GVZC20 Service Manual

SERVICING
31
• Measure static pressure of the return duct at the inlet
of the furnace
• Measure the static pressure of the supply duct at the
outlet of the furnace
•  Both readings may be taken simultaneously
and read if so desired
If the total external static pressure and/or static pressure
drop exceeds the maximum or minimum allowable statics,
check for closed dampers, dirty lters, undersized or poorly
laid out duct work.
AIRFLOW CONSIDERATION
Airow demands are managed di󰀨erently 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 airow
demand. If the indoor unit is responsible for determining the
airow 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 systems, the operating mode, and airow
demand source.
System
System Operating
Mode
Airflow Demand
Source
Cooling Outdoor Unit
Heat Pump Heating
Only
Outdoor Unit
Continuous Fan Thermostat
HP + Electric Heat
Strips
Either outdoor unit or
indoor unit, which has
higher Airflow demand
Electric Heat Strips
Only
Indoor Unit
Gas Furnace Combustion 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 out-
door unit will calculate the system’s cooling airow demand.
The outdoor unit will then send a fan request along with the
cooling airow demand to the indoor unit. Once received,
the indoor unit will send the cooling airow demand to the
ECM motor. The ECM motor then delivers the cooling
airow.
In continuous fan mode, the communicating thermostat
provides the airow demand. The communicating provides
4 continuous fan speeds (25%, 50%, 75% and 100% of
maximum airow). 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 deliv-
ers the requested continuous fan airow.