Install and Operating Manual

LAARS Heating Systems

eld wire area. Single circuit 120-volt units will be eld
connected at the main terminal block labeled "120V
(L1)", "NEUTRAL" and "GROUND". See Figure 15
for over current protection ratings.
Sizes 1750, 2000 (and those which have been eld
retrotted to electrically separate the pump and heater)
require two 120-volt circuits. The heater circuit is found
on the main terminal block and denoted as "120V (L1)",
"NEUTRAL" and "GROUND". The pump circuit is
located on terminal block 1 and is denoted as "120V
(L2)", "N2 (PUMP MTD)", "GROUND 2 (PUMP
MTD)". See Figure 15 for over current protection
ratings.
External Pumps: All non-pump-mounted MT2
units have an internal (dry) contact rated for 20Amps.
A eld-supplied pump can be connected directly to this
contact if it does not draw more than 20 Amps. If more
current is required, a separate pump contactor must be
installed.
To utilize this internal pump contact, supply 120
volt single-phase power to the terminal marked “120V
(L2)” and supply the pump using the terminal marked
“L2 LOAD (EXT. PUMP)”.
NOTE:


Field Interlocks: To install other eld-wired
devices to interlock with the boiler (tank stats, louver
switches, ow switches, etc.), remove the jumper
between terminals 3 and 4 and wire the device in series
across these terminals. These terminals are located in the
upper main control panel.
External Control Wiring: These terminals are
located in the upper main control panel.
Optional Alarm Contacts: As an option, the
Mighty Therm2 is available with an alarm kit. This
alarm kit contains a dry contact for up to 120V which
is available within the 120V eld wiring panel. These
terminals will be clearly labeled "Alarm" and are rated
up to 10 amps.
5.3 External Staging Control Wiring
WARNING





AVERTISSEMENT

un incendie ou une explosion pouvant entraîner des

Ne faire seulement que les connexions conformes au

If controlling the stages of the MT2 from an
external source (multiple boiler control, BAS, etc.),
wire Stage 1 to the terminals marked "Input Stage 1"
and "Stage 1", and wire Stage 2 to the terminals marked
"Input Stage 2" and "Stage 2". The "Local/Remote"
selector switch above the terminal strip must be placed
in the "Remote" position for the external controller to
manage the staging.
If it is intended to use the MT2's on-board 2- stage
controller, no extra wiring is necessary. Ensure the
Local/Remote selector switch is in the "Local" position.
SECTION 6.
Operating Instructions
6.1 Sequence of Operation
Upon initiation of the main power switch, 120V will be
sent to multiple items. Among these are the open contact
fan relays, open contact ignitor relay, open contact pump
relay, ignition control module(s) and open contact lock
out indicators (the optional alarm package uses this). In
addition to these, 24V will travel through the optional gas
pressure switches and optional auto reset high limit and
boiler control relays. The 24V will then travel through the
block ue switch and the manual reset high limit to the
temperature controller.
Upon a call for heat from the MT2’s internal control
or an external staging control, 24V travels through
the eld interlock (if closed) and then to the ignition
control(s) “T-stat” terminal (500-750 models are
equipped with one ignition control and models 1000-
2000 are equipped with two ignition controls).
When “T-stat” on an ignition control is energized, its
fan will energize and the ignition control will seek signal
from the “pressure circuit”. The pressure circuits consist
of the ow switch and proof of fan.
Models 500-1,000 calls for fan, fan prove, initiate
pump then prove ow, after which “pressure circuit” has
been satised.
Models 1250-2000 calls for pump, proof of ow,
initiate fan then prove fan, after which “pressure circuit”
has been satised. The fan prove will only occur for the
active stage that has been called.
When pressure circuit has been satised, proving
combustion air and adequate water ow, the hot surface
igniter will initiate. The ignition control checks that the
igniter current reached a predetermined level then will
dwell momentarily. When dwell time is complete the gas
valve will open.
After a 4-second trial for ignition, the igniter switches
off. Unless a ame is detected by the ame sensor (a
minimum value of 0.4µA), the gas valve will close and
The ignition module will either attempt ignition again
(up to three times) or will lock out if the optional lockout
ignition module is used.
If ame is sensed, the burner will continue to re as
long as there is a call for heat and adequate ame signal.