Honeywell AQ25A Series Technical Reference Booklet page 87

Communications Loss
Because all AQ2000 Series components communicate with
each other via the dedicated AQUATROL network when
controlling a hydronic system, one possible failure mode of the
AQ251 would be loss of communication between the AQ1510
Control Module and any connected Zoning Modules, or
between a Zoning Module and any zone thermostats
connected to the AQUATROL network. In general, the Control
Module:
• Periodically tries to re-establish communication with any
lost components on the network.
• Initializes any component that re-establishes its
communication.
• Displays an error code on the AQ251's System Status page,
until the error is corrected and/or communication is re-
established.
Control Module Reaction
When the AQ1510 Control Module loses communication with
any number of zones for more than one minute (as long as
there is at least one zone communicating on the AQUATROL
network), the AQ251 continues to deliver heat to the other non-
communicating zones.
When communication is lost with all zones, the AQ251 enters
BOILER FREEZE PROTECTION mode, in which it fires the
boiler and then activates the Boiler (supply) pump and zone
equipment for a period of 10 minutes every hour. This should
provide sufficient heat to the system to prevent a building from
freezing up until communication is re-established between the
AQ2000 Series components.
BOILER FREEZE PROTECTION mode is disabled when the
outdoor temperature is above the warm weather shutdown
(WWSD) temperature setting.
Zoning Module Reaction
When a Zoning Module loses communication with the Control
Module (as long as there is at least one other Zoning Module
communicating with the Control Module), the Zoning Module
operates its pumps or valves in a conventional, non-
synchronized zoning fashion. That is, it operates according to
the demands from the thermostats, without zone
synchronization or waiting for permission from the AQ151
Control Module to operate. This allows the zones to extract any
heat provided by the boiler.
When using AQ1000 thermostats and communication is lost
between a Zoning Module and any of its thermostats, that zone
enters Single Zone Freeze Protection mode. In this mode, the
zoning equipment is operated for an amount of time equal to
one-half of the maximum demand of all other zones on the
network. This helps prevent the lost zone from freezing. This is
especially helpful if a zone's thermostat is removed from the
wall while a room is being painted. When used with 2-stage
thermostat operation, only the first stage will be activated in the
freeze protection mode.
When using digital, non-communicating thermostats other than
the AQ1000, the Single Zone Freeze Protection is disabled for
those zones.
AQ251 SERIES BOILER RESET CONTROL PANELS
Single Zone Freeze Protection mode is disabled when the
outdoor temperature is above the warm weather shutdown
(WWSD) temperature setting.
The AQ251 provides Zoning Module diagnostic information via
the DIAGNOSTIC LEDs located above the DIP switches on its
Zoning Module(s), as well as on the System Status menu of the
LCD display.
If a zone becomes disconnected from the Control Module or is
malfunctioning (i.e., lost), this can be seen in the ZONE
COUNT value on the AQ251's Home Page display (Detail view
only). The ZONE COUNT represents the number of zones on
the AQUATROL network at any given time (the second stages
of a heating zone is also counted as a zone). When a zone is
lost, the ZONE COUNT decreases by one. Because an
installer knows how many zones are installed, if there is a
difference between the actual number of installed zones and
the ZONE COUNT, the installer will know to look for the
zone(s).
The identity of lost zones displays in the System Status page
as LOST ZONE A-X:
• A identifies the Control Module of the lost zone, and B, C, or
D identify an AQ25400B Add-A-Temperature Expansion
Control Panel. This prefix can be set or changed on the
AQ254 Panels.
• X identifies the lost zone (1 through 16).
Boiler Freeze Protection
related)
If the boiler supply temperature sensor measures 50°F (10°C)
or less, the boiler fires and the boiler pump, secondary pump
etc. operate until the target temperature is achieved, as
measured by the supply temperature sensor. This target
temperature is the reset temp (if the boiler RESET parameter =
OUTDOOR). Or, it is the AQ251's BOILER HIGH LIMIT (if the
boiler RESET parameter = LOAD or NONE). DHW and Heat
Demand calls are ignored when the system is in freeze
protection mode.
Power Disruption
The AQ251's system and thermostat parameters are stored in
non-volatile memory and are updated as they are changed.
When a power disruption occurs, the system configuration is
retained in memory. When power is restored, the AQ251
Control Panel enters auto-detection mode, reads its
previously-stored settings, and initializes all AQUATROL
network components according to their saved parameters.
Power Disruption Greater Than 2 Hours
If a power disruption lasts for more than 2 hours, the AQ251
will have discharged its internal super capacitor, and the DATE
and TIME OF DAY settings will need to be reset.
Upon restart, the AQ251 displays its clock settings as: YEAR =
2008, MONTH = JAN, DATE = 1. The message PLS SET DAY/
TIME displays and optionally, the AQ251's backlight flashes
repeatedly. This displayed message continues until the DATE
and TIME OF DAY are updated. The AQ251 remains
permanently in OCCUPIED (or Comfort) mode until the DATE
and TIME OF DAY are updated.
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(not network communication-
69-1974—04