In multiple cell units, test each cell separately. Test sealed combustion units either from an access in the discharge line, or outdoors by probing into the discharge pipe. The maximum allowed under ANSI is 400 ppm, however at 400 ppm improper combustion is occurring and corrective measures should be taken. Using a full combustion analyzer, set excess air to manufacturer's specifications. 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.