LAARS Mighty Therm LO-NOx VW (Sizes 250/400) - Installation, Operation and Maintenance Instructions - Manual (Page 26)

LAARS Heating Systems Page 26
1. Touch the other lead of the voltmeter to both
terminals of the manual reset hi-limit switch. The
voltmeter should read 20 to 28VAC at both
terminals.
2. If no voltage is detected at one terminal, reset the
manual reset hi-limit switch by pressing the reset
button. Check the temperature setting.
3. If after pressing the reset button there is still no
voltage indicated at one terminal, replace the
manual reset hi-limit switch. An open switch may
indicate excessive water temperatures or
improper setting.
5.2.5 Testing the Flow Switch
The flow switch is a safety device that senses
water flow through the heater. When the switch senses
adequate water flow, it closes, allowing the heater to
fire. If the water flow is too low, the switch remains
open and prevents the heater from firing regardless of
the temperature control setting.
The flow switch is factory mounted and wired.
Never attempt to repair the flow switch. If the flow
switch is found to be defective, replace the flow
switch. To test the flow switch:
1.
Remove cap from flow switch (see Figure 22).
2. Touch a voltmeter lead to each terminal of the
flow switch.
3. If the voltmeter reads voltage at each of the
terminals, the switch is good.
4. If there is no voltage at one terminal, it indicates
an open switch. Replace the flow switch.
5.2.6 Testing the Fusible Link
(flame roll-out switch)
The fusible link is a thermally fusible element
which shuts down the heater if it detects excessive
temperatures inside the heat exchanger compartment.
To test the fusible link (flame roll-out switch):
1. Remove the electrical wiring from the fuses and
check across each fuse's terminals with a
continuity or ohm meter.
2. Touch the voltmeter lead to the lower terminal on
the terminal block.
3. If the voltmeter reads voltage, the safety fuse and
the fusible link are good.
4. Replace the fuse when an open circuit is detected.
An open fuse indicates overheating in the heat
exchanger compartment.
5. Check for blockage of the heat exchanger.
5.2.7 Testing the Fuse
To test the fuse:
1. Clip a lead of the voltmeter to the grounding
terminal.
2. Touch the other voltmeter lead to terminal
number (1) one on the terminal strip.
3. If there is no voltage, replace the fuse. A blown
fuse is usually an indication of a short in the
24VAC circuit. It is important that the cause of
the short be found and repaired. Do not jumper or
bypass the fuse.
5.2.8 Testing the Ignition Control
WARNING
The ignition control and igniter operate on 120V
power. Keep this in mind while servicing the heater,
and take care to avoid electrical shock, which can
cause serious injury or loss of life.
The ignition control provides power to the
igniter, energizes the gas valve when there is a call for
heat, and senses when a flame is established. To test
the ignition control for spark ignition:
1.
Clip one lead from the voltmeter to the yellow
wire terminal on the transformer (or to terminal
number (12) twelve on the terminal strip).
2. Touch the other voltmeter lead to terminal
number (6) six on the terminal strip.
3. If the voltmeter reads voltage, the temperature
control and the manual reset hi-limit switch are
not keeping the heater from firing.
4. Make sure the flame sensor is clean and
positioned in place, the terminal connection is
tight, and the lead is at least 3/8 inch (9.5 mm)
from the heater chassis and other metal parts.
NOTE: The ignition control cannot be repaired in the
field. If it does not operate properly, replace it.
5.2.9 Combustion Air Blower
This heater uses a fan-assisted combustion
process. For proper operation of the burners, inspect
the air blower for contamination one week after start-
up and every six (6) months thereafter.
1.
To inspect and service the blower, shut off all
electrical and gas supply to the heater.
2. Disconnect the air hose from the air blower (see
Figure 39) and from the air inlet elbow.
3. Remove the screws holding the blower bracket
(see Figure 39) off the inner front panel.
4. Inspect the blower wheel. Clean the blower
housing and its wheel from any contamination or
debris.
5. Check the air hose for any blockage or rupture.
Clean any blockage, if ruptured or holes exist,
replace the hose.
6. Reassemble all parts securely in place.
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