tekmar 374 Universal Reset Control User Manual

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Section C2: DHW with Low Temperature Boilers

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

Setpoint Demand is required in order for the control to provide heat to the setpoint load. The

control registers a setpoint demand when a voltage between 24 and 230 V (ac) is applied
across the

Setp Dem and Com Dem terminals (28 and 31). Once voltage is applied, the

Setpoint Demand pointer turns on in the LCD. The control operates the boiler(s) to maintain
at least the setpoint setting.

DHW

Pump

Primary

Pump

DHW

Dem

Boil

Dem

Com

Dem

Setp

Dem

24 to 230 V (ac)

Setpoint

Device

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(s) 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 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 powered with the DHW Pmp / Vlv contact.

During a DHW Demand, the control closes the primary pump (

Prim P1) contact and the DHW Pmp / Vlv contact. Once the DHW

Demand is removed, or during a DHW priority override, the DHW Pmp / Vlv 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.

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.

Section D1

Setpoint

Section D: Setpoint Operation

Section D1: Setpoint