LAARS Mighty Therm VW-PW (Sizes 175-400) - Installation, Operation and Maintenance Instructions - Manual (Page 27)

Mighty Therm Volume Water Heaters
Page 27
5.2.4 Testing the Manual Reset
Hi-Limit Switch
To test the manual reset hi-limit switch (see
Figure 36):
1. Touch the other lead of the voltmeter to both
terminals of the manual reset hi-limit switch. The
voltmeter should read 24 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.
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. Check flow switch paddles to
determine if they are broken, missing or frozen.
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 heater flue collector
compartment. To test the fusible link (flame roll-out
switch, if installed):
1. Touch the voltmeter lead to the lower terminal on
the terminal block (see Figure 37).
2. If the voltmeter reads voltage, the safety fuse and
the fusible link are good.
3. Replace the fusible link when an open circuit is
detected. An open fuse line indicates overheating
in the flue collector or compartment.
4. Check for leaks of the flue collector, vent pipe,
gaskets, and all connections.
Figure 38. Pilot location (spark ignition).
Figure 39. Pilot location (standing pilot).
Figure 40. High voltage ignition test.
5. Check the flue pipe for blockage.
6. Check heat exchanger fins for partial sooting.