This article originally appeared in ABMA’s Fall 2018 issue of “Today’s Boiler.” The author, Geoff Halley, has more than 25 years of experience as an expert witness, covering some 300 boiler related investigations. He was ABMA’s director of technical affairs for a period of 17 years and over 50 years experience in the boiler industry.
This article focuses on a problem area which has occurred much too frequently in a boiler room. It does not cause property damage, nor does it leave visible evidence. But it does affect the lives of many with significant sickness and, in some cases, death. As you may have guessed, we are talking about the generation of carbon monoxide (CO).
Carbon monoxide is a colorless, odorless, tasteless, non-irritating gas, generated by the incomplete combustion of hydro-carbon fuels. As such, it can only be detected with instrumentation. Even today, there are people connected to the boiler industry who claim that they can “eyeball a flame” and tell whether or not it is generating CO. These statements were apparently based on the belief that a blue gas burner flame did not contain CO. But even in the 1970s this was not true, as mixing of fuel (gas) and air had improved to the point where it was possible to have a blue flame containing large amounts of CO.
Combustion methods designed to reduce nitrogen oxide (NOx) emissions can also produce blue flames containing CO. Additionally, in today’s world, there are emission limits on the amounts of the various pollutants in the products of combustion, CO and NOx among them. These numerical limits demand that properly calibrated instrumentation be used to measure them.
Many building codes now require CO detectors/alarms installed close to or in the sleeping areas of homes and other buildings. This is a smart first step, but it does not address the generation of CO, which takes place in the combustion chamber of heat-producing equipment, such as boilers and furnaces.
There are two basic reasons why the generation of CO occurs in the combustion process:
• Improper mixing of combustion air with the fuel being burned, even with a sufficient supply of combustion air. Given the development and certification process that today’s burners go through, this can be rectified by a competent burner technician making adjustments to various burner components, while checking the products of combustion with a properly calibrated flue gas analyzer.
• The lack of complete burning because of an inadequate supply of combustion air for the fuel input being supplied. This is the scenario that this article will concentrate on.When there is an excess of air, the products of combustion are straightforward oxygen, carbon dioxide, water vapor and nitrogen, which is simply the major inert part of the combustion air, traveling through the process unchanged. However, when there is a deficiency of air, things become more confusing, with a mixture of nitrogen, some water vapor and carbon dioxide, and then some hydrogen, carbon monoxide and the raw fuel.
There are two reasons why there may be insufficient combustion air to completely combust the fuel being supplied, these are:
• The combustion air openings to the boiler room were either improperly designed or installed, or if properly designed, they were subjected to human interference or modification, which decreased the amount of combustion air available.
• The fuel input, in terms of heat input, has been increased significantly, so as to be outside of the capability of the burner combustion control system to properly control the air/fuel ratio.
The following is a brief discussion of incidents covering both of these reasons.
It’s Cold Outside — Deficiency of Air
This incident was discovered by a burner service technician who was called to resolve a problem of soot build-up in the tubes of firetube boilers. Although no lawsuit was filed, it is highly likely that this type of action has resulted in a lawsuit at other locations.
This incident occurred at a metals refining facility in which the boiler room contained two firetube boilers having a combined input capacity of 25 million BTUH firing propane. Upon entering the boiler room, the technician noticed that an 8-ft by 4-ft sheet of plywood had been neatly installed over the combustion air intake louvers.
When asked why this had been done, the boiler operators responded that in winter the boiler room was very cold, due to the inflow of large quantities of outside air. This is never a good answer; the corrective action is either to install an air heater adjacent to the intake louvers or relocate the air intake to a place where operator comfort is not affected in cold weather. The “plywood fix” has been used in other cases where the reason was given as the freezing of water pipes routed near the combustion air intake.
CO generation was not the reason for the service call, but there is no doubt that it was being generated. The only saving grace was that the boilers were located in an isolated boiler room equipped with short, leak-tight stub-stacks, which vented the CO outside to a location where it quickly dispersed.
Negative Pressure Boiler Room — Deficiency of Air
This incident was not the subject of a lawsuit, but had it been allowed to persist, it may well have developed into one. The location was a manufacturing plant that produced household appliances. They were having a problem with carbon buildup in the tubes of a firetube boiler, and requested the help of a burner service technician. The service technician noted that the boiler room was located in the middle of the plant, and as such it was taking its combustion air from inside the plant. A flue gas analysis revealed high levels of CO.
Doors also tended to slam shut due to pressure differentials. It was decided that an inventory of exhaust fans and makeup air fans be installed in the plant building. This inventory revealed that the exhaust out of the plant building was 350,000 scfm, whereas the makeup airflow was only 125,000 scfm. It appears that the exhaust and makeup air flows may well have balanced when the plant was originally constructed; however, several process additions, such as plating tanks, paint spray booths, and environmental upgrades all required added exhaust capacity. Unfortunately, no thought had been given to adding to the makeup airflow to maintain balance.
