Airflow Considerations; Fossil Fuel Applications - Intertek MVM97 Series Installation Instructions Manual

A ComfortNet heating/air conditioning system differs from a non-communicating/traditional system in the manner in which the
indoor unit, outdoor unit and thermostat interact with one another. In a traditional system, the thermostat sends commands to the
indoor and outdoor units via analog 24 VAC signals. It is a one-way communication path in that the indoor and outdoor units typically
do not return information to the thermostat.
The indoor unit, outdoor unit and thermostat comprising a ComfortNet system "communicate" digitally with one another, creating
a two-way communications path. The thermostat still sends commands to the indoor and outdoor units. However, the thermostat
may also request and receive information from both the indoor and outdoor units. This information may be displayed on the
ComfortNet thermostat. The indoor and outdoor units also interact with one another. The outdoor unit may send commands to or
request information from the indoor unit. This two-way digital communications between the thermostat and subsystems (indoor/
outdoor unit) is the key to unlocking the benefits and features of the ComfortNet system.
Two-way digital communications is accomplished using only two wires. The thermostat and subsystem controls are powered with 24
VAC. Thus, a maximum of 4 wires between the equipment and thermostat is all that is required to operate the system.
A C
IRFLOW ONSIDERATIONS
Airflow demands are managed differently in a communicating
system than they are 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 furnace is responsible for determining the air-
flow 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 trans-
mits the demand along with a fan request to the indoor unit.
The furnace then sends the demand to the ECM motor. The
following table lists the various ComfortNet systems, the oper-
ating mode, and airflow demand source.
For example, assume the system is an air conditioner matched
with a furnace. With a call for low stage cooling, the air condi-
tioner will calculate the system's low stage cooling airflow de-
mand. The air conditioner will then send a fan request along
with the low stage cooling airflow demand to the furnace. Once
received, the furnace will send the low stage cooling airflow
demand to the ECM motor. The ECM motor then delivers the
low stage cooling airflow. See the applicable ComfortNet air
conditioner or heat pump installation manual for the airflow
delivered during cooling or heat pump heating.
In continuous fan mode, the CTK0* thermostat provides the
airflow demand. The thermostat may be configured for one of three continuous fan speed settings allow for 25%, 50% or
75% airflow, based on the furnaces' maximum airflow capability. During continuous fan operation, the thermostat sends a
fan request along with the continuous fan demand to the furnace. The furnace, in turn, sends the demand to the ECM
motor. The ECM motor delivers the requested continuous fan airflow.
F F A
OSSIL UEL PPLICATIONS
This furnace can be used in conjunction with a ComfortNet™ compatible heat pump in a fossil fuel application. A fossil fuel
application refers to a combined gas furnace and heat pump installation which uses an outdoor temperature sensor to determine the
most cost efficient means of heating (heat pump or gas furnace). When used with the CTK0* thermostat, the furnace/heat pump
system is automatically configured as a fossil fuel system. The balance point temperature may be adjusted via the CTK0*
thermostat advanced user menus (see CTK0* instructions for additional information).
46
System Operating
System
Mode
Cooling
Air Conditioner +
Heating
Furnace
Continuous Fan
Cooling
Heat Pump Heating
Only
Heat Pump +
Furnace
Auxiliary Heating
Continuous Fan
Cooling
Furnace + Non-
Comm 1stg Air
Heating
Conditioner
Continuous Fan
Airflow Demand Source
Air Conditioner
Furnace
Thermostat
Heat Pump
Heat Pump
Furnace
Thermostat
Furnace
Furnace
Thermostat