The design point is shown for an outdoor design
temperature of 0°F and a boiler design supply
temperature of 160°F. The mild weather point is
shown for an outdoor temperature of 70°F and a
boiler supply temperature of 95°F.
From this, the P
UREFIRE control calculates a boiler
supply temperature of 130°F when the outdoor
temperature is 32°F. Table 8.8 shows common
values for the boiler temperatures based on
different types of heat distribution units.
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.
37
BOILER CONTROL: INTERNAL WIRING & OPERATION
Figure 8.8: Outdoor Reset Operation
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