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© 2010 D 265 - 07/10
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
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
Section D2: DHW with Low Temperature Boilers
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 DHW Priority Override setting. However, if the DHW Demand is not satisfied within the allotted time, the boiler(s) shuts
off and the heat of the boiler is purged into the DHW tank.
Once the boiler supply temperature is sufficiently reduced, the Boiler 3 / DHW 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 MODE 4 OPERATION
In DHW MODE 4, the space heating zones must be prevented from coming on during DHW demands using external wiring. This
can be done using an external relay to remove power from zone pumps or zone valves while a DHW Demand is present. This
external relay is interlocked with the Boiler
3
/
DHW contact.
During a DHW Demand, the control closes the primary pump (Prim P1) contact and the Boiler 3 / DHW contact. Once the DHW
Demand is removed, or during a DHW priority override, the Boiler 3 / DHW contact is opened, and the external wiring should allow
the space heating zones to operate.
There is no mixing purge available in DHW
MODE
4. After DHW priority, the boiler supply water temperature may exceed the design
water temperature of the space heating system and can result in damage to the heating system.
