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.
The graphs below illustrate the approximate cfm going through the unit vs. the differential pressure measured by the airflow switch. Simply measure the differential profile pressure drop at the airflow tubes in the unit and match that value up to the matching unit curve below. This will show the cfms traveling through the burner and will indicate proper airflow or airflow problems (too much or not enough). If the pressure drop is outside of the .15” to .80” range, the blower rpm should be adjusted to fix airflow.
Size 1-3 Heater CFM vs. Burner Profile Pressure
| 10000 |
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| 9000 |
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| Size 2 |
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| 8000 |
| Size 3 |
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| 7000 |
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| 6000 |
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CFM | 5000 |
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| 4000 |
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| 3000 |
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| 2000 |
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| 1000 |
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| 0 |
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| 0.15 | 0.2 | 0.25 | 0.3 | 0.35 | 0.4 | 0.45 | 0.5 | 0.55 | 0.6 | 0.65 |
Burner Differential Profile Pressure (in. w.c.)
Size 4-5 Heater CFM vs. Burner Profile Pressure
| 25000 |
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| 23000 |
| Size 4 |
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| 21000 |
| Size 5 |
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| 19000 |
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| 17000 |
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CFM | 15000 |
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13000 |
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| 11000 |
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| 9000 |
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| 7000 |
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| 5000 |
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| 3000 |
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| 0.15 | 0.2 | 0.25 | 0.3 | 0.35 | 0.4 | 0.45 | 0.5 | 0.55 | 0.6 | 0.65 |
Burner Differential Profile Pressure (in. w.c.)
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 back up safety device is the high temperature limit switch. | High Temperature Limit |
This switch is a mechanical thermostat that measures the temperature |
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inside the unit downstream of the burner. If the |
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of 170°F is exceeded, it will signal the FSC to turn off the burner. This |
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requires a manual reset of the high temperature limit. This insures that |
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the discharge does not exceed 185°F. |
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Operation Summary
With the blower already running and the airflow switch proven;
9The outside air temperature falls below the setting of intake air thermostat or
9The optional remote panel is set to “Manual” and “Heat” mode
•The FSC in energized and the following occurs;
9FSC indicates that it has power by illuminating the OPR CTRL LED
9FSC verifies proper Airflow
17