Service manual

17
Mode 2: Constant Temperature Control at
System Sensor
This mode is designed for hydronic heating.
Once a heat demand is present, the BTC 1
modulates the boiler burner to maintain the
boiler target at the system sensor. A heat
demand is generated when 24VAC is applied
across CD (common demand) and Ht D (heat
demand). Dem 1 on the LCD display is lit.
If the system sensor is ½ (half) of the differential
below the BOIL TARGET, the BTC 1 then
changes the proportional modulation output to
the START modulation setting, the Stage
contact (pins 15 & 16) closes to proceed to trial
for ignition. The burner remains at minimum
modulation until the flame is proved and then the
modulating output changes the boiler burner
output to maintain the programmed boiler target
temperature at the system sensor. If the system
sensor reaches ½ (half) of the differential above
BOIL TARGET setting, the burner shuts off.
Once the external heat demand is removed, the
BTC 1 turns off the appliance and operates the
boiler pump based on the PUMP DELAY setting.
In this case, it is imperative that the system
pump operates continuously in order to provide
constant circulation past the system sensor.
The water temperature is controlled based on a
fixed setpoint (BOIL TARGET).The setpoint for
inlet water is pre-set to 120
o
F and the auto re-
set limit is set to 210°F and is fixed. In addition
to the auto reset limit the factory installs a
manual re-set limit set to 250°F.
Figure 17: Mode 2 Piping & Electrical Layout
Mode 3: Dedicated Domestic Hot Water
Operation
This mode is designed for domestic hot water. A
DHW sensor must be inserted into a
temperature immersion well within the DHW
tank to function properly. The BTC 1 modulates
the boiler based on the boiler inlet sensor to
maintain a tank temperature at the DHW sensor.
An internal heat demand is generated when the
DHW sensor drops ½ (half) of the tank
differential setting below the desired DHW tank
temperature. Dem 1 is lit on the LCD screen.
The BTC 1 then changes the modulation output
to the START modulation setting and closes the
Stage contact (pins 15 & 16) to proceed to trial
for ignition. The burner remains at minimum
modulation until the flame is proved and then
modulating output changes the boiler output to
maintain the programmed boiler target
temperature at the boiler inlet sensor. Once the
DHW tank reaches ½ of the tank differential
above the TANK TARGET setting, the internal
demand is removed and the boiler burner is shut
off. The pump circulates until the PUMP DELAY
timer expires.
The TANK TARGET setting is used to set the
desired DHW tank setpoint. The set-point for
inlet water is pre-set to 130°F and can be
adjusted, the auto re-set limit is set to 210°F and
is fixed. In addition to the auto reset limit the
factory installs a manual re-set limit set to 210°F .
Figure 18: Mode 3 Piping & Electrical Layout
Mode 4: Outdoor Reset using Boiler Inlet Sensor