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DHW MODE & PRIORITY OPERATION (PARALLEL PIPING ONLY)
It is often desirable to have priority for the DHW tank. Priority allows for quick recovery of the DHW tank temperature. This
is achieved by limiting or even stopping the flow of heat to the heating system when the DHW tank calls for heat. The
control allows for DHW priority only when the boiler is piped in parallel to the heating system and MODE is set to 1, 3, or
5. DHW priority is achieved by shutting off the boiler pump contact while turning on the DHW pump contact.
• When DHW MODE is set to 1 there is no DHW priority.
• When DHW MODE is set to 2 there is DHW priority.
DHW FULL & CONDITIONAL PRIORITY
The control provides full DHW priority when BOIL MIN is set to OFF. This is intended for use with condensing boilers.
The control provides conditional DHW priority when BOIL MIN is not set to OFF. Should the DHW and the heat demand
targets be similar in temperature, and the boiler target is maintained at the operating sensor, then simultaneous DHW and
space heating occurs.
DHW PRIORITY OVERRIDE
Should a DHW demand and a heat demand be present at the same time, the control limits the amount of time that DHW
heating has priority over space heating. The amount of time is dependant on the Mode setting.
For Mode 3, the amount of time allowed for DHW priority is based on the measured outdoor air temperature. When the
outdoor temperature is equal to the OUT DSGN setting, the priority time is 10 minutes. When the outdoor temperature is
equal to the OUT START setting, the priority time is 2 hours. As the outdoor temperature changes between the OUT DSGN
and the OUT START setting, the priority time increases linearly.
For Modes 1 and 5, the amount of time allowed for DHW priority is fixed at 1 hour.
Once the DHW priority time elapses, the DHW priority override operation is dependant on the BOIL MIN setting.
While in DHW priority override and the BOIL MIN setting is not set to OFF, the DHW pump and the boiler pump are
operated together in order to provide simultaneous DHW and space heating. The control can resume DHW priority should
the boiler target be satisfied for 5 minutes.
While in DHW priority override and the BOIL MIN setting is set to OFF, the control performs a DHW post purge to lower
the boiler temperature, the DHW pump is shut off, and then space heating is resumed. This operation is intended for oper-
ation with condensing boilers.
DHW POST PURGE
After the DHW Demand is removed, the control performs a purge on the boiler. In MODE 1, 3, or 5 (parallel piping), the
control shuts off the burner and continues to operate the DHW pump. In MODE 2, 4, or 6 (primary / secondary piping),
the control shuts off the burner and continues to operate the DHW pump while the boiler pump is turned on. This purges
the residual heat from the boiler 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 opera-
tion. The control also stops the purge if the operating sensor temperature is close to the current boiler target temperature.
DHW MIXING PURGE
After DHW priority, the space heating zones may have cooled off considerably after being off for a period of time;
meanwhile, the boiler is hot. To avoid thermally shocking the boiler after DHW priority, the control shuts off the burner, but
continues to operate the DHW pump while restarting the boiler pump. This allows some of the DHW tank return water to
mix with the cool return water from the zones and temper the boiler return water.
12
Model SF-10 Control Instructions Manual
Boiler
Outlet
Sensor
Boiler
Outlet
Sensor
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