Keep Outside Combustion Appliances Outside

Every winter I hear a few sad stories of people getting ill or dying because they decided to operate an outdoor combustion appliance inside their house. Most often, this occurs during a power outage. One example, someone decided to bring their charcoal barbeque grill inside the house to cook because it would also provide heat and they wouldn't have to stand out in the cold. They didn’t realize that burning charcoal creates lots of carbon monoxide. Their daughter got deathly ill, so they rushed her to the hospital. Fortunately, that got everyone out of the house and she recovered. Charcoal tells you right on the bag not to do that, but we don’t all read the instructions. You also hear about people running generators inside. Last year in Atlanta, a family died of carbon monoxide poisoning due to carbon monoxide from operating a generator in their basement overnight during a power outage. Gasoline engines should never be operated inside. A simple rule for staying safe is: if it is normally operated outside, keep it out there. Although there are probably many creative ways to use outdoor tools and appliances inside, there is usually a reason they are designed to be operated outside. I am sure most folks involved in HVACR already know this, but many of your friends and acquaintances may not. So do folks a favor, pass the word that charcoal grills, generators, and gasoline powered tools should stay OUTSIDE!  

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. 

Sometimes HVAC/R technicians have an opportunity to do more than make people comfortable, we can save lives. More people are sickened or killed by carbon monoxide poisoning than any other type of poison. The Center for Disease Control and Prevention (CDC) reports that each year more than 500 people in the United States accidentally die from carbon monoxide. An estimated 10,000 people in the U.S. are treated for CO poisoning in hospital emergency rooms annually. It is believed that many more people suffering CO poisoning are misdiagnosed, or never seek medical care. This is because the symptoms of CO poisoning are very similar to influenza symptoms. One big difference is that influenza causes a fever and CO poisoning does not.

Carbon monoxide is an odorless, colorless gas that is highly poisonous. It is formed by the incomplete combustion of carbon based fuels, like natural gas, oil, coal, or wood. Incomplete combustion can be caused by lack of oxygen, improper mixing of the fuel and oxygen, or too low a combustion temperature. A correctly adjusted gas or fuel oil flame should produce very low levels of CO. Ideally a correctly adjusted gas or oil flame should produce no CO, but realistically, most produce at least trace amounts. Solid fuels almost always produce large amounts of CO, that is why charcoal comes with a warning that it is not to be used inside. Even people that should know better sometimes overlook the obvious. A friend of mine was conserving heat during a cold winter power outage by using his charcoal grill inside. His daughter became very ill and had to be rushed to the hospital where they correctly diagnosed her condition. This story ended well, she recovered and is doing well. Unfortunately there are many stories about CO that do not end well.

HVAC/R technicians are in a position to help. We can make sure all combustion appliances in the home are burning correctly, insure there is enough combustion air for proper combustion and venting, and finally by making sure the vent system is adequate and working correctly. For gas and oil furnaces also remember to inspect the heat exchanger for leaks. The heat exchanger separates the combustion products from the air circulating in the home. Although a defective or cracked heat exchanger can contribute to CO poisoning, more obvious problems are frequently to blame. Stopped vents, loose or leaky vents, and lack of combustion air are common causes of CO. While every technician should learn to look for conditions that can lead to problems, testing is required to verify that a system is operating at safe levels of CO and that there is no CO in the house. Every technician should have an accurate CO tester. Household alarms are not a substitute. While every house with gas or oil appliances certainly should have CO alarms, they are not a replacement for an accurate tool for diagnosis. I highly recommend a seminar done by Bob Dwyer for COSA (Carbon Monoxide Safety Organization) Make sure and take advantage of the opportunity if you have a chance to attend one of his CO Safety Seminars.

There are many units in Fundamentals of HVAC/R to help explain how to achieve safe, efficient combustion for gas and oil furnaces, including

• Unit 37 Gas Fired Heating Systems
• Unit 38 Warm Air Furnaces
• Unit 40 Gas Furnace Installation, Startup, Checkout, and Operation
• Unit 41 Troubleshooting Gas Furnaces
• Unit 42 Oil Fired Heating Systems
• Unit 43 Oil Furnace and Boiler Service
• Unit 44 Residential Oil Heating Installation
• Unit 45 Troubleshooting Oil Heating Systems

There are many good web sites for more research on carbon monoxide poisoning.

