2
LOW-HIGH-LOW (STEP-MODULATION) FIRING
Advantages: There are several strong advantages to low-
high-low firing in both heating and process applications:
1. Smooth ignition both in natural and forced draft opera-
tions.
2. Closer control than with a single input.
3. Less strain or wear on the burner, boiler and combustion
chamber components:
a. Longer cycles.
b. Gradual changes; less thermal shock.
4. Lower fuel consumption:
a. Higher efficiencies of both low and high fire.
b. Lower stand-by loss due to longer ON cycles and to
the closed air damper during OFF cycles.
Operation: A true low-high-low burner controls both air
and oil flow rates. A low fire start burner (low-high-off) con-
trols only the oil rate and has a fixed air setting with none
of the advantages of 1, 2, 3, or 4 above. The low-high-low
burner cycle operates as follows:
1. Call for heat. Burner motor and ignition are energized. Air
is in the low fire position. The low fire oil valve admits oil
to the low fire nozzle and its spray ignites, burning clean
with proper air/fuel ratio.
2. If demand exceeds low fire, the damper motor is en-
ergized through a high fire operating control (aquastat,
airstat, pressure-trol, or outdoor thermostat.) As the
damper motor approaches a wide open position, the
auxiliary end switch energizes the high fire valve and the
full fire with open air. Burns clean with high CO
2
and high
efficiency.
3. As the high fire input begins to exceed the demand of
the high fire operator, the operating control opens to
de-energize the damper motor. As the damper motor
returns the air shutter to low fire the auxiliary end switch
in the damper motor opens and de-energizes the high
fire valve.
4. If the demand exceeds low fire, the high fire operating
control would again call for more heat as in Step 2 and
then followed by Step 3.
5. If, after returning to low fire, the load should drop to
below the low fire output, the operating limit would shut
off the burner completely.
ASSEMBLING THE BURNER (TWO-PAK)
1. Remove the air tube and nozzle line assembly from the
smaller carton. If nozzles are not installed, see instruc-
tions under (4).
2. Remove the main housing assembly from the larger
carton.
3. Install air tube assembly in housing using set screws
provided. Be sure air tube is fully seated against step in
housing
4. Install and tighten the proper nozzles (see Tables 5 &
6. page
4
) in the adapter. Be careful not to damage the
electrode insulators or to bend the wires.
5. Check the electrode settings.
FIG. 1
6. Swing open the transformer and slide the nozzle line
assembly into the air tube. On Model 801CRD, the
flame retention ring must be lifted and guided through
the throttle ring (a reduced diameter) in the end of the
airtube. DO NOT FORCE IT.
7. Fasten the high tension leads to the transformer termi-
nals.
8. Place the nozzle line yoke in the groove in the adjusting
screw.
9. Swing the transformer to the closed position.
10. Connect the flared fitting on the copper oil lines to the
nozzle lines and tighten.
11. See sections on page 4 for adjustments of combustion
head and combustion air.
DIMENSIONS
MODEL
STD.
FLANGE
B.C.
FORCED DRAFT
FLANGE
B.C.
O.D.
701CRD
59642
6
3
⁄
4
-7
5
⁄
8
59444
9-10"
11"
801CRD
59683
6
7
⁄
8
-7
5
⁄
8
59535
10"
11"
MODEL
A
B
C
D
1
D
2
E
F
L
W
X
701CRD
7
1
⁄
2
9
1
⁄
2
10
4
1
⁄
8
5
7
⁄
8
13
9
10
11
⁄
16
-14
11
⁄
16
19
1
⁄
2
2
1
⁄
2
801CRD
8
12
11
5
1
⁄
4
5
7
⁄
8
14
1
⁄
2
10
1
⁄
2
10-15
23
3