Thomas & Betts LDAP 1200 - Manual (Page 17)

Form RZ-NA-I-LDAP, P/N 207733 (Rev 2), Page 17
18. High Limit
Control
Each heat section is equipped with a temperature activated, manually reset high limit
control. The high limit control is located at the top of each heat section. It is factory
set and is non-adjustable. If the setpoint is reached, the high limit control acts to
interrupt the electric supply to the gas valve in that heat section. If the high limit
control activates, identify and correct the cause before resetting the switch. Refer to
16. Combustion
Air Proving
[Pressure
Switch(es)]
17. Limit Control Each heat section is equipped with a temperature activated auto reset limit control.
The control is factory set and is non-adjustable. If the setpoint is reached, the limit
control will interrupt the electric supply to the gas valve in that heat section. This
safety device provides protection in the case of motor failure or lack of airflow due to
a restriction at the inlet or outlet. (For location, see FIGURE 22, page 33.)
CAUTION: The auto reset limit control will shut down the heater
until the cause is corrected. Do not bypass the limit control;
hazardous conditions could result. See Hazard Intensity Levels, page
2.
The combustion air proving switch is a pressure sensitive switch that monitors air
pressure to ensure that proper combustion airflow is available. The switch is a single
pole/normally open device which closes when a differential pressure is sensed be-
tween the venter housing and the flue collection box. Each section in the heater has a
pressure switch. (For switch location, see FIGURE 22, page 33.) In addition, heaters
with more than one heat section (Size 800 and Size 1200) have a main pressure switch
which senses the negative pressure in the main venter housing.
On startup when the heater is cold, the sensing pressure is at the highest level, and as
the heater and flue system warm up, the sensing pressure becomes less. After the
system has reached equilibrium (about 20 minutes), the sensing pressure levels off.
If a restriction or excessive flue length or turns cause the sensing pressure to be out-
side the switch setpoint, the pressure switch will function to shutoff the burner. If the
main combustion air 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.
The Table below lists the approximate water column differential pressure settings of
the heat section pressure switch(es) and the negative pressure readings of the main
pressure switch.
Pressure Switch Settings
DANGER: Safe operation of this heater requires proper venting flow. NEVER bypass combustion
air proving switch(es) or attempt to operate the heater without the venter running and the proper
flow in the vent system. Hazardous conditions could result. See Hazard Intensity Levels, page 2.
NOTE: For high altitude,
follow instructions in
Paragraph 5 to change heat
section pressure switch(es).
The main combustion air
pressure switch (Sizes 800
and 1200 only) does not need
to be changed for a high
altitude installation.
Start-up Cold
Equilibrium
Hot
Setpoint
OFF
Setpoint
ON
Start-up
Cold
Equilibrium
Hot
Setpoint
OFF
Setpoint
ON
400 1 1.80 to 1.50 1.05 to 0.85 0.65 0.83 Yellow 197028
800 -1.30 to -1.00 -0.85 to -0.65 -0.15 -0.33 Gray 205445 2 1.90 to 1.60 1.10 to 0.90 0.65 0.83 Yellow 197028
1200 -1.40 to -0.90 -0.97 to -0.59 -0.15 -0.33 Gray 205445 3 2.40 to 1.90 1.55 to 1.00 0.65 0.83 Yellow 197028
Start-up Cold
Equilibrium
Hot
Setpoint
OFF
Setpoint
ON
Qty
Start-up
Cold
Equilibrium
Hot
Setpoint
OFF
Setpoint
ON
400 1 1.75 to 1.45 1.00 to 0.80 0.60 0.78 Lt. Blue 197029
800 -1.25 to -0.95 -0.80 to -0.60 -0.15 -0.33 Gray 205445 2 1.85 to 1.55 1.05 to 0.85 0.60 0.78 Lt. Blue 197029
1200 -1.35 to -0.85 -0.92 to -0.54 -0.15 -0.33 Gray 205445 3 2.35 to 1.85 1.50 to 0.95 0.60 0.78 Lt. Blue 197029
Sea Level Pressure Switch Settings (Applies to elevations up to 6000ft/1830M)
Model LDAP
Model LDAP
Heat Section Pressure Switch(es)
Heat Section Pressure Switch(es)
Qty
Main Pressure Switch
Label
Color Switch
Part No.
Label
Color Switch
Part No.
Negative Pressure (in. w.c.) Differential Pressure (in. w.c.)
High Altitude Pressure Switch Settings, Units Above 6000ft/1830M
Switch
Part No.
Negative Pressure (in. w.c.) Differential Pressure (in. w.c.)
Main Pressure Switch
Label
Color Switch
Part No.
Label
Color