LAARS HP-M2 Series - Installation, Operation and Maintenance Instructions - Manual (Page 23)

H Series Page 23
11I. Transformer
The control transformer accepts 120 VAC power
and provides 40 VA of 24 VAC power for the boiler
control only. It is not capable of supplying control
power for external devices such as zone valves. They
must have their own separate power supply.
11J. Low Limit
(HW-M2 Series Only)
The H Series Mark 2 may be equipped with a
strap-on Low Limit. The Low Limit control performs
two functions. It controls the temperature of the water
in the transfer tank and it prevents the house circulator
from operating when insufficient tank water
temperature causes the domestic hot water delivery
temperature to drop below 140°F. The Low Limit
Control has a fixed differential of 15°F and is set to
turn the burner on at 140°F tank water temperature
and to turn it off when the sensor reaches 155°F.
Because of the thermal lag in the sensor, the tank
water temperature will reach about 180°F before the
burner actually is turned off. The boiler temperature
may reach operating temperature due to this thermal
lag.
If replacement is necessary, shut off the 120 volt
power. Simply remove and attach wire for wire.
Clamp the new Low Limit to the hot outlet pipe as
close to the domestic exchanger plate as possible and
cover area with tank insulation blanket. Return boiler
to service.
11K. Blower
The Combustion Air Blower is a high head
centrifugal blower. It is designed to provide about 2"
w.c. of suction at 30 CFM. This performance is
necessary to operate the gas valve reliably, to
overcome induction system friction losses and to
eliminate any sensitivity to wind striking the vent
terminal. It is powered by a 120 volt motor which
draws about 1.3 amps at rated load. It is powered by
the integrated boiler control whenever there is a call
for heat and 30 seconds thereafter. If a blower change
is required, turn off the 120 volt power and unplug the
power wires from the blower motor. Remove the three
nuts from the blower discharge flange and the four
nuts from the blower inlet flange. The blower may
now be deflected enough to permit its removal. Install
the new blower using new gaskets, in the reverse order
from which the old blower was removed. The four
inlet flange nuts, however, should only be finger tight
initially and then tightened with a wrench after all
other operations have been completed. The
combustion should be checked for correct air-fuel
ratio whenever the blower is replaced (see Burner
Adjustment).
11L. Priority Relay - R1
(HW-M2 series only)
The Priority Relay is a normally open single pole
relay which accepts a 24 VAC signal from the low
limit aquastat and the T-T wires to provide 120 VAC
power to the system circulator when the tank
temperature remains above 150°F.
11M. Boiler Control
The Integrated Boiler Control Module controls
the combustion process, the gas valve, the igniter, the
blower, the unit pump and the system circulator. It
provides blower prepurge as well as burner flame
sensing. When replacing the boiler control turn off
disconnect switch and press in tabs on each end of
plugs to remove from control. All plugs are color
coded and it is not possible to miswire the control.
11N. Stack Switch
The Stack Switch is a normally open single pole
switch which is operated by the pressure difference
across the air orifice. It is set to close when a static
pressure difference of 1 in. w.c. is generated by the
combustion air blower. Its function is to prove airflow
and to inhibit burner operation in the event of flue or
chimney stoppage. The switch is wired directly to the
boiler control. It is located on the inside of the jacket
back panel adjacent to the boiler bracket.
11O. Transfer Tank and Domestic Hot
Water Coil (HW-M2 series only)
The transfer tank contains approximately 20
gallons of boiler water. It functions as an energy
storage vessel to reduce boiler cycling on small output
heating zones and to provide heat for domestic hot
water through the domestic hot water (DHW) coil
immersed in it. The DHW coil may be removed from
the tank if either tank or the DHW coil must be
replaced. The coil is secured to the tank with (6) -3/8"
- 16 bolts which are replaceable if they are broken or
stripped.
11P. Thermostatic Union
(H(P)-M2-Series only)
The thermostatic union is a 1¼" NPTF union
which has a thermostatic element on the inside. The
element has two small bypass holes to allow some
water to flow into the system when the element is
closed. When the boiler first starts and cold system
water is returning, the element is closed and boiler
water is recirculated back to the return until the supply
water reaches 160°F and the element opens. To
replace the element shut off and drain the section of
the system that the thermostatic union is installed in.
Open the union and replace the element with a new
one. The element should be installed so that its spring
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