Check out this interview with brilliant scientist Brett Singer (seriously, he's a genius) from Lawrence Berkeley National Laboratory on our misconception about worst case CAZ Depressurization testing.

Tags: bpi, brett, case, caz, podcast, singer, testing, worst

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Hi Corbett,

An interesting topic and one where I think I disagree with Mr Singer, but being hard of hearing I'm not able catch enough of his discussion to be specific.  If a text version is or becomes available please post.  IMO, CAZ is very important, to the point where it should be handled by a separate entity trained and certified in all aspects.  When an energy auditor or hvac contractor comes onto a job site, they should have in hand the results and recommendations of a proper CAZ evaluation.  Far too many responsibilities are being placed upon a simple energy auditor for which the training and experience is insufficient.

I do thank you for the post and look forward to some comments.


I was interested in his contention that backdrafting caused by exhaust fans is going to be diluted by the makeup air being pulled into the house. But wouldn't part of that "makeup" air be the actual gas that's getting pulled back down the flue?

So, on a related note, when a house is really good and tight, where do we assume makeup air for exhaust fans is coming from? Is there ever a time when the exhaust fans don't work because of the tightness of the house?

on a related note,

Exhaust fans flow into a duct that offers 25 Pa of resistance say.  If the house depressurized to -25 Pa as a result of the exhaust fan running the flow would drop by say 50% but would never stop.  Even a Passiv Haus requires 100 CFM to get to 50 Pa; meaning the tightest possible house would not stop the exhaust fan from working at all. But then that house wouldn't/ couldn't have any naturally vented appliances. 

Okay, thanks.  Good to know.

It's about time that somebody challenged all this worst-case depressurization test hysteria. Mr. Singer does a great job of explaining why these first-generation protocols are too crude and need to be updated. This podcast is definitely worth a listen, but I also found a PowerPoint that begins to illustrate some of these same ideas.

Improving the alignment between home performance testing protocols and actual safety risks will help boost the perceived value proposition for these services. That's an upside we all have to look forward to. I just hope it doesn't take us too long to get there.

Thanks for posting this Corbett.

So glad you enjoyed the show, Tom!

Brett brings up a number of very good points about the limitations of current CAZ testing, but there are a few points that warrant further discussion. 

When considering the byproducts of natural gas combustion, we need to consider the impurities in the supply.  In my region (the Front Range of Colorado) our natural gas contains lots of stuff besides methane.  The presence of these other chemicals (mostly other hydrocarbons) leads to a variety of combustion complications that in turn result in a variety of chemicals in the exhaust gases.  Further complicating matters, these impurities vary over time and are not easy to quantify at the consumer level.  The overall result is that complete combustion of all the hydrocarbons is exceptionally rare. 

Brett states early in the interview that the focus of combustion safety is the prevent people from dying from Carbon Monoxide poisoning.  While this is certainly the top goal, it is (as he points out) fairly rare and almost always the result of several problems all happening at the same time.  That said, low levels of CO are being implicated in a wide variety of non-lethal health problems ranging from suppressed immune systems, fatigue, migraines, and a number of auto-immune related ailments such as asthma.  These more diffuse effects are hard to study so it may be a while before we know with clarity where safe thresholds can be found, but there is clearly cause for concern. 

Brett also puts a lot of emphasis on the overall dilution of these indoor air pollutants as a reason to not worry about occasional spillage.  In my experience, this dilution takes place over time, and there is often opportunity for localized exposure that is much more intense.  Usually I have seen this issue in basements with a bedroom located near a mechanical room, or a family room dominated by video game playing kids.  The overall exposure level in the house may seem fine, but there is an unhealthy level of exposure happening anyway. 

This is a very important topic and I applaud Brett bringing up some of the limitations about current testing methods.  Our industry is getting quite good at building tight homes, and we need to pay close attention to how our combustion appliances integrate into these homes. 

- Ron

I'd like to make 2 quick points.

First, we need to stop talking as if only 'tight' houses might have issues with CO and other by-products of incomplete combustion.  We need to remember that 'leaky' houses are not safe houses where these issues are concerned.  In a tight house we can have control over ventilation in a leaky house we are at the whim of nature.  I'd be very interested to know what the ACH was of the homes where we know the cause of death was CO poisoning.  None of the CO deaths I've read about sounded like homes that had been air sealed.

Second, to answer a question raised in the podcast, code does allow natural, atmospherically vented combustion appliances that are in confined spaces to be connected to an interior unconfined space such as the main body of a house.  Not a good idea but it is allowed by code:

All air from indoors, with 1 Upper vent and 1 Lower vent Venting to another interior room/ space

2 indoor vents: When volume of a room/space containing an appliance is too small, an Upper and a Lower vent may be installed into adjacent space with sufficient volume.
Each vent must provide 1 sq. in. NFVA per 1,000 Btuh input, with a minimum size of 100 sq. in. [NFVA = 100 + # of KBtuh above 100]

We need to remember that 'leaky' houses are not safe houses where these issues are concerned. 

Here here.  "The house is leaky so the risk is low" is a perspective that needs nipping in the bud.  

Do you know where the dominant leakage is?  Did the roofer prescriptively add a ridge vent to that leaky house, causing the basement to be more negative under common or uncommon conditions?  Does use of the fireplace cause that water heater to fail (Not so uncommon problem after installing ridge vents...)?  

We need to get rid of natural draft appliances, but that's not going to happen today or tomorrow.  So in the meantime we need to have SOME idea how those appliances perform as installed. 

Frankly, I believe it's unconscionable that atmospheric appliances are still allowed by Code. When we moved into our current (100-year-old) house the VERY first change I made was to replace the atmospheric water heater with a power-vent unit.

Half a dozen or eight years ago, our office put together a white paper and tried to convince our state Code officials to outlaw atmospheric water heaters. 

The builders all said, "TOO EXPENSIVE! Consumers can't afford that expensive water heater."

Forget the hazard reduction -- We pointed out that the house would get back the two square feet of floor area being used to contain the flue AND save the cost of framing and drywalling the chase. (No one builds houses in Wisconsin with an 80% furnace, so the chase usually only holds a water heater flue.) They'd pay for 10 feet of PVC instead of 20 feet of flue. They'd have one less penetration through the ceiling and the roof. Our best guess was that most builders would be able to save more in construction cost than the added cost of the power vent water heater. And they'd cut down the space heating load, due to the reduced air infiltration.

None of it seemed to matter. Just reinforced my conclusion that "built to Code" translates to "the worst house I can get away with."

I'm glad to see that folks are getting interested in this. Ron, great point about chronic low-level exposures. These are certainly of concern, and are on the radar of folks at LBNL. Also, Judy, GREAT point about localized exposure in basement play areas; I had not thought of that, despite having played in one for hours-on-end as a youngster. I think the main point/issue is that while there may be some very rare instances of real problems with CAZ, our current time-consuming, expensive protocols are not very good at identifying them. As for occasional spillage and other CO leakage moments, the cooking with unvented gas appliances in the kitchen almost without question contributes more to overall exposure.      

As home energy experts we had better take this report seriously. We should not be making recommendation for new equipment based on the inaccuracies that it seems BPI wants to continue to support.

I for one want to see just how fast BPI reacts to this and when they will change the protocol. I can hear it now BPI is going to “state we only recommend we don’t mandate”. Well if there is no science to support the position then what is the position based on. We in the industry need to encourage BPI to review their protocols and action tables accordingly before we are accused of being fear mongers praying on a families concern for their children health and safty.


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