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Viessmann AirflowPLUS AH2A Installation, Operating and Service
Control Function
Temperature sensors
The Viessmann AirflowPLUS integrated Control includes
five temperature sensors. Temperature sensors are 2-wire
devices that are connected to the control board with a
two pin connector allowing field replacement of individual
sensors. In the event of a sensor failure for longer than
2 minutes, the alarm beeper will sound 2 beeps every 8
min. after 24 hr. of continuous power on.
Circulator
The circulator connection will support up to 5A 115VAC
pump motors, activated by control logic on call for heat.
Air conditioning relay (24VAC)
The Air Conditioning relay is on when the A/C cooling
function should be active. It is a 24VAC signal that will
energize the A/C power contactor.
Fan relay (24VAC 5A Max dry contact)
The fan relay is on at any time the fan is running. The
relay presents a contact closure that may be used to
activate external air cleaners, humidifiers, etc that should
be active while the fan is running.
Heat relay (24VAC 5A Max dry contact)
The heat relay is on when the thermostat calls for heat.
Alarm relay (24VAC 5A Max dry contact)
The alarm relay provides a dry contact closure that is
used to indicate an alarm condition. The contacts are
closed during an alarm condition.
The alarm relay contacts may be used to activate an
external device to alert the user that an alarm condition
exists.
Supply Water Temperature (T1)
Freeze Protection Temperature (T3)
A/C Discharge Air Temperature (T4)
Alarm conditions include:
• Heat exchanger freeze protection limit exceeded
• Return air temperature is too low
• Exhaust air temperature is too cold
Cooling lockout
If a freeze protect cycle is initiated 3 times within a single
cooling cycle, the system goes into cooling lockout mode.
The cooling cycle is stopped and new cooling cycles are
blocked. The alarm relay is turned on and the LED will
blink an error code.
Freeze protection mode
During cooling the Viessmann AirflowPLUS monitors for
freeze protection.
The A/C delta T rule is used when the return air
temperature is below 60°F (16°C). The difference
between the return air temperature (and the discharge
air temperature) is monitored as an indication of possible
core freezing (A/C delta T). While the A/C delta T exceeds
the threshold the normal modulation control is bypassed.
When the A/C delta T is greater than 24°F (13°C) the
A/C relay is turned off for 5 minutes.
The sensor is mounted at the supply end of the water
heating coil.
The sensor will be mounted above the water heat
exchanger to detect freezing temperatures from the
A/C heat exchanger that could damage the water heat
exchanger. The exhaust air is also monitored during the
cooling mode.
The sensor is installed in the discharge air path of the
Air Handler, downstream of the air handler and/or air
conditioning assembly. Refer to pages 14 and 15.
Installation
25
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