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Properly commissioning gas fired high efficiency equipment — Pt. 5

Sept. 1, 2011
This month I want to address the natural gas fired, forced draft appliance, most of which have a finned tube heat exchanger of one design or another.

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.

Last month we looked at the results from an atmospheric appliance. This month I want to address the natural gas fired, forced draft appliance, most of which have a finned tube heat exchanger of one design or another. There are numerous manufacturers on the market, and most manufacturers have numerous models that utilize this design. They are typically limited to a thermal efficiency of around 85%, and are not considered a condensing appliance. The flue pipes serving these units may be required to be compatible with the possible production of condensation, and some are not, depending upon how they are set up and operated. In any case, each manufacturer has a specific set of instructions that must be followed in order to guarantee proper and safe operating conditions. These appliances pressurize the combustion zone, and depending upon the actual field application, may or may not require the use of a barometric damper to control the draft being exerted upon the combustion process. If barometric dampers are required, it is extremely important that safety switches be incorporated into the safety circuit should excess flue gas spillage occur.

I was recently involved in the inspection of numerous relatively new gas fired appliances of this category. Of eight appliances tested, all of them had issues of one sort or another, with most of them producing carbon monoxide well outside of acceptable limits. None of them had the required barometric draft relief damper. The installation people evidently didn't feel the need to review the manufacturers' installation manuals. Even after correcting this deficiency, the carbon monoxide was still well outside of acceptable limits, and required additional adjustments in order to bring them into compliance. One factor that always trips up most appliance manufacturers is the fact that the natural gas at this location (central Denver) has been intentionally de-rated by pumping air into the gas. This has to be brought to the attention of the appliance manufacturer and they will make a prescriptive recommendation to assure proper operation. They (rightfully) are assuming we have 1,050 BTU/Cubic foot gas here, which is the norm for the rest of the world. In reality, our gas contains around 830 BTUs per cubic foot. In any case, the installers decided to "plug-n-play" these appliances, and never even bothered to run a combustion analysis. As I've overheard in the field many times, "If it's hot, don't mess with it." Fortunately, no one was injured, but all it takes is a wind from the right direction under the right conditions to create a situation that can go south in a hurry. If you don't test, you don't know, and if something should go wrong, you will be holding a very heavy bag of liability.

I will only address the worst case scenario on these boilers to save page space, and to keep the readers from dozing off and or going cross eyed.

Oxygen = 15.9%
Diluted Carbon Monoxide = 550 ppm
Combustion Efficiency = 82.3%
Carbon Dioxide = 2.8%
Gross stack temp = 177°F
Air temperature = 72°F
Net stack temperature = 105°F
Measured excess air = 250%
Air Free Carbon Monoxide = 2,301 ppm

This appliance required the following service: removed excess weight from barometric damper counter balance (approximately 3 pounds of washers); increase fuel pressure per factory recommendations; and decrease combustion air by adjusting slide shutter at blower inlet.

Post service adjustments:
Oxygen = 8.8 %
Diluted Carbon Monoxide = 57 ppm
Combustion Efficiency = 87%
Carbon Dioxide = 6.8%
Gross stack temp = 179°F
Air temperature = 73°F
Net stack temperature = 106°F
Measured excess air = 65 %
Air Free Carbon Monoxide = 99 ppm

As can be seen, there was a significant increase in combustion efficiency, and a significant decrease in the production of carbon monoxide that was related to a decrease in excess air.

No one knows who initially set this boiler up, and it is obvious, based on readings obtained from the field, that the contractor didn’t understand what was supposed to be done in order to insure safe and proper operation. It is my belief that this appliance received the "If it's hot, don't mess with it" analysis.

Unfortunately, this is typical of my findings on eight boilers of the same age and make on this property. If you don't have the time to do the job right the first time, where in the world are you going to find the time and money to do it again, and again and again? I should note that many of these boilers were experiencing service lockout conditions.

Tune in next month as we continue to look at more of the actual readings obtained in the field, and what actions were taken to influence those readings. Until then, happy stoichiometric hydronicing!

All Mark Eatherton material on this website is protected by Copyright 2011. Any reuse of this material (print or electronic) must first have the expressed written permission of Mark Eatherton and CONTRACTOR Magazine. Please contact via email at: [email protected].

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