Bradford White RE3 Service Manual page 7

Non-Simultaneous and Simultaneous Operation
Double element electric water heaters are designed to operate in either Non-Simultaneous or Simultaneous mode.
Non-Simultaneous Mode: Allows only one heating element to operate at a time. For example, when the tank is cold,
the upper element is energized first, heating the top of the tank. Only when the upper thermostat is satisfied, the upper
element is de-energized and power is directed to the lower thermostat, energizing the lower element and heating the
bottom portion of the tank until the lower thermostat is satisfied. As hot water is drawn off the tank, it is replaced with cold
water delivered through the diptube to the bottom of the tank. When the tank cools at the lower thermostat level, the
lower thermostat will call for heat, energizing the lower element. If enough hot water is drawn from the tank, the top
portion of the tank cools and the upper thermostat will call for heat, de-energizing the lower element and allowing only
the top element to energize until the upper thermostat is satisfied.
Simultaneous mode: allows both heating elements to operate at the same time. That is, if either thermostat (upper or
lower) is calling for heat, the corresponding heating element is energized independent of the other.
Sequence of Operation- Double Element, Non-Simultaneous Operation, Single Phase.
1
Line voltage is applied across terminals
L1 & L3 of the upper thermostat. ECO is
closed, so there is voltage at terminal L4
and to one side of the upper and lower
elements.
2 Tank is cold. Therefore, the thermostats
are closed at terminals T2 & 2 (calling for heat).
The circuit is complete through the upper
thermostat only, allowing current to flow through
upper element.
3
When the upper thermostat is satisfied, it opens at
terminal T2, interrupting current flow through the
upper element. Terminal T4 closes, allowing voltage
to pass to terminal 1 of the lower thermostat. This
completes the circuit through the lower thermostat
and allows current flow through the lower element.
4
When the lower thermostat is satisfied, it opens at
terminal 2, interrupting current flow through lower
element. The system is now in stand-by mode
waiting for the next call for heat.
5
The lower thermostat/element
combination will generally
cycle on and off more often
than the upper. In some cases,
such as a cold tank or in high
demand periods, the upper
thermostat will call for heat
(opening at terminal
T4 and closing at
terminal T2) prior to the lower
thermostat being satisfied. This
will interrupt current flow
through the lower thermostat
and element and allow current
to flow through the upper
element only. When the upper
thermostat is satisfied, it
resumes operation as
described in sequence #3
above.
3
Thermostat closed
at terminal T4
Upper
T'stat
Upper
Element
Lower
T'stat
Lower
Element
SEQUENCE OF OPERATION
1
ECO
Closed
Upper
T'stat
Upper
Element
Lower
T'stat
Lower
Element
4
Upper
T'stat
Upper
Element
Thermostat open
between terminals
1 and 2
Lower
T'stat
Lower
Element
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2
Thermostat closed
at terminal T2
Upper
T'stat
Upper
Element
Thermostat closed
at terminal 2
Lower
T'stat
Lower
Element
5
Thermostat closed
at terminal T2
Upper
T'stat
Upper
Element
Thermostat closed
between terminals
1 and 2
Lower
T'stat
Lower
Element
7