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Airflow Limits and Equipment Protection
The maximum airflow allowed into a zone is based on the relative
size of the zone determined by the duct assessment and the airflow
limits selected for each zone. Airflow limits are set at High as
factory default. This means that 250% of assessed airflow is
allowed into the zone.
If the system determines that it cannot deliver the airflow into a
zone that needs conditioning, and that zone has an airflow limit
selected, the system will take the following 4 steps:
Step 1 — Reduce airflow if possible
a. 275 CFM per ton minimum in cooling. 175 CFM/ton
for low stage cooling if 2- -stage unit is installed.
b. Comfort Heat airflow is minimum for heat pump heat-
ing (3.5 X Outdoor Temp + 137) CFM/ton.
c. No adjustment for furnace heating
Step 2 — Dump air to unoccupied zones
a. Unoccupied zones can be conditioned up to the most
conditioned setpoint.
Step 3 — Dump air to zones with less conditioned
setpoints
a. Zones with lower setpoints in heating and higher set-
points in cooling may be conditioned to within 3_F
(1.7_C) of the most conditioned setpoint.
b. Increase or decrease unoccupied zones 0.75_F (.4_C).
Step 4 — Single stage cooling
a. System will reduce airflow to 275 CFM/ton in single
stage cooling systems.
Step 5 — Stage down equipment
a. Equipment stage down or shut off if necessary.
b. Fault history will record an event of "AIRFLOW LIM-
ITED STAGEDOWN OCCURRED" on stagedown and
"EXCESS STATIC PRESSURE" if shut down.
c. If shut down occurs, other zones need to call before
equipment will resume operation.
Dehumidify Operation
Once a target cooling humidity setpoint is selected in the Advanced
Setup COOLING HUMIDITY screen, two other setup options
affect Dehumidify
operation: COOLING
DEHUMIDIFY.
COOLING AIRFLOW: Setting this to COMFORT or QUIET
will enable the system to use low airflow to help dehumidify the
space. If duct sweating becomes an issue, setting DEHUM
AIRFLOW to HIGH may resolve the problem.
DEHUMIDIFY: Located in the Cooling Humidity screen of the
Advanced Setup, this option has 2 settings: ON and OFF.
If DEHUMIDIFY is set to ON (factory default), then the cooling
unit will be allowed to overcool the space up to 3_F (.6_C) if the
humidity level is above the cooling humidity target setpoint.
The
amount
of
over- -cooling
dehumidification demand, the cooling demand, and the actual
space temperature. More over- -cooling is allowed with greater
dehumidification demand.
When the space temperature is at or above 75_F (24_C) and the
dehumidify demand is high, over- -cooling up to 3_F (1.7_C) is
allowed. As the space temperature approaches 70_F (21_C), less
over- -cooling is allowed. At 70_F (21_C) space temperature, no
more over- -cooling is allowed no matter how great the dehumidify
demand. This is done to protect the equipment.
AIRFLOW
and
allowed
varies
with
the
Hybrid Heat Setup / Operation
Furnace Lockout Operation — (in HYBRID HEAT SETUP
menu) is the outside temperature above which the furnace will not
run except for defrost (otherwise known as the aux heat lockout).
Heat Pump Lockout Operation — (in HYBRID HEAT SETUP)
is the outside temperature below which the heat pump will not run
(otherwise known as the balance point).
These values can be set identical to each other. If they are not
identical, the system will stage up and down normally from heat
pump to furnace when the outside temperature is between these
settings. The User Interface will not allow the heat pump lockout
setting to be above the furnace lockout setting.
The factory default settings for both of these is NONE (no
lockouts). Even though a furnace lockout temperature may be set,
the system will still use the furnace in defrost operation, and may
stage back down to heat pump when defrost is completed after a 2
minute delay.
Hybrid Heat Defrost — (in HYBRID HEAT SETUP) When the
outdoor unit needs a defrost cycle, the furnace will run during
defrost regardless of lockout temperature, unless told not to in the
Hybrid Heat Setup screen. After defrost, the system may stage
down to heat pump after a 2 minute delay.
If the room temperature falls below 40_F (4_C) and a furnace
lockout is in enabled, the furnace lockout will be overridden to
bring on the furnace.
17
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