tekmar 268 Boiler Control - Manual (Page 13)

13 of 32 © 2010 D 268 - 11/10
CONDITIONAL DHW PRIORITY
The Conditional DHW Priority Override applies to DHW MODE 2 and 4. 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 and the heating and DHW targets are
similar, 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 system and boiler 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 two 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 is close to the current boiler target temperature.
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. The length of DHW priority time is determined using
the Priority Override setting. Once the allowed time for priority has
elapsed, the control overrides the DHW priority and resumes space
heating.
To provide external DHW priority in DHW Mode 4, the space heating
zones must be interlocked with the Relay 9 / DHW contact. During
DHW demands, the Relay 9 / DHW contact must remove any power to
all space heating zone valves or zone pumps.
DHW MODE 4 – DHW in Primary / Secondary with Priority
When a DHW Demand is present, the Relay 9 / DHW contact (terminals
30 and 31) is closed and the primary pump contact is closed. Priority
can only be obtained using external wiring. During a priority override,
the Relay 9 / DHW 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
Pump
Primary
Pump Boiler
Pump
Disable
Using External
Wiring
DHW MODE 4
External Priority
Interlock
Power to External
Boiler Zones
N.C.
L
DHW
Pump
N
N.O.
L
N
COIL
Power from DHW
Pmp/Vlv contact
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.
NUMBER OF BOILERS USED FOR DHW GENERATION
The number of boilers used for DHW generation can be selected from one to the maximum number of boilers using the BOIL
DHW setting. This applies when only a DHW Demand is present. If there are other demands present, the control does not limit
the number of boilers operated.
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 in
parallel with priority (DHW MODE 2), the control shuts off the boiler(s),
but continues to operate the DHW 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
Pump
Primary
Pump Boiler
Pump