Wrong Gas Orifice — Excess of Fuel
Unfortunately, this has been the reason for several lawsuits in recent times, including sickness and fatalities. In all cases where I was involved, there were many similarities.
• All the boilers were at the low capacity end of the boiler size range, used in either residential or small commercial applications.
• All were high-efficiency condensing hot water boilers.
• All were available for purchase through local supply dealers.
• Because these boilers typically fired on natural gas, the burners were set up with a natural gas orifice installed; however, a propane orifice was included in the auxiliary parts package.
• The installation manuals provided in the parts package provided details on how to install the flue gas vent piping, in terms of maximum length of straight pipe, maximum number of elbows, etc. It also provided the dimensions of the water trap to be located at the bottom of the vent piping to drain the condensate.
• The boilers were purchased by the local plumber, HVAC contractor, or in one case, school maintenance personnel.
• Future events suggested that none of the installing individuals were in possession of either a flue gas analyzer or a CO analyzer.
Below is an overview of one incident, which caused three fatalities. The boiler in question met all the criteria listed above and was located in the basement of a large home. The vent piping was at the maximum in terms of length and number of elbows, meaning it was at or close to the maximum pressure drop allowed by the boiler manufacturer.
With condensing boilers under normal operation, the condensing water vapor from the combustion process must be continually drained from the base of the vent through a water trap (“P” trap). The height of the water trap is crucial to stopping the products of combustion entering the boiler room and must exceed the pressure drop across the vent piping in inches of water column, otherwise the trap will be blown and products of combustion will enter the boiler room. In this case, the legs provided to support the boiler were too short to accommodate a trap of the required height. The installing contractor shortened the trap to a height only slightly greater than the pressure drop through the vent piping, thus it could be easily blown by something as simple as a gust of wind over the top of the vent pipe.
At some point in time, roofers were working on this home and noted an obnoxious odor (obviously aldehydes) coming from a roof vent, and as a quick fix they taped a plastic bag over the top of the vent, which happened to be the boiler exhaust. The result was that the water trap at the bottom of the vent was blown and could not refill itself. As a result, the products of combustion (including CO) could now enter the basement of the home, where they were distributed throughout the home by the HVAC system. The result was three fatalities.
An extensive investigation was undertaken, which eventually concluded that:
• The amount of CO in the flue gas was 80,000 ppm.
• This amount of CO being emitted from the blown trap into the basement was sufficient to produce lethal levels of CO in the home in less than 20 minutes.
• The boiler was being operated on propane with a gas orifice that was only slightly smaller than a natural gas orifice, the orifice change having been made incorrectly by the installing contractor. Thus, more than double the required amount of fuel was being fed into the boiler.
• It is obvious that no attempt was made to use a CO analyzer to measure the amount of CO in the flue gas at startup. With an analyzer, the issue would have been known and presumably corrected by installing the correct orifice for propane firing, and the fatalities would have been avoided.
With proper equipment and education, these situations can be avoided and lives can be saved. All three of the incidents discussed above were caused by lapses on the part of installers, operators, or startup/service technicians.
It is also evident that airflow is critically important to avoid impacts of carbon monoxide. Factors affecting the supply of combustion air happen on both the input side of the combustor and the output (venting) side.
We have seen the effect of reducing the size of air intake openings or even closing them completely, and we have also seen how the addition of power venters without compensating for their effect by adding makeup air can affect the generation of CO. However, the addition of pressure-causing devices or effects on the discharge side of the combustion chamber can reduce combustion air fan performance, causing CO to be generated.
These devices could be the addition of economizers or fire-tube turbulators, whereas naturally occurring events might include a bird’s nest plugging vents or snow drifts covering the termination of side-venting stacks.
The massive generation of CO due to the wrong gas orifice being used on propane-fired boilers seems to be a phenomenon of recent times. It would appear to be due to several reasons.
• Installation and start-up personnel who lack sufficient training to work on the equipment in question.
• Lack of use of flue gas analyzer. In some cases, the personnel probably feel they do not do enough boiler work to justify the cost of a flue gas analyzer, which brings us back to point one above. However, these items can be rented for a reasonable price. I personally would make this a major requirement in hiring people to perform this type of work. An eyeball is not a good way to monitor CO formation and never was.
• A lack of knowledge of the various installation codes and standards, such as NFPA 54, the National Board Inspection Code (NBIC) Part 1 Installation, and ASME CSD-1 — plus, of course, the boiler manufacturer’s installation manual.
While some progress has been made in protecting against CO poisoning by requiring CO alarms in or near sleeping areas, and recent requirements on the part of the State of Texas and NBIC to install CO alarms in boiler rooms in which condensing boilers are installed, there has been little effort to ensure that CO is not being generated in the combustion chamber of the boiler.
However, projects are being initiated to require that a flue gas analysis, including CO, to be measured and recorded as a part of the startup procedure.