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BOILER WITH TANKLESS COIL:
Aquastat high limit controller:
The aquastat control’s high limit contacts open and
turn off the burner when the boiler water temperature
reaches the control’s high limit
set point. The high limit contacts
automatically reset after the
boiler water temperature drops
past the set point by 10°F, which
is a xed differential.
Aquastat low limit and circula-
tor control:
With the adjustable differential at
the minimum setting of 10°F:
• On a boiler water temperature
rise above the low limit set
point, the burner circuit
contacts (RB) break and
the circulator circuit contacts
(RW) make
• On a boiler water temperature
drop of 10°F below the low
limit set point, the RB contacts
make and the RW contacts break.
With the adjustable differential setting greater
than 10°F:
• When the boiler water temperature is at (or below)
the low limit set point, the RB contacts make and
re the burner and the RW contacts break leaving
the circulator off. The burner will re until the boiler
water temperature reaches the low limit set point
plus the adjustable differential setting minus 10°F.
At this point the RB contacts break stopping the
burner and the RW contacts make. With no call
for space heating, operating in this way keeps the
circulator off and allows the boiler to give prefer-
ence to the domestic water heating requirements.
Example: Consider the boiler water temperature
drops to 150°F with the low limit set at 160°F and
the differential is set at 15°F. The burner will begin
to re. When the aquastat probe senses a boiler
water temperature of 165°F (I5 °F-10°F = 5°F, then
160°F +5°F =165°F) the RB contacts break and the
RW contacts make.
When the boiler water temperature rises above
the low limit / adjustable differential setting and
the thermostat controls call for domestic space
seQUeNCe OF OperaTiONs
heating. The sequence of operation for the burner
and circulator is as follows:
1. Thermostat calls for heat, completing circuit be-
tween terminals T & T on the aquastat controller,
energizing the 1K relay coil.
2. With the 1K relay coil energized,
contacts 1K1 and 1K2 are closed.
Terminal B1 is energized, provid-
ing power to the oil burner primary
control. This, in turn, powers the
ignition coil and burner motor.
3. With the primary control en-
ergized, the burner operation
starts and remains running as
long as the cad cell senses
flame. In the event of flame
failure or the ame is not fully
proven within the trial for ignition
period, the primary control will
lockout and open the burner cir-
cuit. This will require a manual
startup of the burner.
4. As long as ame is proven
through the cad cell relay, the
burner will remain on until the
circuit is interrupted by the boiler water tem-
perature reaching the aquastat’s high limit setting
(opening high limit contacts RB), or the thermostat
is satised breaking the TT circuit.
5. Circulator pump is powered through terminal C 1 and
will run when boiler water temperature is above the
low limit setting and there is a call for heat from
the thermostat. If the boiler water temperature
reaches the aquastat’s high limit setting and high
limit contact RB is de-energized, the burner stops
but the circulator pump will continue to run, as long
as, the thermostat calls for heat.
6. Whenever the boiler water temperature reaches the
aquastat’s high limit setting, the high limit contact
RB is de-energized shutting the burner off. Then the
boiler water temperature must fall 10°F below the
aquastat’s high limit setting for the high limit contacts
RB to close and the burner to be energized.
7. When the thermostat is satised, the call for domestic
space heating is ended. Relay coil 1K is de-energized,
opening 1K1 and 1K2 contacts. Both the burner and
circulator pump operation stop.
NOTE: High limit setting must be a minimum of
20º higher than the low limit setting.
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