Bosch Heatronic 4000 Installation & Operating Instructions Manual page 15

Priority Override - -- - - - - - ------- -------- -------- --------- --------------------------- ----------- --
Priority override applies when IDHW Priority is set to ON &
prevents the building from cooling off too much or the possibility
of a potential freeze up during IDHW priority.
When set to auto, the priority time is calculated based on outdoor
temperature. At or below the design outdoor temperature, 15
minutes is allowed for IDHW priority. At or above the design
indoor temperature, 2 hours is allowed for IDHW priority. The
time allowed for IDHW priority varies linearly between the
above two points. There is a manual setting also available in
the Setup menu.
The priority timer does not start timing until priority is selected
& both a DHW Call & a Heat Call exist together. Once the
allowed time for priority has elapsed, the control overrides the
DHW priority & resumes space heating.
Requirements : Auto is only available when the application
mode is set to RSET.
Automatic Priority Override
Indoor Design
Temperature
Conditional IDHW Priority -------------- ------- ----------
If the boiler supply temperature is maintained at or above the
required temperature during IDHW operation, this indicates that
the boilers have enough capacity for IDHW & possibly heating
as well. As long as the boiler supply temperature is maintained
near the target, IDHW & heating occurs simultaneously.
IDHW Post Purge - - - - - - - --------- -------- ------------ --- - - - ------------------------------------
After the DHW Call is removed, the control performs a purge.
The control shuts off the boilers & continues to operate the
IDHW Pump & the primary pump if applicable. This purges the
residual heat from the boilers into the DHW tank. The control
continues this purge until one of the following occurs:
1. A Heat Call is detected.
2. The boiler supply drops 20°F (11.0°C) below the DHW
target temperature.
3. The DHW tank temperature rises above the DHW setpoint
plus 1/2 DHW Differential.
4. Two minutes elapse.
IDHW Mixing Purge - - - - - --- ----------- ---------- ------- - - - - - - --- -----------------------------
After IDHW operation, the boiler is extremely hot. At the same
time, the heating zones may have cooled off considerably after
being off for a period of time. When restarting the heating
system after a DHW Call with priority, the control shuts off the
boiler & continues to operate the IDHW pump while the primary
pump is turned on. This allows some of the DHW return water
to mix with the cool return water from the zones & temper the
boiler return water.
BTC461503101C | 6720810369 (2018/03)
2 hours
15 mins
Outdoor Design
Temperature
----- ----------------- ---------
IDHW with Low Temperature Boilers -- -- -- - - -- -- -- -- -- --
If DHW heating is to be incorporated into a low temperature
system such as a radiant floor 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 Call.
This may result in damage to the low temperature heating
system.
The control is capable of providing IDHW heating in such a
system while minimizing the chance that the temperature in the
heating system exceeds the design supply water temperature.
In order to do this, the following must be true:
• All available boilers are set to condensing.
• IDHW Location set to PRIM.
• Primary Pump DURING IDHW set to OFF.
• IDHW Priority set to ON.
On a DHW Call, the control provides IDHW priority by turning
off the primary pump for a period of time. The length of time is
based on the outdoor air temperature, or selectable time, as
described in the IDHW Priority Override section. However, if
the DHW Call is not satisfied within the allotted time, the boiler
shuts off & the heat of the boiler is purged into the DHW tank. A
IDHW mixing purge occurs in order to reduce the boiler water
temperature & once the boiler supply temperature is sufficiently
reduced, the IDHW Pump contact shuts off. The primary pump
is allowed to turn on for a period of time to prevent the building
from cooling off. After a period of heating, & if the DHW Call is
still present, the control shuts off the primary pump & provides
heat to the DHW tank once again.
IDHW Boil COUNT IDHW - -- -- -- - -- -- -- -- -- - -- -- -- -- - -- --
Selects the number of boilers to be used for IDHW operation.
This applies when only there is a requirement for IDHW. All
available boilers are allowed to operate if there is both a
requirement for space heating & IDHW.
15 of 60
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Heatronic 4000