16.Combustion Air Proving [Pressure Switch(es)]

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.

Pressure Switch Settings

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.

Sea Level Pressure Switch Settings (Applies to elevations up to 6000ft/1830M)

Model LDAP

 

Main Pressure Switch

 

 

 

 

Heat Section Pressure Switch(es)

 

 

Start-up Cold

Hot

OFF

ON

Label

Switch

Qty

Cold

 

Hot

 

OFF

ON

 

Label

Switch

 

 

Equilibrium

Setpoint

Setpoint

 

 

 

Start-up

 

Equilibrium

 

Setpoint

Setpoint

 

 

 

 

 

 

 

 

Color

Part No.

 

 

 

 

 

 

 

 

Color

Part No.

 

Negative Pressure (in. w.c.)

 

 

Differential Pressure (in. w.c.)

 

 

 

 

 

 

 

 

 

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

 

 

 

 

 

 

 

 

 

 

 

 

High Altitude Pressure Switch Settings, Units Above 6000ft/1830M

 

 

 

 

 

Model LDAP

 

Main Pressure Switch

 

 

 

 

Heat Section Pressure Switch(es)

 

 

 

Hot

OFF

ON

 

 

 

Cold

 

Hot

 

OFF

ON

 

 

 

 

Start-up Cold

Equilibrium

Setpoint

Setpoint

Label

Switch

Qty

Start-up

 

Equilibrium

 

Setpoint

Setpoint

 

Label

Switch

 

 

 

 

 

Color

Part No.

 

 

 

 

 

 

 

 

Color

Part No.

 

Negative Pressure (in. w.c.)

 

 

Differential Pressure (in. w.c.)

 

 

 

 

 

 

 

 

 

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

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.

17. Limit Control

The control is factory set and is non-adjustable. If the setpoint is reached, the limit

 

Each heat section is equipped with a temperature activated auto reset limit control.

 

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.)

18.High Limit Control

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.

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

Form RZ-NA-I-LDAP, P/N 207733 (Rev 2), Page 17

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Thomas & Betts LDAP 1200 Combustion Air Proving Pressure Switches, High Limit Control, Pressure Switch Settings
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LDAP 1200 specifications

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