Can a Single CO Reading Really Confirm Combustion Safety?
Guest post by David Richardson of Thermal Performance Services in the Lexington, Kentucky area. David is an EVER rater, HVAC contractor, and combustion safety instructor for the National Comfort Institute. This is his second guest post in the EV Blog, the first one discussing his education in combustion safety. ~ab3
In the BPI Standards, measuring carbon monoxide (CO) in undiluted flue gas of combustion equipment is a critical part of combustion safety. This is one of several measurements that allow us to gauge the safety of an appliance and ensure that it operates properly.
According to BPI Standards, if a natural draft appliance passes worst case spillage, the building analyst is to take a CO reading after 10 minutes of operation or after the equipment reaches steady state operation. But, does a single CO reading really give us enough information to verify that a piece of equipment is really operating safely?
The information available in watching carbon monoxide readings can be eye-opening. One thing that always holds true about proper combustion is that the longer a piece of equipment burns, the cleaner the burn should get. This is critical to remember when using CO readings as a diagnostic tool; the burn should always get better, not worse. If the burn gets worse as the combustion process continues, there are underlying problems that need to be uncovered.
CO testing is similar to watching your favorite sporting event - If you’re able to watch only a couple of minutes in the middle of the game, you’re likely to miss not only the most exciting parts of the game, but also the game-changing moments as well. CO testing should be no different. If a building analyst relies on a single-point CO test, they’re likely to miss some of the most important parts of the combustion process.
If you’re wondering where I’m going with this, let me remind you that the burn should get cleaner as the combustion process continues. When a piece of equipment is operating safely and properly, the CO readings should be stable and stay in a range of acceptable levels. This tells us that the flue gases are leaving the equipment properly and that combustion air is making its way into the burner compartment. Smoke testing the draft hood can’t tell us if this is occurring as it only shows if room air is moving up the flue.
The CO numbers should never go up or continue to climb once the initial components have warmed up. If the CO numbers continue to climb, the equipment is operating in a deteriorating condition. This pattern of numbers tells us that the combustion process is breaking down inside the equipment itself. When rising CO numbers are present, combustion air is not making its way into the burner compartment and flue gases are not leaving the heat exchanger.
Rising CO numbers are one of the most dangerous and overlooked conditions in CO testing and cannot be discovered with a single-point CO test. The reason this condition is so dangerous is that you have no idea of where the CO numbers will stop. There have been many instances where rising CO numbers have gone from numbers that would look acceptable in a single-point CO test to very dangerous in mere seconds due to the deteriorating condition inside the equipment.
It doesn’t take as long as you would think to find this problem if it exists. Watching the pattern of the CO numbers will reveal if this problem is present. The next time you perform a combustion safety test, watch the whole combustion process instead of just a portion of it. You might be surprised at what you find.