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Operation
Indicators
Flashing LED
°F = °C x 1.8 + 32
e.g. 24 x 1.8 + 32 = 75.2°F
During normal operating mode the LED will blink to
indicate the temperature reading of the five sensors.
The LED will blink RED to indicate the sensor number.
It will then blink the temperature in GREEN.
All temperature readings are in Celsius. Example, while
sensor T3 is reading 75°F (24°C) the LED will blink
RED 3X then GREEN 2X then GREEN 4X then RED 4X
followed by the reading for the next sensor.
Normal Operation
Heating mode
On a call for heat from the thermostat, the circulating
pump will turn on. Giving the pump 15 sec. to heat the
coil, the fan will activate heating the home until the
thermostat is satisfied.
Once the thermostat is satisfied, the circulating pump
will shut down and fan will continue to run for 2 min.
During the heating cycle the hot water temperature is
monitored. If the hot water temperature drops below
a threshold the fan will be turned off. The hot water
temperature continues to be monitored after the fan
is shut off. When the temperature rises to 10°F (6°C)
above the selected threshold the fan will be restarted.
The threshold may be set according to the system select
switch. The threshold may be set at 110°F or 85°F
(43°C or 29°C)
38
Viessmann AirflowPLUS AH2A Installation, Operating and Service
A bicolour LED is used as a system status indicator to
indicate alarm, test mode, normal operation, etc.
The indicator will display the temperature sensor values
during normal operation. When abnormal behavior is
noted the LED will display an indication to indicate the
condition.
LED flash code will indicate the sensor location in RED
and temperature in GREEN.
e.g. 3RED then 2GREEN, pause, 4GREEN = Sensor T3 is
75°F (24°C)
• An 8RED then 8RED flash indicates a bad reading or
defective sensor, this may repeat several cycles.
Note: If a defective sensor is present, the audible alarm
will sound. For corrective action refer to
troubleshooting.
Sensor 1 should see GREEN flashes of 4GREEN – 3GREEN
110°F (43°C) to 8GREEN –2GREEN 180°F (82°C)
depending on the temperature of the hot water source.
With 120°F (49°C) to 180°F (82°C) entering water
the reading would be 3GREEN – 6GREEN 97°F (36°C)
to 7GREEN – 1GREEN 160°F (71°C), after the correct
number of red flashes to indicate the sensor #.
Refer to page 25 and 26 for sensor numbers and
description.
Anytime a fault condition exist, the specific fault
condition will be displayed represented by a specific LED
flash code (refer to page 42) approximately 2 seconds
after the display of temperature reading for temperature
sensor T6.
Cooling mode
On a call for cooling from the thermostat, if the fan is on,
it will turn off and the A/C relay will be energized. The
outdoor condenser will turn on and flood the evaporator
coil with refrigerant. The fan will turn on and deliver cool
air to the system. After approximately 5 min. (this gives
the condenser and coil time to normalize pressures), the
microprocessor control will cause multiple fan speed
adjustments in order to target the latent load first
(humidity) before speeding up to capture the sensible
load. Sensor feedback allows this enhanced blower
fan feature to protect the outdoor unit from refrigerant
slugging caused by lower fan speeds, dirty filters, poor
duct design or poor installations.
Summary
In heating mode, most error codes and/or improper
unit operation are caused by too low of supply water
temperature or sensors not installed correctly. Cooling
mode errors are caused by too low or too high of
refrigerant charge or gross over sizing of equipment.
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