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LDAP-IOM (01-22) 207733-A
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At startup when the heater is cold, the sensing pressure is at the most negative level, and as the heater and flue
system warm up, the sensing pressure becomes less negative . After the system has reached equilibrium (about
20 minutes), the sensing pressure levels off .
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If a restriction or excessive flue length or turns cause the sensing pressure to be outside the switch’s setpoint, the
pressure switch will function to shut off the burner . If the flue collection box section pressure switch opens, it will
interrupt the electric supply to all gas valves . If a heat section pressure switch opens, it will interrupt the electric
supply to the gas valve in that heat section . The burner(s) will remain off until the system has cooled and/or the
flue system resistance is reduced .
•
lists the approximate water column differential pressure settings of the heat section pressure switch(es)
and the negative pressure readings of the flue collection box section pressure switch .
Table 13. Pressure Switch Settings
Heat Section Pressure Switches
Unit
Size
Elevation
(Feet (Meters))
Differential Pressure (IN WC)
Label
Color
Startup Cold
Equilibrium Hot
Setpoint OFF
Setpoint ON
400
≤
6000 (
≤
1830)
1 .80 to 1 .50
1 .05 to 0 .85
0 .65
0 .83
Yellow
>
6000 (>1830)
1 .75 to 1 .45
1 .00 to 0 .80
0 .60
0 .78
Light blue
800
≤
6000 (
≤
1830)
1 .90 to 1 .60
1 .10 to 0 .90
0 .65
0 .83
Yellow
>
6000 (>1830)
1 .85 to 1 .55
1 .05 to 0 .85
0 .60
0 .78
Light blue
1200
≤
6000 (
≤
1830)
2 .40 to 1 .90
1 .55 to 1 .00
0 .65
0 .83
Yellow
>
6000 (>1830)
2 .35 to 1 .85
1 .50 to 0 .95
0 .60
0 .78
Light blue
Flue Collection Box Section Pressure Switch
Unit
Size
Elevation
(Feet (Meters))
Negative Pressure (IN WC)
Label
Color
Startup Cold
Equilibrium Hot
Setpoint OFF
Setpoint ON
400
≤
6000 (
≤
1830)
—
>
6000 (>1830)
800
≤
6000 (
≤
1830)
−1 .30 to −1 .00
−0 .85 to −0 .65
−0 .15
−0 .33
Gray
>
6000 (>1830)
−1 .25 to −0 .95
−0 .80 to −0 .60
1200
≤
6000 (
≤
1830)
−1 .40 to −0 .90
−0 .97 to −0 .59
>
6000 (>1830)
−1 .35 to −0 .85
−0 .92 to −0 .54
High Temperature Limit Controls
⚠ WARNING ⚠
The high temperature limit control will continue to shut down the heater until the cause is corrected.
Never bypass this control as hazardous conditions could result.
All units are equipped with temperature-activated, high temperature limit controls that are factory-set and non-
adjustable . If the setpoint is reached, the controls interrupt the electric supply to the gas valve . This safety device
provides protection in the case of motor failure or lack of airflow due to a restriction at the inlet or outlet .
Automatic-Reset High Temperature Limit Control
The automatic-reset high temperature limit control (see
for location) provides protection in the case of
motor failure or lack of airflow due to a restriction at the inlet or outlet .
Manual-Reset High Temperature Limit Control
⚠ WARNING ⚠
If the manual-reset high temperature limit control activates, identify and correct the cause before
resetting the switch.
The high limit control is located at the top of each heat section (see
for location) . If the setpoint is reached,
the control interrupts the electric supply to the gas valve in that heat section . If the control activates, identify and
correct the cause before resetting the switch (refer to
section) .
