Boiler Minimum: The boiler minimum is a value that
is used to limit the minimum temperature that the
boiler can target as a supply setpoint.
This is useful if the boiler is used in conjunction with
fan coil units, kick space heaters, unit heaters or other
instances where a low boiler supply temperature can
cause uncomfortable conditions.
Boiler Maximum
: If the boiler is connected in a
situation where a temperature that is too high can
cause damage to tubing or floors, this will limit the
maximum temperature that can be achieved.
Unlike the boiler design temperature, the control will
not let the boiler run if the supply is at this temperature.
The default for this temperature is 180°F (82°C).
Boost Function
: The boost function increases the boiler
supply target temperature by the value, “Temp”, if the
demand is not satisfied in the value, “Time.” Both of
these values are selected by the installer.
The default for these values are as follows:
Temp = 18°F and Time = 20 minutes.
This function is designed to help the boiler recover
from a setback condition. It will also help if too low of
a value is chosen for the boiler design and/or mild
weather temperature.
This value will continue to increase by the Temp value
if the demand continues through consecutive Time
periods.
The value is allowed to exceed the design temperature
but can never exceed the boiler maximum temperature.
Warm Weather Shutdown
: In any CH mode utilizing
outdoor reset, the warm weather shutdown feature
prevents the boiler from operating if the outdoor
temperature exceeds the selected temperature. The
default value for this function is 70°F (21°C).
4. Other Features:
Pump Post Purge
: The PUREFIRE
®
control allows the
installer to set post purge times for the CH and/or
General Pumps. The circulator will continue to run for
the time selected after the call for heat has ended.
These values are settable from 0 to 900 seconds and
the factory default is 60 seconds.
Ramp Delay Step Modulation
: This feature is useful
for systems with very small zones which can cause a
boiler to short cycle. The Ramp Delay Step
Modulation function only applies to Central Heating
and is not applied to DHW demands.
After the ignition sequence, the control modulates
down to 20% of input. Then the input rate is
gradually increased until the supply temperature
approaches its target. At this time, the control
modulates normally.
Anti-Cycling
: The PUREFIRE
®
boiler control allows the
installer to set a minimum off time to prevent short
cycling of the boiler. This only affects short cycling
due to reaching the temperature limit setpoint. If the
CH or DHW demand cycles, the control will cycle to
match the demand.
“The minimum off time can be set from 0 to 15
minutes in 1 minute increments. It has a default of 3
minutes.
46
BOILER CONTROL: INTERNAL WIRING & OPERATION
Figure 8.6: Outdoor Reset Operation
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