In general, if excess air is less than 30 percent the unit should be cleaned and adjusted. Many manufacturers require installers to make the final gas pressure adjustment after installation. Because the gas pressure regulators on these units are not adjusted at the factory, it is critical the installer make this required adjustment. Not all problems are noticeable from observing the unit, especially carbon monoxide production. The correct pressure, to the nearest 0.01 inches of water column, will be on the nameplate.
Most manufactures provide a pressure tap on the manifold or the gas pressure regulator.
The gas pressure must be set exactly to the manufacturers' specifications, including any adjustments for high elevations. Typically, manufacturers' specify that units must not be overfired to reduce the risk of carbon monoxide production. Some under-firing is acceptable on some units. Consult the manufacturer for acceptable limits. Use either a liquid manometer or a sensitive, calibrated mechanical gauge to measure and adjust manifold pressure.