Manifold Pressure Adjustment
A suitable manifold pressure is important for efficient furnace operation. If the gas pressure is too low, it will cause rough ignition, incomplete and inefficient combustion, and incorrect fan control response. An excessively high manifold pressure may cause the burners to overfire the heat exchanger. Overfiring the heat exchanger not only reduces the life of this component, but it also may result in repeated cycling of the burner on the high limit control.
Manifold pressure should be set at 3.5 in W.C. for natural gas. Control manufacturers normally preset the pressure regulator in natural-gas valves at the time of manufacture so that natural-gas units are fired at this 3.5 in W.C. rate. Only small variations in the gas flow should be made by adjusting the pressure regulator. This adjustment should never exceed plus or minus 0.3 in W.C. Major changes in the gas flow should be made by changing the size of the burner orifice (see below).
The manifold gas pressure can be tested by using a U-tube water manometer (Figure 11-40). The manometer is connected to the manifold through an opening covered by a 1⁄8-in plug cap. The pressure test must be run while the unit is operating. On Dunham- Bush gas-fired furnaces, the test is run through an opening in the gas valve (Figure 11-41). The furnace manufacturer’s installation literature should contain instructions for testing the manifold pressure.
Once you have determined the manifold pressure, you may find it necessary to adjust it for better operating characteristics. Manifold pressure can be increased by turning the adjusting screw clockwise, and decreased by turning it counterclockwise.
LP gas units are fired at 11 in W.C. manifold pressure. LP gas (propane and butane) is heavier than natural gas and has a higher heating value. As a result, LP gas needs more primary air for combustion. Thus, a higher manifold pressure is required to induce the greater volume of primary air into the burner than is the case in natural gas units An LP furnace does not have a pressure regulator in the gas valve. Pressure adjustments are made by means of the regulator at the supply tank. These adjustments must be made by the installer and regularly checked by the serviceman for possible malfunctions.
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