CO Safety and the HVACR Tech

There is more to carbon monoxide safety than leaky furnace heat exchangers. Venting problems may actually cause more CO safety issues than leaky heat exchangers. I am not suggesting that you overlook the importance of checking furnace heat exchangers for leaks, but rather that you expand your CO safety horizons a bit. I want to talk about CO safety concerning the technician – you. A combustion appliance operating in an unsafe condition can spill vent gasses containing CO out into the room where it is operating. This can create a safety hazard for technicians going into that area to service the furnace. You should carry a CO detector that displays the CO level in the area you are working in. If the ambient CO goes above 50 ppm – you should leave. 50 ppm is the OSHA standard for the maximum allowable concentration for an 8 hour work day.

Ray Wohlfarth recently published an article in the October 2015 issue of Plumbing & Mechanical magazine in which he discusses a story of an electrician being overcome by CO fumes. The electrician was working in a mechanical room on something completely different than the boilers, but the boilers were in that room and not operating properly. When the electrician failed to report back to the school administration, the secretary was sent to see what was taking him. She found him passed out on the floor – ran and called 911 to get emergency medical help. This undoubtedly saved his life. Suppose that had been a service technician working on a furnace with nobody home? By the time someone found the tech – it would probably be too late.

Before you go to a big box store to get a home CO detector, consider that they are typically built to UL 2034 standards, which allows 15 minutes before alarming at levels above 400 ppm. This is just not fast enough in the case of high levels. The UL standard is weighted to prevent false alarms, but this also means that a UL listed device can fail to alarm at dangerous levels until it is really too late. Plus, typically these do not show what the CO level is. Kidde, a primary manufacturer of these types of CO monitors, points out that their devices are for continuous monitoring – not short term detection.

Single gas, battery operated CO detectors which display the CO level are available for $100 - $150. Alternately, you could use the CO setting on your combustion analyzer. If you don’t have a combustion analyzer, you could put that $150 you would normally spend on a single gas CO detector towards one of the lower cost $500 combustion analyzers. You will be doing both yourself and your customers a favor.

What is Air-Free CO

If you have used a combustion analyzer or CO detector, you may have come across the term “air-free  CO reading.” A common question is “What is the difference between the regular CO reading and the air-free CO reading?” You may have noticed the air-free reading is always higher than the “regular” reading. Basically, the “air-free” reading is calculated to determine what the CO concentration in flue gas WOULD BE if all the excess air were removed. CO readings tell how many parts per million of the sampled gas are CO. If you add a bunch of gas that  was not part of the combustion process, it dilutes the CO reading because now all those other gas “parts” are being counted, even though they had nothing to do with the combustion processs. All gas appliances are designed to operate with some excess air. Excess air is left over in the flue gas after the combustion process – thus its name; excess air. The purpose of excess air is to insure adequate combustion air by providing more than is needed. Lack of combustion air leads to CO production, so having more than enough air helps reduce CO levels. Older, natural draft furnaces have higher levels of excess air than induced draft burners; but all should have some excess air in the flue gas. Excess air is necessary for safe operation. The problem is that the excess air dilutes the flue gas, lowering the CO reading. The air-free calculation corrects for this dilution effect.

Basically, the CO reading is multiplied by the ratio of the atmosphere’s oxygen percentage (20.9)  to the excess oxygen percentage. The formula is
Air-Free CO  ppm = Measured CO ppm x (20.9 / (20.9 – O2% in flue gas))
 So if the measured CO ppm is 50 and the measured oxygen in the flue gas is 10.5%
Air-Free CO = 50 x (20.9/(20.9 – 10.5)) = 100 ppm (approximately)
If the CO reading is exactly the same, but the O2 reading is 14%
Air-Free CO = 50 x (20.9/(20.9-14))= 150 ppm  (approximately)
The second furnace is producing much more CO than the first, but the CO meter reads the same because the extra excess air in the second furnace has diluted the flue gas. This is why you should use air-free readings whenever checking flue gas CO levels. Most digital flue gas analyzers will do this for you. If you are using the old hour glass bubblers, you will need to do the math yourself.  

For a more detailed look at air-free CO and carbon monoxide in combustion gas, take a look at this article by Richard Karg http://www.karg.com/pdf/coairfree_article.pdf

It is actually about ovens, but the processes and science are relevant.