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Building Science

Do Combustion Safety Testing Protocols Need Fixing?

Dr. Vi Rapp of Lawrence Berkeley National Lab says one test in particular doesn't help as much as we think it does

Combustion safety inspections and testing are critical in preventing carbon monoxide poisoning. This water heater has a venting problem that should be caught by a visual inspection.
Image Credit: Energy Vanguard
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Combustion safety inspections and testing are critical in preventing carbon monoxide poisoning. This water heater has a venting problem that should be caught by a visual inspection.
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.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


  1. GBA Editor
    Martin Holladay | | #1

    Looking at Image #1
    I assume that Image #1 illustrates a problem with inconsistent types of flue pipe (and suspect flue pipe joints) -- right?

    I don't think that the photo shows any evidence of backdrafting. The dust on the top of the water heater appears to be drywall sanding dust, not soot or particles attributable to backdrafting.

  2. ntisdell | | #2

    causes of posioning
    In MN it seems most of the headlines i hear about someone getting sick/dead is from not improper makeup or backdrafting or a tight house.

    It is from a running car, ice plugged flue, or some other type of catastrophic failure. That should have been detected by law required CO detectors. Except in cases where it is in a garage, icefishing house, passengers of large boats (CO pockets by back seat or while slow tubing)

    I have tested with a meter while in my boat for CO and found at idle or no wakes speeds CO of 15-80 if going with the wind or calm winds. Worse then while in garage with a running car.

  3. wjrobinson | | #3

    Radiant gas heaters
    I will google but does anyone know the amount of CO a gas range gives off during a typical evening cooking session? And question two, how much CO does a radiant gas heater give off per btu size?

    I have radiant gas heaters and have a digital home style CO detector. The detector never reads above 0. A friend heats their store with a large unit all day every day all winter. The owner is still alive and has not had to go the the ER for CO.

    The cases around here seem to be faulty burns.... yellow flames bad, blue flames good and clean?

    So can a radiant heater burn gas with no CO just CO2 and H2O? Same for cars with working catalytic converters? Maybe the CO2 is the danger more than the CO when these are functioning correctly?

    Edit; just did a quick read... looks like the depletion of oxygen in an area where the heaters are used throws the CO count up to hazardous levels. So a air leaky old home may have no problems and a tight home or closed off room is a death trap. Dilution as this blog talks of is key to use or disuse.

    Edit add 2; ODS oxygen depletion sensors... save lives. Best type to own of the portable propane heaters. Just read an article that said very few deaths have come from the use of portable units which is contrary to what Allison states above. Most have come from heating too small a space with the lack of air leakage resulting in lowered oxygen levels along with CO and no ODS on the heating device. Very interesting, blog as I do have quite a few site heating devices from salamanders to radiant propane tank top heaters which are great for warming the hands first thing in the AM.

    So make up air is key along with ODS for portable gas heat.

  4. heidner | | #4

    RE:Radiant Gas Heater

    I have gas stove, so long as it is operating correctly (including the oven), the CO levels are generally lower (undetectable) than the CO from a bag of fresh ground coffee. Running the fan over the stove with a known make up air (crack a window).. and the CO from stove isn't likely to be the significant health problem in the kitchen. It is worth noting that the last time the oven ignitor failed, I pulled the bottom oven tray out - and discovered the flame spreader had failed... and was lying flat on one of the burners. I can't imagine that the combustion process was working as designed. I fixed the flame spreader at the same time as the ignitor.

    Hot water heaters are roughly 30,000 btu/hr.. Gas dryers are about the same. Either appliance is deadly vented into the room. The gas stove, with oven going.. can easily produce 30,000 btu/hr. About the same - even in new condition. Unvented stoves are unhealthy without the stove fan running. However, the stove isn't the only problem. it could be the other products from cooking...

    CO is fatal in small quantities..(well under 200ppm) CO2 in the same quantity is mostly non-impacting to we humans. In larger quantities > 3000 ppm the CO2, may give you headaches (cognitive ability may be impacted before that..) but we pretty much recover from "high" residential CO2 levels. Our bodies have adapted and the blood is more willing to give up CO2 and not hang on to it until we die. CO on the other hand, latches on and doesn't let go unless really forced to by using a hyperbaric chamber with oxygen. Navy submariners are trained and tested to endure well over 10,000ppm for extended periods... (they do pay for it - eventually, loss of calcium, sleep difficulties, etc... but those are from long duration exposures).

    I always worry about the catalytic heaters -- in part because more of them are now being made off shore. Few of us have the ability to test if the correct catalytic coating were applied and have not since been poisoned by some household chemical. I suppose if the catalyst is gone - you would smell the propane or NG and not use the heater, at least that's the theory.

    People that have cardiac problems, lung problems, or previously had brain injuries can be more sensitive to high CO2 levels. Others in the house would probably describe the house a stuffy before they would be seriously impacted. There are CO2 sensors which can be used to measure and track the approximate concentration. They are much lower cost than O2 sensors if you can find them for houses.

    People that have cardiac, lung or previously had brain injuries are also likely to be more sensitive to O2 depletion... If you use a combustion device in the house -- make it direct vent, with intake air... or provide the equivalent..

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