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Why Do Natural Draft Water Heaters Have The Potential To Be So Dangerous?
A couple of days ago nearly 150 people were CO poisoned at a middle school in the small town of Girard Illinois. Those who were poisoned ranged from children to elderly adults. The first symptoms noticed were dizziness and nausea, some started fainting. By that time you know the ambient carbon monoxide levels are high.
The cause of the CO poisoning was narrowed down to a gas fired water heater that had a flue which had broken loose from the appliance causing the carbon monoxide “leak”. This same water heater was checked and passed an inspection just this spring.
After reading various stories on this incident, the malfunctioning equipment has also been called a boiler and a furnace. In this entry I’m going to focus on water heaters as that’s what has been reported the most. By the way, boilers fit this category too.
How is it that a water heater which had been declared safe several months back has now sent close to 150 people to the hospital? Let’s look at why natural draft water heaters have the potential to be so dangerous and what can be done to ensure this type of incident stops occurring.
The Drafthood Influence
When I see headlines which state a flue that was broken loose was the cause of a CO poisoning it immediately throws up a red flag to me. Every natural draft piece of equipment has a flue that is broken loose by design from the factory. You may be more familiar with this designed break as a drafthood.
ASHRAE defines a drafthood or draft diverter in chapter 34 of its 2008 handbook as serving the purpose of isolating equipment from venting disturbances and allows combustion to start without venting action. These disturbances are updrafts, downdrafts, and blocked vent conditions.
This looks like a pretty significant break doesn't it?
Drafthoods are there to allow the equipment to operate even if it means that it will dump 100% of the flue gas into a building (or school). Flue gases, which include carbon monoxide, can spill freely with nothing to stop them.
Many have been taught that the main purpose of a drafthood is to allow additional dilution air into the flue. As you can see that’s not the only function. By design, its purpose is to isolate the equipment from the flue or chimney. It is a designed break in the venting system. Does this concern you? It should and is one of the reasons natural draft equipment has the potential to be so dangerous.
Burner Location
When you look at perfect combustion the two main byproducts in flue gas are Carbon Dioxide (CO2) and Water Vapor (H2O). If these components of flue gas leave the equipment properly and are replaced with Oxygen (O2) for the burner flame, the conditions exist for safe operation.
When the flue gases comprised of CO2 and H2O don’t leave the equipment properly, the combustion process starts to break down inside of the equipment. The molecular weight of CO2 is heavier than air. As this heavier CO2 begins to “back up” in the heat exchanger oxygen to the burner flame is slowly displaced creating a deteriorating condition. Remember, if the flue gases don’t leave the equipment, combustion air can’t get into the equipment.
This is a big deal with natural draft water heating equipment as the burner in this style of equipment is located at the lowest point in the equipment. CO2 is able to deteriorate the combustion process as it backs up in the heat exchanger with nowhere to go. The burner is already located at the lowest point in the equipment so the combustion process breaks down more rapidly. This influence can be tested and measured to see if it is occurring but few protocols specify how to do it.
Lack of Safeties
There are no safeties on drafthood equipment unless they have been retrofitted. It is also difficult to effectively retrofit drafthoods with safety controls to prevent unsafe operation. You may see a spill switch on one side of the hood to give you that feeling of safety but what happens if the flue gas spillage is occurring from the burner compartment or from the opposite side of the drafthood? The spill switch will remain unaffected and this type of incident will occur again.
Flue gas spillage can occur at the burner compartment as well. Notice the discoloration?
The Building Influence
Another reason natural draft equipment has the potential to be so dangerous is the building influence. The environment the equipment is located in has a direct correlation to how safely it operates. In order for natural draft equipment to operate safely, it needs to operate in a fairly stable environment that is free from factors which negatively influence its source of combustion air. The majority of natural draft equipment allows its source of combustion air to be controlled by Mother Nature and it is never stable.
Depending on how stack effect is influencing a building, the flue could easily become a make-up air duct. As air is changed in a building through exfiltration and infiltration, air will take the path of least resistance. This could be through a number of locations that communicate to the outside such as flues, combustion air openings, fireplaces, framing connections that weren’t sealed before drywall, etc….
When other factors such as duct leakage and exhaust fans start to compete for air with a natural draft water heater, the fans will win every time and safety is compromised. Unfortunately, air can’t read a code book and will continue to follow its natural tendencies regardless of what we assume it will do. The building influence has to be eliminated.
How Do You Uncover This Issue?
Multiple carbon monoxide readings of the equipment’s flue gas hold the key to predicting if this situation will occur or not. Unfortunately the methods currently being utilized by many who are testing combustion safety will cause them to walk right past this issue. Typically, only one CO reading of the flue gas is taken instead of multiple readings during the run cycle. That’s if any readings are taken at all!
A single CO reading after 5 – 10 minutes of equipment operation, or at a stabilization point of the equipment will miss this problem every time. The problem can be uncovered though by looking at the pattern of the CO readings over the run cycle of the equipment. During the run cycle, if equipment CO readings continue to climb, even a bit, you have a deteriorating condition on your hands. That equipment will eventually poison someone.
With rising CO readings you don’t know where the CO numbers will peak out. The sad fact is, if these readings continue to rise after 5 to 10 minutes of run time they will not self-correct. They only get worse over time.
How Do We Stop This?
You might be wondering how this type of problem can be corrected, there is natural draft equipment everywhere. The simple answer is to not use natural draft equipment, but that isn’t a likely solution in many situations. The other solution is to make sure that the stack effect doesn’t occur in buildings and that all air movement in the building is balanced. Unfortunately what we have to deal with in the real world is far from perfect.
Another solution many think will solve this problem is the installation of UL 2034 rated CO alarms. Big mistake with this assumed solution too. ASHRAE dropped the ball with this one in the updated 62.2-2013 standard. If you want real protection get a low-level CO monitor. Don’t kids deserve the best in protection instead of a false sense of security?
Over the counter CO alarms are not the answer in prevention when they can be subjected to dangerous levels.
In the end these assumed solutions are all trying to treat an effect with an effect instead of going after the root cause which is the drafthood itself. Most codes frown upon any modification of equipment even if it is to make it safer. Go figure. There are those who think all natural draft equipment should be replaced and that this will solve the problem. That won’t cure this either. Many of the current CO alarm laws for states are there due to poisonings from direct vent equipment, not natural draft.
Any fuel-fired piece of equipment needs to be tested properly to assure its safe operation instead of assuming it based on a visual inspection and checklist. How many people just turned their fuel-fired heating systems on this week in the north and assumed they would operate safely?
Hopefully through cooperation and education, contractors and code officials will be able to work together to make sure what’s in the field is truly safe. Not just assumed to be safe because a group of industry leaders said it should be safe because it looks good on paper. How many people will continue to be poisoned while the solutions are assumed?
Posted 3 weeks ago by David Richardson
