In last months article, we ended by telling you we would look at the tales of the tape from combustion analyzer readings to analyze the analysis, and make a determination as to corrective actions. I realize that the title of the article is relating to the newer gas fired high-efficiency equipment, but the reality of the matter is that there are more atmospheric appliances out there than there are high-efficiency appliances, and a person needs to understand the basics of combustion before they can move into the high-efficiency arena.
The combustion process in an atmospheric appliance is fairly straight forward. In most cases, when there is a call for heat, provided that all safety circuits are closed, then the main gas vale is opened, and natural gas is metered into the burner inlet. At this point, the air necessary for initial combustion is mixed with the fuel, and burned at the face of the burner. The ability of the burner to induce primary air flow into the burner can be dictated by the orifice size, gas velocity, primary air shutter adjustment, and the draft potential through the combustion chamber and flue gas elimination system. If any one of these variables are significantly off of their required settings, the combustion process can be upset, and fuel can be wasted and carbon monoxide generated.
Orifice sizing is based on the total firing capacity of the appliance. It is affected by the appliances final location as it pertains to altitude. The appliance must be de-rated at the rate of 4% per 1,000 feet above sea level. This is due to the difference in atmospheric density. Not properly sizing the orifices to the altitude is a guaranteed path to carbon monoxide production. After this has been done, then it becomes necessary to adjust the primary air shutter if possible. I use the term “IF” because many newer atmospheric gas fired burners do not have any adjustment to them on the primary side. They are dependent upon draft established through the fire box to make them work, which is the next item to be checked.
In order to maintain and sustain proper combustion, it is necessary to draw the proper amount of secondary air through the combustion chamber. At a minimum, it is necessary to establish and maintain a slight negative pressure of -.02” Water Column (WC) at the flue gas collector. If this pressure is too low, inadequate secondary and primary air are drawn through the combustion process, and will result in improper combustion numbers at the flue gas breaching.
If draft is in excess, the flame can be cooled down due to excess air being drawn through the combustion process, and will result in a high oxygen content in the exiting flue gas, and the flue gas temperature may be excessively high, resulting in wasted fuel. This is because the flue gas is being drawn through the heat exchanger at a rate that is too fast to allow reasonable heat exchange to occur. Carbon monoxide will also be produced as a result of the cooling effect on the flame.
If the draft is low, the primary and secondary air intake will be low, and the carbon monoxide will be high due to incomplete combustion. This again is due to improper fuel to air ratios being drawn through the combustion chamber.
Both of these high and low draft conditions can be addressed with the proper application of a barometric damper installed down stream of the fixed draft relief hood found on most appliances. This will require that the fixed hood be blocked off in order to control the air flow through the combustion processes. A word of warning here: most manufacturers will not warranty an appliance that has been significantly modified without factory permission. Always consult the appliance manufacturer prior to making any modifications, and make certain that the modifications are done with UL or AGA listed components, and always install the proper flue gas spill detection switches to avoid dumping products of combustion into the living space. Once installed, adjust the breaching negative pressure to the required negative .02” WC pressure, and see what effect it has on your combustion numbers.
I should note that all flue gas samples on natural gas fired appliances must be taken before any draft relief devices, otherwise the flue gas samples will be diluted.
Tune in next month as we continue to look at the actual readings obtained in the field, and what actions were taken to influence those readings. Until then, happy Independence Day hydronicing!
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