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© 2011
D 406 - 12/11
DHW Priority Override Time
Increasing Time
Decreasing Outdoor Air Temperature
DHW Priority Override
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To prevent the building from cooling off too much or the pos-
sibility of a potential freeze up during DHW priority, the control
limits the amount of time for DHW priority during cold weather.
During warm weather the DHW priority is 120 minutes. During
cold weather the DHW priority is 15 minutes. Once the allowed
time for priority has elapsed, the control overrides the DHW
priority and resumes space heating for 15 minutes. It will then
revert back to DHW Priority and repeat.
DHW Post Purge
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After the DHW Call is removed, the control performs a purge
on the boiler. The control shuts off the boiler and continues
to operate the DHW pump and the Boiler System pump if
applicable. This purges the residual heat from the boiler
into the DHW tank. The control continues this purge for a
maximum of two minutes. The control also stops the purge if
the boiler supply temperature drops below the current boiler
target temperature.
Boiler Operation
Section G
The 406 is able to operate a single on-off or modulating boiler.
For proper operation of the boiler, the 406 must be the only
control that determines when the boiler is to fire.
*Important note:
The operating control in the boiler, also
known as an aquastat, remains in the burner circuit and acts
as a secondary upper limit on the boiler temperature. The boiler
operator temperature setting can be adjusted above 210°F
(99°C), which is the absolute maximum target on the 406.
Boiler Target Temperature
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The boiler target temperature is the temperature which the
control is trying to operate the boiler. There are four items
that determine the boiler target temperature:
+1/2 Differential
- 1/2 Differential
Boiler Target
Boiler On
Boiler Off
Boiler Differential
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Both on-off and modulating boilers are operated with a dif-
ferential. In some cases, a modulating boiler must be operated
with a differential while operating at low fire because this
indicates the load is smaller than the minimum modulation of
the boiler. For modulating boilers, the differential no longer
applies once operating the boiler above low fire.
The differential operates by closing the boiler contact when the
boiler supply water temperature is 1/2 of the differential below
the boiler target temperature. As the supply temperature reaches
1/2 of the differential above the boiler target temperature, the
boiler is shut off.
Off
Differential
Time
Sy
st
em L
oad
On
In order to decrease temperature swings and increase boiler
efficiency, the Auto Differential feature automatically adjusts the
operating differential of the boiler based on the heating load.
As the load increases, the differential will decrease to minimize
temperature swings. As the load decreases, the differential
will increase to prevent short cycling. This can significantly
improve operating efficiency and prevent equipment failures
through excessive cycling.
Boil Call
DHW Call
•
•
Setpoint Call
Backup Mode
•
•
If the control receives more than one of these at the same
time, it will operate the boiler at the highest target.
The control displays the boiler target in the View Menu. If
the control does not presently have a boiler target, ‘– – –’ is
displayed on the LCD.
On-Off Boiler Operation
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The 406 operates a single hot-water on-off boiler to maintain
the boiler target within a differential. The boiler target is the
average temperature and the boiler supply temperature can
fluctuate by 1/2 of the differential above and below of the
boiler target.
