Image Credit: Energy Vanguard If this natural draft furnace backdrafts, flue gases (including, in some cases, carbon monoxide) could get into the home's air.
Image Credit: Energy Vanguard This is the equation for combustion appliance draft, where: Da is the actual draft of the appliance; Dt is the theoretical draft; ΔpLoss is the pressure loss due to bends, birds nests, weather, etc.; Dp is the depressurization.
Image Credit: Dr. Vi Rapp BPI and RESNET depressurization limits
Image Credit: Dr. Vi Rapp Real worst-case depressurization curve, indicating that as the depressurization increases beyond the spillage point, the pollutants in the air may decrease.
Image Credit: Energy Vanguard Ambient CO concentration decreases as depressurization increases.
Image Credit: Dr. Vi Rapp
Burning fuels inside a house can lead to serious health and safety problems. That’s why energy auditors perform a variety of combustion safety tests to find potential hazards and recommend fixes.
A couple of weeks ago at the Dry Climate Forum, I heard Vi Rapp, PhD, from Lawrence Berkeley National Lab (LBNL) make an argument for changing the way we do combustion safety testing. It turns out that one of the tests we do may not be as helpful as many people think it is.
How many people are affected by carbon monoxide poisoning?
You see reports about carbon monoxide poisoning on the news, and it seems that lately there have been a bunch of them. But how many people die of CO poisoning? According to the Center for Disease Control (CDC), 439 per year people died in the U.S. from unintentional, non-fire-related CO poisoning in the period from 1999 to 2004.
The Consumer Products Safety Commission has a different number. They say 170 people per year die in the U.S. “from CO produced by non-automotive consumer products.”
The small number of deaths, however, is not the only metric to look at. A report in The Journal of Emergency Medicine estimated that about 40,000 people per year in the U.S. get medical attention for CO poisoning. Jim Davis of the National Comfort Institute wrote recently that the National Fire Protection Association (NFPA) published a figure showing that number had more than doubled from 2003 to 2010 and is now over 80,000. In addition to those who seek medical attention, it’s likely that a whole lot more live with the symptoms of low-level CO poisoning without every getting treatment.
We’re talking about a lot of people here, and we should definitely be concerned with doing what we can to reduce the number. The BPI motto, after all, is, “Do no harm.” So if you look only at deaths, it doesn’t seem that significant. If you look at CO poisoning cases, it’s something we need to understand. But do we?
What causes CO poisoning?
Energy auditors spend a lot of time testing for backdrafting, one way that CO can enter a home. When the pressure inside the home is low enough, air can come down the flue of a combustion appliance, preventing the exhaust gases from going up the flue. The main candidate for backdrafting is a natural draft combustion appliance, like the water heater shown above (see Image #1) or the furnace shown below (see Image #2). Other combustion appliances can backdraft, too, but it’s less likely.
If a natural draft combustion appliance is inside the conditioned space of a home, backdrafting can create serious a health and safety risk. A backdrafting appliance is more likely to create CO than one operating normally. If the exhaust gases, including CO, get into the house, people can breathe them in and get CO poisoning.
And that’s the big emphasis for energy auditors: carbon monoxide from backdrafting. But how many of those 170-439 deaths or 40,000 CO poisoning cases each year result from backdrafting? I haven’t found statistics on that, but I’d bet the majority of cases are from using unvented space heaters indoors or from running generators, and not from backdrafting.
The problems with the worst-case depressurization test
OK. So the number of deaths seems to be low, the number CO poisoning cases is significant, and the number of backdrafting events leading to CO poisoning is probably a small fraction of all the CO poisoning cases. A big part of the combustion safety testing protocols of both BPI and RESNET is the worst-case depressurization test. It’s what takes the most time, and it’s the part that Dr. Vi Rapp of LBNL took a close look at last week at the Dry Climate Forum.
Their work began with a thorough review of the existing research on combustion safety, codes, and test procedures, including the worst-case depressurization test. They wrote up what they found in 2012, and you can download the 87-page document from the LBNL website.
What they found, according to Dr. Rapp, led them to question the worst-case depressurization test because it was “over-predicting the number of spillage-prone appliances and may be missing some problematic appliances.” Here’s the short version of what they found.
Problem #1. How well a combustion appliance drafts depends on three factors, as shown in the equation from her presentation (Image #3).
Depressurization, as you can see, is only one factor that affects whether an appliance will backdraft or not. You can have depressurization and still have good draft. The good news here is that the energy auditor measures the draft in the worst-case depressurization state, so we have some idea here about what’s going on. Sort of.
One of the factors that affects a combustion appliance’s ability to draft normally is the weather, yet that’s not included in the test protocols. An appliance may draft poorly when it’s tested in the summer but be fine when it’s operating in the winter, or vice versa.
Problem #2. BPI and RESNET don’t agree on what amount of depressurization is allowable and don’t indicate what the real risk is (Image #4)
Problem #3. The big issue here is that even if there is enough depressurization to cause backdrafting, there may or may not be a problem. Dr. Rapp had a slide in her presentation showing that the risk associated with depressurization depends on three factors:
- The probability that the depressurization leads to backdrafting
- The probability that the combustion appliance is operating when the depressurization occurs
- The probability that the backdrafting leads to increased pollutants and an indoor air quality problem if 1 and 2 both occur.
There’s a lot going on in problem #3. How often will worst-case depressurization actually occur in a home? Where are the appliances located? How does weather affect it? How likely is it that the backdraftable combustion appliance will be operating when the house is depressurized sufficiently to cause problems? And even if it backdrafts, might the pollutants be sucked right back out because of the depressurization?
The graph below (Image #5), from Dr. Rapp’s presentation, shows that the most likely “real worst case” occurs when the appliance first starts to backdraft. The pollutants enter the indoor air and build up to higher concentrations. As the depressurization level increases, more pollutants get pushed out of the house and more outdoor air gets pulled in to dilute the pollutants (Image #6).
So it’s not a simple matter of having bad indoor air quality and the potential for CO poisoning anytime the house exceeds the depressurization limit. The answer, as usual, is, it depends.
So what should we do?
Combustion safety is serious. We absolutely need to address it. Low-level CO exposure can lead to chronic health problems. Higher levels can lead to acute health problems or even death. What the folks at LBNL are doing is not to say we shouldn’t do combustion safety testing or that we should abandon the worst-case depressurization test. What they’re really doing is asking, What’s the best way to bring the number of CO poisoning incidents down?
We certainly can be smarter about how we do that. The worst-case depressurization test may be salvageable with some changes to the protocol, but there are definitely other things we can do better at now. Dr. Rapp concluded her talk with these recommendations:
- Remove unvented space heaters; ask if they ever use the oven as a heater and advise them against doing so if they do.
- Check for gas leaks
- Visual inspection of appliance and venting for damage or problems
- Advise homeowner to install and/or use the range hood
- Confirm that the range hood fan exhausts air.
Just to be clear, Dr. Rapp did not say that we shouldn’t do combustion safety testing. She did say that testing for draft and for CO are good tests and we should still do them. What she questioned was the effectiveness of the worst-case depressurization test, especially without accounting for variability due to weather and the total risk when you factor in the three probabilities I covered in Problem #3 above.
We need to be asking hard questions and constantly evaluating the protocols we train energy auditors to use in homes. Dr. Rapp’s work at LBNL is important, and you can read more about it in her upcoming Home Energy magazine article (online only) on the myths and facts of combustion safety. Look for it on 1 March.
In the meantime, let’s all ask the hard questions about what it is we do… and what we’re aiming to do.
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