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Air Flow Switch
There are both high and low airflow switches contained within one housing measuring the pressure drop across the burner. This is to insure that there is proper airflow (.15 in. w.c. to .95 in. w.c.
) across the burner and proper combustion at all times. Both switches are wired in series and have single pole double throw (one common contact, one normally open contact, and one normally closed contact) switches that are ‘switched’ by air pressure. There are two airflow tubes in the heater, located near the burner and profile plate assembly (profile plates surround the burner and control air into the burner section). In the case of clogged filters, blocked intake, excessive duct static pressure, or a broken belt, the correct burner differential pressure may not be achieved, not allowing the low airflow switch to close. The high airflow switch protects against profile plate failures that cause excessive airflow through the burner. In the event that the pressure drop across the burner is not in the range of the airflow switch, gas flow to the burner is stopped by the Flame Safety
Control.
Unit Type
80/20
Air Flow
Switch
Part
Number
DDP-106-256
Low
Setting
In. W.C.
0.15”
High
Setting
In. W.C.
0.95”
Target Pressure
Drop In. W.C.
Natural Gas
0.45-0.65
”
Air Flow Switch
Airflow Switch
Target Pressure
Drop In. W.C.
Propane Gas
0.45-0.65
”
Modulating Gas System
The second system, the Maxitrol modulating gas system, consists of a temperature selector dial, a discharge air sensor, an amplifier, and a modulating gas valve. The two types of Maxitrol systems used are the Maxitrol 14 series or the Maxitrol 44 series. The Maxitrol 14 utilizes a discharge air sensor and modulates the Maxitrol gas valve to provide discharge air to match the selected temperature on the temperature selector. The Maxitrol 44 utilizes a room temperature sensor to control room temperature as well as a discharge air sensor in order to control the discharge air temperature. The modulating gas valve controls the amount of gas flow to the burner based on the temperature rise needed. When the modulating gas valve is all the way open and achieving the maximum BTUs and temperature rise of the unit, it is called “high fire”.
Maxitrol 14
Amplifier
High Temperature Limit
One of the safety devices is the high temperature limit switch. This switch is a mechanical thermostat that measures the temperature inside the unit downstream of the burner. If the factory-set temperature is exceeded, it will signal the FSC to turn off the burner. This requires a manual reset of the high temperature limit. This insures that the discharge does not exceed 185°F.
High Temperature Limit
Electric Cabinet Heater
On units shipped with an optional electric cabinet heater, ensure that the heater is wired to a separate
120V, 15 amp input, the thermostat sensing bulb is mounted correctly in the control vestibule where the heater is located, and the thermostat set to 0 Degrees Fahrenheit.
28
ELECTRICAL VESTIBULE
1
2
3
5
4
6
10
13
17 15
16
14
8
9
11
12
7
1. IGNITION TRANSFORMER
2. INTAKE AIR THERMOSTAT
3. FLAME SAFETY CONTROL
4. MODULATING AMPLIFIER
5. BLOWER SERVICE SWITCH
6. BURNER SERVICE SWITCH
7. CONTROL TRANSFORMER
8. FREEZE CONTROL
9. HIGH TEMPERATURE LIMIT
10. POWER TRANSFORMER
11. MOTOR STARTER
12. MOTOT OVERLOAD
13. CLOGGED FILTER SWITCH
14. AIR FLOW PROVING SWITCH
15. 120 VOLT TERMINALS
16. 24 VOLT TERMINALS
17. MINIMUM OUTDOOR AIR SETPOINT
18. MINIMUM RETURN AIR SETPOIN
MANIFOLD VESTIBULE
18
9
8
7
6
5
4
1. GAS INLET
2. 1 ST MAIN GAS VALVE
3. 2 ND MAIN GAS VALVE
4. BURNER SHUT OFF VALVE
5. MODULATING GAS VALVE
3 2
1
6. PILOT SHUT OFF VALVE
7. PILOT REGULATOR
8. PILOT GAS VALVE
9. 80/20 DAMPER ACTUATOR
29
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