© 2010 D 270 - 08/10
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DHW MODE 2 OPERATION
On a call for DHW, the control provides DHW priority by shutting off the primary pump (
Prim
P1
) for a period of time. This time
is based on the outdoor air temperature as described in the DHW Priority Override section. However, if the
DHW
Demand
is not
satisfied within the allotted time, the boiler shuts off and the heat of the boiler is purged into the DHW tank.
Once the boiler supply temperature is sufficiently reduced, the
DHW Pmp / Vlv
contact shuts off. The heating system is turned on
for a period of time to prevent the building from cooling off. After a period of heating, and if the
DHW
Demand
is still present, the
control shuts off the heating system and provides heat to the DHW tank once again.
For correct operation, close attention must be paid to the mechanical layout of the system. When the control turns off the primary
pump (
Prim
P1
), flow to the heating system must stop. If flow is not stopped, the temperature in the heating system can exceed the
maximum desired temperature and can result in damage to the heating system.
DHW
Pump
Primary
Pump
Boiler
Pump
CONDITIONAL DHW PRIORITY
The Conditional DHW Priority Override applies to
DHW MODE 2
and Boiler Minimum is not set to OFF. If the boiler supply
temperature is maintained at or above the required temperature during DHW generation, this indicates that the boiler has enough
capacity for DHW and possibly heating as well. As long as the boiler supply temperature is maintained near its target, DHW and
heating occurs simultaneously.
DHW POST PURGE
After the
DHW Demand
is removed, the control performs a purge on the boiler. The control shuts off the boiler 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 into the DHW tank. The control continues this purge for a maximum of four 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 drops below the current boiler target temperature.
DHW MIXING PURGE
After DHW operation, the boiler 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 after DHW in
parallel with priority (
DHW MODE 2
), the control shuts off the boiler, but
continues to operate the DHW pump 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 DURING UNOCCUPIED
When an external
DHW Demand
is present, the control can either continue operation of the DHW system as it would during the
occupied period or the control can ignore a call for DHW as long as the control is in an unoccupied mode.
Section E2: DHW with Low Temperature Boilers
If DHW is to be incorporated into a low temperature system such as a
radiant heating system, a mixing device is often installed to isolate the high
DHW supply temperature from the lower system temperature. If a mixing
device is not installed, high temperature water could be supplied to the
low temperature system while trying to satisfy the DHW demand. This may
result in damage to the low temperature heating system. The control is
capable of providing DHW in such a system while maximizing the chance
that the temperature in the heating system does not exceed its allowed
Boiler Maximum
setting.
To prevent high temperature water from being introduced into the heating
system, the primary pump (
Prim
P1)
must be turned off during a call for
DHW. To do this, the control must be set to
DHW MODE
2 or DHW
MODE
4
and
Boil
MIN
must be set to
OFF
.
DHW
Pump
Primary
Pump
Boiler
Pump