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DHW MODE 4 - DHW IN PRIMARY / SECONDARY WITH
PRIORITY
When a DHW Demand is present, the Relay 9 / DHW con-
tact (terminals 30 and 31) is closed and the primary pump
contact is closed. Priority can only be obtained using exter-
nal wiring. During a priority override, the Relay 9 / DHW
contact is opened until the heating system has recovered
before returning to DHW operation, see Figure 19. 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 pos-
sibility of a potential freeze up during DHW priority, the con-
trol limits the amount of time for DHW priority. The length of
DHW priority time is determined using the Priority Override
setting. Once the a llowed t ime 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, Figure 20. During DHW demands, the Relay 9 /
DHW contact must remove any power to all space heating
zone valves or zone pumps.
CONDITIONAL DHW PRIORITY
The Conditional DHW Priority Override applies to DHW
MODE 2 and 4. If the boiler supply temperature is main-
tained 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 boil-
er supply temperature is close to 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 consid-
erably after being off for a period of time. To avoid thermal-
ly shocking the boiler(s) after DHW in parallel with priority
(DHW MODE 2), the control shuts off the boiler(s), but con-
tinues to operate the DHW while restarting the heating sys-
tem. This allows some of the DHW return water to mix with
the cool return water from the zones and temper the boiler
return water (FIG. 21).
DHW DURING UNOCCUPIED
If the control receives a DHW Demand during an unoccu-
pied 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.
FIG. 19
FIG. 20
FIG. 21
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