Bradford White DB-80R3DS - Manual (Page 9)

SEQUENCE OF OPERATION
Sequence of Operation- Double Element, Non-Simultaneous Operation, 3 Phase.
Line voltage is applied across terminals L1
& L3 of upper thermostat. Likewise, Line
voltage is applied to terminal L3 of lower
thermostat. ECO in both upper & lower
thermostat is closed, so there is voltage at
terminal L4 of both thermostats and to one
side of both upper & lower elements.
Tank is cold therefore both thermostats
are closed at terminal T2 (calling for heat).
The circuit is complete through the upper
thermostat only allowing current to flow
through the upper element.
1
2
When the upper thermostat is satisfied, it
opens at terminal T2 interrupting current
flow through upper element, and closes at
terminal T4 allowing voltage to pass to
terminal L1 of lower thermostat. This
completes the circuit through the lower
thermostat allowing current flow through
lower element.
When the lower thermostat is satisfied, it
opens at terminal T2 interrupting the
current flow through the lower element. The
system is now in stand-by mode waiting for
the next call for heat.
3
4
The lower thermostat/element combination
will generally cycle on and off more often
then 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.
5
ECO
Closed
1 2
Thermostat closed
at terminal T2
Thermostat closed
at terminal T2
3
Thermostat closed
at terminal T4
Thermostat closed
at terminal T2
4
Thermostat closed
at terminal T4
Thermostat open
at terminal T2
5
Thermostat closed
at terminal T2
Thermostat closed
at terminal T2
ECO
Closed
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