A few are listed below.

http://www.bbc.co.uk/health/conditions/carbonmonoxide1.shtml

http://www.cosafety.org/Aboutco.htm

http://www.clima-tex.com/consumer/carbonmonaction.html

http://annhyg.oxfordjournals.org/cgi/content/abstract/18/1/79

http://www.coheadquarters.com/colimits1.htm

http://www.coheadquarters.com/CarbonMonoxideHQ.com/index.html

Flue Season is Here!

No not the swine type, the furnace type! The weather is getting cold enough in many parts of the country that people are starting up their furnaces. Now is a good time to teach your students to check furnace flues during fall seasonal checks. The purpose of this article is not to discuss CO poisoning, but it deserves a mention since furnace flue problems can lead to carbon monoxide poisoning and CO poisoning shares many symptoms with influenza. One big difference is that influenza is normally accompanied by a fever and CO poisoning is not. For more information on CO poisoning check out the Carbon Monoxide Safety Organization web page.

A good place to start discussing furnace flues is to describe the four categories of vented appliances: Categories I, II, III, and IV. These categories are determined by the static pressure in the vent and the temperature of the vent gasses. For reasons of manufacturing and application limitations, Category II and III furnaces are rare. Most furnaces fall into either Category I or Category IV; 80% furnaces are category I while 90% furnaces are category IV. Category I furnaces are vented with type B gas vent. Category IV furnaces are usually vented using PVC. The combustion gas is cool enough to be safely vented through PVC and PVC is relatively easy to seal air tight.

In practical terms the vent gasses in a properly operating Category I furnace will not leak out small cracks because the vent gas pressure is less than the surrounding air. Vent gasses will generally not condense in a Category I flue because the temperature of the flue gas is considerably above dew point. Even though most 80% furnaces manufactured today have induced draft blowers, they still operate with a non-positive pressure vent because of the buoyancy of the hot combustion gas. However, the combustion gas coming from an 80% induced draft furnace is far more likely to condense in the flue than with older natural draft appliances. Oversized vents, single wall vents, masonry vents, or some combination of these can lead to condensation in the flue. Flue condensation can corrode metal vents and cause masonry vents to crack. Severe condensation can return water to the furnace and cause pre-mature heat exchanger failure. These situations most often occur when an older existing furnace is replaced with a newer, higher efficiency furnace. Even though the newer furnaces are designed for regular type B gas vent, they can not necessarily be connected to the old furnace flue. The extra heat in the combustion gas and the dilution air from the draft diverter of the older furnaces combined to make large single wall vent connectors and masonry vents work without condensation. The cooler combustion gas and lack of dilution air in the fan assisted furnaces makes their vent gas more susceptible to condensation. I have seen a single wall vent connector on an 80% induced draft furnace in a crawl space rust completely through and fall on the ground in a single year of operation. The furnace replaced a previous natural draft furnace that operated for many years without problems on the same type of vent. To prevent similar results when replacing an older furnace I recommend using only double wall vent and installing a metal flexible chimney liner when venting into a masonry chimney. An alternative to lining the masonry chimney is to vent horizontally using a power venter and not using the masonry chimney. More information on power venters is available from Field Controls

Another important step is to size the vent. The existing vent for the older natural draft furnace being replaced is often larger than is required for the 80% induced draft furnace. An oversized vent can also lead to condensation. You can download a pdf file on vent sizing from Hart & Cooley

Visual cues that furnace combustion or venting needs attention include: rust on metal vents, condensation weeping from vent joints, or carbon buildup anywhere in the vent system. Your students may run across non-condensing furnaces that were vented using a rigid plastic vent material called high temperature plastic vent, HTPV. This material has been recalled and should be replaced whenever it is found. HTPV recall If they see any of this material they should contact the furnace or vent material manufacturer or to find out what replacement vent material is recommended.

Fundamentals of HVAC/R can help your students prepare for flue season. Details on gas combustion can be found in Unit 37 Gas Fired Heating Systems. Furnace categories are discussed in Unit 38 Warm Air Furnaces. Vent sizing is discussed in Unit 40 Gas Furnace Installation, Startup, Checkout, and Operation. Gas combustion and venting problems are discussed in Unit 41 Troubleshooting Gas Furnaces.