Quick Answer - -0.2” of water column, with a tolerance of +/- 0.15” (same for both natural and LP applications)
Overview:
The light commercial rooftop series of products operate with a negative pressure regulated (redundant) gas valve (GV). The concept of the negative pressure regulation, while not new to the industry, is not a common application. This concept actually allows the reduction of combustion air proving devices, such as centrifugal switches, vane or paddle switches and pressure differential switches, notorious for causing nuisance heat problems.
The concept of this operation is such that there will be no gas flow, if no suction is present at the outlet of the gas valve (GV). The outlet of the gas valve (GV) is tied to the inlet of the combustion blower motor (CBM), which creates a negative pressure or suction at the outlet of the gas valve (GV). So in the event of a combustion blower motor failure, there will be no gas flow even if the valve is electrically energized.
Negative pressure burners rely on the combustion fan to “pull” the gas through the gas valve as opposed to positive pressure burners that “push” the gas through the gas valve. In order to properly measure negative pressure gas flow, the gas gauge must be able to read down to a negative 4 inches water column (-4.0”w.c.). A digital Magnehelic is a preferred instrument for accurate measurements, but an analog Magnehelic will also work.
When the combustion fan is running, and before the gas valve opens, the pressure reading on the outlet side of the gas valve will be in the range of –2.5” to –3.5” w.c. When the gas valve opens during normal ignition, the pressure will increase upwards to –0.2” w.c. The maximum allowable pressure for proper ignition is –0.05” w.c. From the factory, the valve should not require readjustment.
The inlet pressure is regulated by a field supplied and installed external pressure regulator. The inlet pressure for natural gas applications should be no less than +2.5 or +3.5” of water column (model specific), while the maximum for all models and applications is +14.0” of water column (1/2 PSIG). The optimum inlet pressure setting is in the middle of the regulation window, or approximately 7.0” of water column, this helps eliminate nuisance problems in areas where gas supply pressures may vary or fluctuate during peak demand. The inlet pressure for LP applications should be no less than 8.0” of water column, while the maximum is again 14.0” of water column (1/2 PSIG). Here again the optimum inlet pressure setting is in the middle of the regulation window, or approximately 11.0” of water column.
The outlet or manifold pressure is measured at the outlet pressure tap, and is factory set to -0.2” of water column, with a tolerance of +/- 0.15” of water column (Range = -0.05” to -0.35” of water column). This setting remains the same for both natural and LP applications. The outlet or manifold pressure may be measured with a Magnehelic gage, or with a “U” tube, inclined, or digital manometer.
In the event that a manifold pressure regulation adjustment is necessary, the tamper resistant cap (which covers the regulation adjustment screw) must be removed. This cap utilizes two round indentations that a “Spanner” wrench will fit, in the event that a spanner wrench is unavailable, a pair of snap ring pliers, a
jumbo paper clip (cut and bent to fit), or two small pointed devices may be used to remove the cap. Once removed, the regulator can be adjusted with a small flat tip screw driver. Turning the adjustment screw clockwise (CW) will increase the manifold pressure (bring it closer to -0.05” of water column) and richen the mixture. Turning the adjustment screw counterclockwise (CCW) will decrease the manifold pressure (become increasingly negative and bring it closer to -0.35” water column) making the mixture leaner.
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WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.