LAARS SUMMIT SMW-250 - Install and Operating Manual - Manual (Page 24)

Page 24 LAARS Heating Systems
ADJUSTMENT OF THE OFFSET PRESSURE
MUST ALWAYS BE DONE WITH THE
APPLIANCE AT FULL OUTPUT.
Note: During the prepurge cycle, the gauge will
indicate several inches W.C. negative pressure.
7.2.3 Safety Limit Switch
The Safety Limit Switch is a manual reset switch
with a fixed set point of 245°F (118°C). To replace
the switch, shut off the 120-volt power and valve off
the appliance. Remove the switch from the jacket top
and disconnect the two wires from the switch. Drain a
gallon of water from the appliance drain and reclose.
Loosen the compression nut at the mixing valve and
remove the sensing bulb. To replace, perform the
same operations in reverse. When tightening the
compression nut, do not overtighten. After isolation
valves have been opened, bleed air from the appliance
through the air vent on top of the mixing valve.
Switch on electrical power and restart appliance.
7.2.4 Appliance Control, Printed Circuit
Board (PCB)
The Integrated Boiler Control Module controls
the ignition process, the combustion process and the
supply temperature and provides space/hot water
heating on demand. The supply temperature may be
reset by an optional sensor such as an outdoor air
sensor to improve efficiency.
To do this, the control receives inputs from two
(one optional) temperature sensors, an overheat
thermostat (safety limit) and external heating control
switch, such as a room thermostat, zone valve end
switches, circulator relay contacts or aquastat. It then
controls the pump, blower, ignitor and gas valve
sequencing.
It constantly displays the appliance outlet
temperature (Bft) when the appliance is operating in
one of its demand modes. It also allows the user to
find up to 8 previous faults stored in memory. The
table of detectable faults is included in Section 8.3.
If replacement of PCB is necessary shut off the
120-volt power and disconnect the wire connectors
from the PCB. Remove the PCB from its standoffs.
Replace in reverse order ensuring that the connections
are correct.
7.2.5 Ignitor / Flame Sensor Assembly
The ignitor is a “Hot Surface” type. It is
energized whenever there is a call for heat and
switched off when ignition is established and the
flame has been sensed.
The flame sensor is a remote sensing flame rod
that is connected directly to the circuit board “J5”
terminal.
If the ignitor fails and the assembly must be
replaced, always install a new ignitor gasket with the
replacement assembly.
For sequence of operation, see Section 8.1
Sequence of Operation.
Caution
Ignitor gets hot.
7.2.6 Transformer
The control transformer accepts 120 VAC line
voltage and provides 40 VA of 24 VAC control
voltage for the boiler control ONLY. It is NOT
capable of supplying control voltage for external
devices such as zone valves, which MUST have their
own separate power supply.
Should the transformer require replacing, shut
off the 120-volt power. Unplug the transformer wires
from the PCB, remove the mounting screws and
remove the transformer.
Replace transformer in the reverse order.
7.2.7 Blower
The combustion air blower is a high-pressure
centrifugal blower. It is powered by a line voltage,
variable speed motor; the speed of which is controlled
by the PCB. Its speed varies according to the
temperature of the appliance discharge temperature.
If a blower change is required, turn off the 120-
volt power and unplug the wires from the blower
motor. Remove the four nuts from the blower
discharge flange and the two screws that secure the
venturi to the blower housing. Remove the complete
assembly. Replace fan in reverse order, ensuring that
all joints are made correctly and sealed.
After replacement, check the combustion by
following Section 6.3 Operating the Burner and Set Up.
7.2.8 Mixing Valve
The Mixing Valve is a thermostatically
controlled device that prevents the Primary Heat
Exchanger (HX) from operating in the condensing
mode. When the water temperature returning to the
appliance is below 140°F, the mixing valve mixes
outlet water with return water to maintain the Primary
HX operating temperature above the condensation
point (dew point) of the flue products.
To change the thermostatic element, valve off
the appliance and switch off electrical power. Open
the appliance drain and drain about a gallon of water
and reclose. Remove the three cap screws from the top
of the mixing valve and remove the top. Remove the
thermostatic element and sealing ring. Remove the
sealing ring from the element and install on
replacement element. Replace the new thermostatic
element and sealing ring (with barrel up). Replace the
o-ring on the top, replace the top and cap screws and
tighten. Turn valves on and bleed air from the air vent
on the top of the valve. Switch on electrical power and
restart appliance.