tekmar 374 Universal Reset Control - Manual (Page 11)

11 of 36 © 2009 D 374 - 03/09
DHW Pump
Primary
Pump
Mix
System
Pump
Disable
Using External
Wiring
DHW
Pump
Primary
Pump
DHW MODE 4 - DHW in Primary / Secondary with External Priority
When a DHW Demand is present, the DHW Pmp / Vlv contact is closed and the primary pump is operated. Priority can only
be obtained using external wiring on boiler zones. The mix system pumps continue to operate but the variable speed pumps(s)
is shut off or the mixing valve(s) is closed. This allows DHW priority over the mix zones. During a priority override, the
DHW
Pmp / Vlv 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 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. As the outdoor air temperature becomes colder, the
length of time that the control provides DHW priority is reduced. Once
the allowed time for priority has elapsed, the control overrides the DHW
priority and resumes space heating.
To provide external DHW priority, the boiler temperature space heating
zones must be interlocked with the DHW Pmp / Vlv contact. During DHW
demands, the DHW Pmp / Vlv contact must remove any power to all
boiler temperature space heating zone valves or zone pumps.
Increasing Air Temperature
Incr
easing Time
DHW priorit
y demand time limit
Outdoor Air Temperature
DHW Priority Override DHW Priority Override
DHW MODE 4
External Priority
Interlock
Power to External
Boiler Zones
N.C.
L
DHW
Pump
N
N.O.
L
N
COIL
Power from
Boiler 3 / DHW
Contact
CONDITIONAL DHW PRIORITY
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, DHW and heating occurs simultaneously.
DHW POST PURGE
After the DHW Demand is removed, the control performs a purge 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 primary 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 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(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
priority, the control shuts off the boiler(s), 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
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.