Low Water; Mrhilimit - A.O. Smith 100 Series Service Handbook

DISPLAYED MESSAGE/CONDITION/INDICATES

Low Water

The Control System has detected that water is not being
sensed by the optional LWCO (Low Water Cut Off) device
sensor.
Important Service Reminder:
When performing any troubleshooting steps outlined in
this manual always consider the wiring and connectors
between components. Perform a close visual inspection
of all wiring and connectors to a given component before
replacing any electrical part or component.

MRHiLimit

The Control System has detected the manual reset high
limit switch is open.
Operational Notes:
The Manual Reset High Limit switch is a bimetal switch
built into the Outlet Probe (two red wires from probe). This
is a normally closed switch that opens on a temperature
rise at 244°F (118°C) ± 5.4°F (± 3°C). The purpose of this
switch is to prevent excessive outlet water temperatures.
The setting is non adjustable/fixed. When the outlet water
temperature from the boiler reaches 244°F the switch
will open its contacts. The Control System monitors the
switch contacts and will declare a fault condition any time
the contacts open during a heating cycle. The Control
System will display the MRHiLimit Error Message on the
UIM's LCD display.
To reset the lock out condition, press the Select Button
on the UIM while the Error Message is displayed. Keep
in mind the outlet water temperature must cool down to
approximately 180°F (82.2°C) before pressing the Select
Button will reset the control.
52 Servicing should only be performed by a Qualified Service Agent
CHECK/REPAIR
• Ensure there is water in the lines/boiler.
• Remove and clean LWCO sensor. The LWCO sensor
is located inside the upper compartment of the Boiler
threaded into the upper head of the Heat Exchanger.
One red wire leads from the LWCO Circuit Board on
the Control Panel, see page 17.
• If the SW1 #6 dipswitch is configured for LWCO
present when the optional control is not installed
the Control System will display a false a Low Water
error message. If there IS NOT a LWCO installed
on the Boiler ensure the SW1 dipswitch is set to the
off position, see page 23. Power to the Boiler must be
cycled off and on again for the configuration change
to take effect.
Service Note:
SW1 dipswitch settings can be confirmed in the Config
Settings menu, see Control System Navigation on
page 40 and Control System Menus on page 44.
• Check all wiring, plugs, and sockets between the
J4 Socket on the MCB board and the LWCO circuit
board. Ensure wiring is not pinched or cut, ensure all
sockets and plug connections are mating properly
and providing good contact - repair/replace any
damaged or worn wiring or parts.
• Call the toll free support phone number on the back
cover of this manual for further assistance if the
problem has not been corrected after performing the
procedures outlined here.
• Check all wiring, plugs and sockets between the
MCB J7 Socket and the Outlet Temperature Probe.
Ensure all connectors, plugs and sockets are making
good contact. Repair/replace any damaged wiring or
connectors. See MCB - Section E on page 24.
• Check the outlet water temperature, ensure the outlet
water temperature from the boiler is not reaching
244°F (118°C).
• If the outlet water temperature is not at or above
244°F (118°C) disconnect the MCB J7 wiring harness
plug (page 24) and check for continuity between pins 1
and 2 on the plug end (two red wires). If the circuit is
open, replace the Outlet Temperature Probe.
• Ensure the outlet water valve has not been throttled
too far in the closed position during water flow set up
and causing excessive outlet water temperatures,
see Water Flow Setup on page 37.
• Check temperature rise (ΔT / Delta T) through the
boiler at 100% firing rate. Ensure ΔT is not above
30°F. De-lime boiler if necessary - lime accumulation
within the boiler (over time) can reduce water flow
and cause excessive temperature rise.
• Call the toll free support phone number on the back
cover of this manual for further assistance if the
problem has not been corrected after performing the
procedures outlined here.