand
Electric Heat Strips
Only
Indoor Unit
Gas Furnace
Combus
�
on Heat
Indoor Unit
Any
Air Handler or
Modular Blower
For example, assume the system is an outdoor unit matched
with an indoor unit. With a call for cooling, the outdoor unit
will calculate the system’s cooling airflow demand. The out
-
door unit will then send a fan request along with the cooling
airflow demand to the indoor unit. Once received, the indoor
unit will send the cooling airflow demand to the ECM motor.
The ECM motor then delivers the cooling airflow. In con
-
tinuous fan mode, the communicating thermostat provides
the airflow demand. The communicating provides 4 contin
-
uous fan speeds (25%, 50%, 75% and 100% of maximum
airflow). During continuous fan operation, the thermostat
sends a fan request along with the continuous fan demand
to the indoor unit. The indoor unit, in turn, sends the demand
to the ECM motor. The ECM motor delivers the requested
continuous fan airflow.