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© 2007
D 422 - 08/07
DHW with Low Temperature Boilers
Section I
If DHW heating is to be incorporated into a low temperature
system such as a radiant floor 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 heating in such a
system while minimizing the chance that the temperature
in the heating system exceeds the design supply water
temperature. In order to do this, the control must be set
to DHW MODE 2 or DHW MODE 4 and Boil MIN must be
set to OFF.
On a call for DHW, the control provides DHW priority by
sending a message on the boiler temperature bus to the tN4
thermostats to shut off the heating zones for a period of time.
The length of 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. A DHW mixing purge occurs in order to reduce
the boiler water temperature and once the boiler supply
temperature is sufficiently reduced, the DHW Pump contact
shuts off. The heating system zones are allowed to turn 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.
Setpoint Temperature Operation
Section J
Setpoint
The control can operate to satisfy the requirements of a
setpoint load in addition to a space heating load and a
DHW load. A setpoint load overrides the current outdoor
reset temperature and WWSD setting in order to provide
heat to the setpoint load.
Setpoint Demand
A Setpoint Demand is required in order for the control to
provide heat to a setpoint load.
The control registers a setpoint demand when a voltage
between 24 and 230 V (ac) is applied across the Setpoint
Demand terminals (55 and 56). Once voltage is applied,
the Setpoint Demand segment turns on in the LCD.
Conditional DHW Priority
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 the 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 the DHW pump and the primary pump
(P1) 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 (-6.5°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. When restarting the
heating system after a DHW demand with priority, the control
shuts off the boiler and continues to operate the DHW pump
while the primary pump is turned on. 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
OFF
