SEQUENCE OF OPERATION
MII Series Commercial Electric Water Heaters can use either immersion thermo stat (contactor models) or
surface mounted thermostats. Sequence of operation for each confi guration is explained below. It would be impractical
to show all wire diagrams applicable to both configurations. A “typical wiring di agram” is illustrated to aid in
understanding the principles of the operating sequence.
Immersion Thermostat (Contactor Models): Configured to use a single immers ion thermostat to control one or more
contactors to energize or de-energiz e all elements simultaneously. In addition, a separate high limit (ECO) control with
manual reset is wired in series with the thermostat. Both controls use a direct immersio n bulb inserted into the tank to
sense water temperature. Immersion thermostat and high limit control ar e mounted to provide temperature adjustment
and manual reset access from the ex terior of the unit.
Surface Mounted Thermostats: Surface mounted thermostats are mo untedi nside the lower control box, in a bracket
above each heating element. The bracket holds the thermostat against the si de of the tank responding to tank surface
temperatures to sense a call for heat, set point temperature and high limit (ECO) act ivation. As each element is controlled
by a dedicated thermostat, it is possible to sequence the elements by varying the se ttings on the thermostat. However,
600V models w/ surface mounted thermostats operate contactors in the same man nera s an immersion thermostat
model, only using surface thermostats to sense the call for heat.
Sequence of operation for each system is explained below.
Sequence of Operation:
Immersion Thermostat (Contactor Models).
Line voltage is applied across terminals of fuse block
or a terminal block. Line voltage continues down and
connects to terminals L1, L2 & L 3 of one or more
contactors.
A) Contactor is open (no call for heat), so there is no
voltage across terminal T1, T2 & T3
of contactor.
B) The contactor is controlled by the control circuit
consisting of the immersion thermostat,
ECO (hi limit) and contactor coil.
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OR Terminal Block
Fuse Block
Line Voltage
Contactor
Contactor
Coil ECO
(High Limit)
T’stat
When the thermostat calls for heat, contacts close
inside the thermostat completing the circuit through
the ECO and energizing the contactor coil.
The system has two distinct circuits.
1. Power circuit (shown solid line) .
2. Control Circuit (shown dotted li ne).
The energized contactor coil causes the contactor to
close energizing the elements from terminals T1, T2
& T3 of the contactor.
3
When the temperature setting of the thermostat is
reached, the contacts in the thermostat open. This
interrupts current flow through the control circuit de-
energizing the contactor coil.
4
The de-energized contactor coil causes the contactor
to open, interrupting current flow through the
elements. The heater is now in stand-by waiting for
the next call for heat.
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Heating Elements
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