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BRADFORD WHITE CORP.
Page 42
firing rate is controlled by the DHW setpoint
temperature and the water temperature at the heat
exchanger outlet. Once the DHW call is supplied the
control starts the boiler and DHW pumps and begins the
ignition process. The ignition process consists of a
prepurge, trial for ignition, and run period. The prepurge
period starts on a call for heat once the fan RPM reaches
a preset level. The trial for ignition period follows once
the start-up RPM of the fan is reached. The trial for
ignition period lasts 4 seconds during which the direct
spark ignition system and gas valve are energized. At
the end of the trial for ignition period the flame signal is
compared with the preset flame threshold value. If the
flame signal is higher than the flame threshold the Brute
Elite enters "run". If the flame signal is below the
threshold the Brute Elite returns to prepurge and starts
the ignition process over. In "run" the Brute Elite
monitors the flame signal, call for heat, safeties, and
water temperatures modulating as needed between
100% and 20% of rate to satisfy the call for heat.
9.9 Domestic Hot Water Heating Using
External Modulation Control
When using the Brute Elite for DHW heating with
external modulation control a call for heat must be supplied
to the "T-T or Interlock" terminal. By supplying a call to
the "T-T or interlock" terminal the operating temperature
setpoint defaults to the hydronic heating setpoint, so all of
the temperature adjustments must be made to the hydronic
setpoint. Once the call is supplied, the control starts the
boiler and system pumps and begins the ignition process.
The ignition process consists of a prepurge, trial for
ignition, and run period. The prepurge period starts on a
call for heat once the fan RPM reaches a preset level. The
trial for ignition period follows once the start-up RPM of
the fan is reached. The trial for ignition period lasts 4
seconds during which the direct spark ignition system and
gas valve are energized. At the end of the trial for ignition
period the flame signal is compared with the preset flame
threshold value. If the flame signal is higher than the flame
threshold the Brute Elite enters "run". If the flame signal is
below the threshold the Brute Elite returns to prepurge and
starts the ignition process over. In "run" the Brute Elite
monitors the flame signal, call for heat, safeties, and water
temperatures. The heater setpoint is used to limit the
maximum water temperature leaving the boiler only. The
modulation rate is controlled by a 4-20mA (0-10Vdc using
converter) signal supplied by an external control. When
setting up a system using an external control care must be
taken to set the external control algorithms to prevent the
boiler from short cycling or "hunting " to prevent prema-
ture component failure.
9.10 Domestic Hot Water Heating Using
Local Lead-Lag/Cascading Feature
When using multiple Brute Elites to supply
Domestic hot water only the heaters should be setup as
stated in Section 8.12 “Lead/Lag Cascading,” and
Section 9.5 “Hydronic Heating Using Local Lead/Lag
Cascading Feature.” When the heaters are installed in
this fashion, all of the setpoints and adjustments are
done through the Lead Lag menus.