© 2010 D 265 - 07/10
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DHW
Pump
Primary
Pump
Boiler
Pump
DHW
Pump
Primary
Pump
Boiler
Pump
DHW MODE 3 - DHW in Primary / Secondary no Priority
When a
DHW Demand
is present, the
Boiler
3
/
DHW
contact (terminals
21 and 22) is closed and the primary pump contact is closed.
This mode can be used if a DHW tank is piped in direct return and
a DHW valve is installed.
DHW PRIORITY OVERRIDE
The DHW
Priority Override
applies to
DHW MODE
2
and
4
. To prevent the building from cooling off too much or the possibility of
a potential freeze up during DHW priority, the control limits the amount of time for DHW priority. The length of DHW priority time
is determined using the
Priority Override
setting. Once the allowed time for priority has elapsed, the control overrides the DHW
priority and resumes space heating.
To provide external DHW priority, the space heating zones must be interlocked with the Boiler 3 / DHW contact. During DHW
demands, the Boiler 3 / DHW contact must remove any power to all space heating zone valves or zone pumps.
CONDITIONAL DHW PRIORITY
The Conditional DHW Priority Override applies to DHW MODE
2
and
4
. If the boiler supply temperature is maintained at or above
the required temperature during DHW generation, this indicates that the boiler(s) has enough capacity for DHW and possibly
heating as well. As long as the boiler supply temperature is maintained near its target and the heating and DHW targets are similar,
DHW and heating occurs simultaneously.
DHW POST PURGE
After the DHW Demand is removed, the control performs a purge (PURG) on the boiler(s). The control shuts off the boiler(s) and
continues to operate either the DHW pump or the DHW valve and the system and boiler pump if applicable. This purges the residual
heat from the boiler(s) into the DHW tank. The control continues this purge for a maximum of two minutes or until the boiler supply
water temperature drops 20°F (11°C) below the boiler target temperature during the DHW operation. The control also stops the
purge if the boiler supply temperature is close to the current boiler target temperature.
DHW MIXING PURGE
After DHW operation, the boiler(s) is extremely hot. At the same time,
the heating zones may have cooled off considerably after being off for
a period of time. To avoid thermally shocking the boiler(s) after DHW in
parallel with priority (DHW MODE 2), the control shuts off the boiler(s), but
continues to operate the DHW while restarting the heating system. This
allows some of the DHW return water to mix with the cool return water
from the zones and temper the boiler return water.
DHW
Pump
Primary
Pump
Boiler
Pump
Disable
Using External
Wiring
DHW MODE 4 - DHW in Primary / Secondary with Priority
When a
DHW Demand
is present, the
Boiler
3
/
DHW
contact (terminals 21 and 22) is closed and the primary pump contact is
closed. Priority can only be obtained using external wiring. During a priority override, the
Boiler
3
/
DHW
contact is opened until
the heating system has recovered before returning to DHW operation.
This mode can be used if a DHW tank is piped in direct return and a DHW valve is installed.
DHW MODE 4
External Priority
Interlock
Power to External
Boiler Zones
N.C.
L
DHW
Pump
N
N.O.
L
N
COIL
Power from
DHW Pump / Vlv
Contact
DHW DURING UNOCCUPIED
If the control receives a
DHW Demand
during an unoccupied period, the control can either continue operation of the DHW system
as it would during the occupied period or the control can ignore a DHW Demand for the duration of the unoccupied period.
NUMBER OF BOILERS USED FOR DHW GENERATION
The number of boilers used for DHW generation can be selected to either one or two using the BOIL DHW setting. This applies when
only a DHW Demand is present. If there are other demands present, the control does not limit the number of boilers operated.
