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34-55 08/14
9.5 Modulating
Firing
Sequence
The boiler is fully modulating and initiates combustion at the lowest firing rate. It will modulate between this lowest
possible rate and full rate as the boiler loop temperature rises and falls in relation to the operating set point. The
Parameters
involved with the modulation control sequence are
St1
,
Hy1
St4,
and
SR.
LABEL
DEFAULT VALUE
NET TEMPERATURE
NET TEMP DETEMINED BY
St1
165
165°F
LED display value
Hy1
-8
157°F
St1 + Hy1
St4
-10
155°F
St1 + St4
SR
-10
145°F
St1 = St4 + SR
•
St1:
The system’s desired maximum setpoint (when all firing ceases). This is the temperature setpoint of the
appliance.
The factory setpoint is 165°F.
•
Hy1:
The differential from
St1.
This is the Burner-On threshold. The burner Call-For-Heat is initiated and
establishes the firing rate at low fire.
The factory default is -8°F
and is adjustable to
-1°F.
This is the drop in temperature °F below the setpoint that the boiler must drop for the heat load to re-establish
following a heat cycle (heat cycle; heat load is established, unit comes on, heat load is satisfied, unit goes
off). Boiler will hold at low fire until parameter St4 is reached; causing burner to modulate higher or parameter
St1 is reached; causing the burner to shutoff.
•
St4:
The Modulation Threshold. This additional differential from the St1 setting is the value when the change
from low fire to modulation occurs.
The factory default setting is -10
°F and is field adjustable.
•
SR:
The Modulation Band. This parameter is added to the St4 setting and establishes the lower end
temperature of the modulation range where the burner will be operating at 100% of rate. The firing rate will
increase or decrease proportionately between 100% and low fire depending upon the differential between this
setpoint and St4. If loop temperature drops below the modulation range, boiler will hold at high fire.
The
factory default setting is -10
°F and is field adjustable.
•
A to B:
A system with a setpoint of
165°F (St1)
has a heating demand placed upon it and the system
temperature begins to fall. No action.
•
B:
If the sensed temperature at Probe-1 falls to
157°F (Hy1)
, the “BURNER ON THRESHOLD” is activated.
This initiates the burner Call-for-Heat with a firing rate established at low fire.
•
B to C:
If further temperature fall occurs, approaching
155°F
, the firing rate remains at low fire.
•
C:
Temperature drop at
155°F
(St4)
enters the “MODULATION THRESHOLD”.
