Originally Posted by
ray jackson
Only take this course if you are willing to re-think everything you thought you knew about combustion. Having taken the class, anything over 100ppm is not good. Now having said that, good luck finding an hvac contractor that can tune the furnaces to operate properly (if they haven't taken this course, they won't be able to).
This course is great for general information and knowledge but, just like any training course, I would caution against taking all the information presented as gospel. Some of the techniques and adjustments presented by NCI are against manufacturer recommendations.
The ANSI Z21.47 standard for gas-fired central furnaces allows up to 400 ppm of CO in the vent. Realistically, this is at way far edge of the design
standard and no manufacturer would ever design to that as a minimum level.
Because of the 400 ppm limit, and the fact the so many factors affect CO, there really never has been a published "acceptable" or "normal" level of CO in the vent. One thing causes high CO; incomplete combustion. But, many factors can contribute to that incomplete combustion.
The sensor(s) in a combustion analyzer are only good for X number of months. After that, the sensor(s) may not read properly. Also, the batteries should be fresh. A bad sensor or weak batteries can affect your results. Meters should be calibrated outdoors
When testing for CO, the following conditions must be first met, or your readings will be high. Most will need to be checked by a qualified HVAC tech, as they are out of the capabilities and responsibilities of even a good inspector)
The burners must be clean and lint free.
The unit must be fired within 2% of the rating plate. (high fire on 2 stage furnaces)
The unit must be at the mid-point of the temperature rise or
slightly above
The unit must be at steady state conditions, which means it has to have run for at least 15 minutes.
For 90% furnaces, the drain trap must be clean and condensate must run free. Water trapped in the secondary HX reduces the excess air levels and results in poor combustion performance.
You need to make sure you have sufficient combustion air to the room.
And for any CO test, you need a background reading of CO in the room or area from which the combustion air is being drawn. You may have high CO in the combustion air. This is especially true if combustion air is being pulled from a garage; very likely when dealing with an 80% non-condensing furnace.
For 80% furnaces, the sampling probe should be put in the vent pipe 12 inches above the draft hood or inducer outlet.
90% furnaces can be checked at the termination outlet outdoors, but more accurate results will occur if you drill a hole in the exhaust pipe 12 inches away from the inducer outlet.
You should seal the hole around the probe in the vent pipe to prevent room air from diluting the sample.
