Bradford-White Corp BNTH, BNTV Domestic Hot Water Heating Using External Modulation Control

Page 42 BRADFORD WHITE CORP.

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.

9.9Domestic 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.

9.10Domestic 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.