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. Thanks to an alert reader, it was brought to my attention that I neglected to mention the critical need for fuel pressure settings, and vent stack draft readings. Please see the Letter to the Editors section for my response to him, and again, thanks for bringing it to my attention. Fuel pressure and exhaust stack pressures are critical on ALL gas fired appliances, regardless of the end use. With the newer negative pressure induction gas valves, there is a lot more room for adjustments and deviation as it pertains to fuel pressure, but it should still be one of the first parameters that are tested under two conditions, static (appliance off) and one with the appliance running.
I was recently involved with the commissioning of a large residential mod-con boiler in a massive home here in Colorado. The building was new, less than a year old, so I (wrongly) assumed that all critical fuel pressure parameters were properly adjusted. When attempting to fire the appliance, it would not light. I should mention that this particular boiler was being used on a liquefied propane gas system. This particular appliance manufacturer used a gas valve that was compatible with natural gas as well as propane, and did not require an orifice change to make the conversion from natural gas to propane, just a gas valve adjustment.
After numerous tries at getting the appliance to fire, I finally had to break down and call the manufacturer and speak with a technician to get it to fire. It seems you had to be turning the fuel adjustment screw at the same time the appliance was attempting ignition. In any case, I was finally able to get the appliance to fire off, but the combustion numbers were terrible, and the carbon monoxide was off the charts. It literally was so high, that it was causing my combustion analyzer to lock up (over 3,000 ppm CO). As I approached reasonable numbers, the fuel mixture was so lean that the flame would self snuff, and I would have to start the process all over again, adjusting the fuel adjustment screw while trying for ignition so final adjustments could be made.
Nothing made sense, and I could not achieve my goal, so I started looking at the fuel supply pressure. The static fuel supply pressure was right in line with the manufacturers requires, approximately 12 inches of water column. However, when it went for a try for ignition, the pressure was dropping way below the allowable minimum pressure. It had every indication of a fuel supply issue, and caused us to start evaluating the fuel distribution system as a whole. The distribution system was made of Corrugated Stainless Steel Tubing. Using the CSST installation manual, and known lengths, and fuel demand factors etc., we determine that the fuel line we had should have been capable of providing the required fuel to satisfy the appliance, but we were still suffering significant pressure drop. We replaced the secondary regulator, thinking possibly it was at fault, but it didnâ€™t make much difference. Subsequently, we arrived at the decision to increase the size of the fuel line to eliminate any possible unseen kinks or other anomalies that would create this kind of condition.
Even after increasing the fuel line size, we were STILL experiencing significant pressure drops at the appliance that was negatively affecting the combustion process. This particular appliance was a truly sealed combustion mod-con boiler. It had a 3-in. exhaust, and a 3-in. combustion air pipe, which reduced to a 2-in. flexible aluminized vent pipe that connected directly to the intake horn of the gas induction blower of the appliance. In a fit of desperation, I disconnected the flexible vent connector, and suddenly, the appliances combustion products cleaned right up and came into the manufactures recommended parameters. What brought this to my attention was that when the boiler would self snuff with the combustion air flex connector connected, this flex pipe would jump. With the factoryâ€™s acknowledgement, I poked a small ÂĽ-in. hole in the side of the flexible vent connector, as I had seen numerous other appliance manufacturers with the same configuration do, and reconnected the flexible connector back to the intake horn, and the numbers stayed within the manufacturers recommended parameters.
If I had taken the usual lower road, and plugged and played this appliance, and just gotten it to run, the owners would have been exposed to extremely dangerous carbon monoxide levels, even though it was a sealed combustion appliance. The products of combustions outside were so strong, that it burned my eyes when standing anywhere near the vent terminus. You can not plug and play this, or any other appliance, and sleep with a clear mind. You must follow the manufacturersâ€™ recommendations as it pertains to the combustion analysis parameters, and it will require you to make some adjustments in order to correct any deficiencies.
There are too many mod-con appliances on the market at this time to list all the possible adjustments that can change these parameters, so the responsibility lies directly with the installer/commissioning agent to check these parameters, and make field adjustments as required to bring the appliance into compliance with the manufacturers recommended parameters. And as I found out, some times you have to observe, and think outside of the box to correct situations not seen in the laboratory. The flex connector was acting as a "choke" and was actually creating enough of a negative pressure at the gas inlet to cause too much fuel to be drawn into the appliance, causing it to produce excess carbon monoxide.
The bottom line is, if you don't test, you won't know. If you don't know, you are rolling some pretty serious litigation dice that may come up snake eyes. Tune in next month as we start looking at a collaborative conservation effort that reduced the energy consumption of a light commercial building by 90%, and they havenâ€™t even touched the physical plant boilers yet.
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: firstname.lastname@example.org.