LAARS NeoTherm NTV600 - Install and Operating Manual - Manual (Page 44)

LAARS Heating Systems
Page 42
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 NeoTherm enters
"run". If the flame signal is below the threshold the
NeoTherm returns to prepurge and starts the ignition
process over. In "run" the NeoTherm 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 NeoTherm 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 NeoTherm enters "run". If the flame signal is
below the threshold the NeoTherm returns to prepurge and
starts the ignition process over. In "run" the NeoTherm
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 Neotherms 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.