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Domestic Hot Water Operation
Section E
DHW operation is only available when the Pump Sequencer
DIP Switch is set to Off.
DHW Demand
DHW Demands come from one of three sources: an external
aquastat, a DHW tank sensor, or a tN4 DHW control.
Once the control detects a DHW Demand, the DHW Demand
segment is displayed in the LCD. If an External Powered
DHW Demand is applied while the DHW sensor is enabled
in the 274, an error message is generated and both demands
are ignored.
A DHW demand from a tN4 Setpoint Control can coexist with
another DHW demand without generating an error message.
The 274 will then use the higher of the two targets.
Powered DHW Demand
The control registers a DHW Demand when a voltage
between 20 and 260 V (ac) is applied across the DHW
Demand terminals 23 and 24. An aquastat or setpoint control
is used to switch the DHW Demand circuit. Program a DHW
Exchange temperature for the Occupied and UnOccupied
events in the Adjust Menu.
• DHW Sensor must be set to Off.
DHW Sensor
The control can register a DHW Demand when A DHW
Sensor is wired to terminals 5 and 6. Once the DHW Sensor
drops 1/2 of the DHW Differential setting below the DHW
Setpoint, the control registers a DHW Demand. Program a
DHW Tank temperature for the Occupied and UnOccupied
events in the Adjust Menu.
The DHW Sensor must be set to On. There cannot be
an externally powered DHW demand when using a DHW
sensor.
tN4 Setpoint Control in DHW Mode
The control can register a DHW Demand when a tN4 Setpoint
Control in DHW Mode is wired to terminals 1 and 2. The
DHW Demand is sent over the tN4 communication bus when
the Setpoint Control calls for heat. Program a DHW tank
temperature for the Occupied and UnOccupied events and
the desired supply water temperature required on the tN4
bus in the Adjust Menu of the tN4 Setpoint Control.
•
Boiler Alarm
For the Boiler Alarm item to appear in the Adjust menu, the
Relay must be set to Alert. If no temperature increase is
detected at the boiler supply sensor within this delay period,
the Alert relay will close and the control will display the Boiler
Alarm error message. All boilers continue to operate if this
error is present. To clear the error, press and hold the up and
down buttons simultaneously for 5 seconds while viewing the
error message in the View menu.
Combustion Air (C.A.) Damper
When the Relay is set to Damper, terminals 15 and 16 operate
a combustion air damper / fan motor or power vent motor. The
Relay closes once a demand is received and the control has
determined that one or more boilers need to be turned on.
Combustion Air Proof Demand Setting in Adjust Menu
The proof demand can be used to prove a combustion air
or venting device if set to C.A. Boiler operation cannot occur
until the proof demand is present. If the proof demand is lost
during operation, the boiler plant is sequenced off.
Combustion Air Proof Demand Delay Setting in Adjust
Menu
The control includes a time delay that is associated with
the proof demand feature in order to determine if the proof
device is functional. Once the C.A. relay closes, the control
allows for this delay to receive the proof demand. If the proof
demand is not received within the delay time, the control will
display an error message.
Combustion Air Damper Delay Setting in Adjust Menu
If the Proof Demand function is set to F P (flow proof) or
OFF, boiler sequencing only occurs once a user adjustable
time delay elapses.
Combustion Air Post Purge
There is a fixed 15 second post purge of the C.A. relay
after the last boiler has turned off, or demand is removed. If
there is a heat demand still present once the last boiler has
turned off, the control can look at the error and determine
if sequencing is to occur in a “short” period of time. If the
control does anticipate staging, the C.A. relay will remain
on. Otherwise, the C.A. relay will be turned off once the 15
second post purge elapses.
Combustion Air Proof Demand Test
The control includes a C.A. proof demand test in order to
determine if the proving device has failed. If the C.A. damper
contacts are opened, the flow proof demand should not be
present after 4 minutes. If the flow proof demand remains,
the control will display an error message.