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  1. #1
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    Default 2 blower door questions

    1. I have read that humans need 15 CFM of fresh air. How does that number translate to blower door test results for an average house? In other words, what is the ASHRAE-accepted cutoff point in a blower door test at which mechanical ventilation would be required? Assume a family of 4 if necessary.

    2. Is there a website through which I can locate a local, reputable, qualified blower door contractor?

  2. #2
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    Default Re: 2 blower door questions

    Charles,
    I'll try to get you started by re-posting comments I made to a similar question a while back.

    We have to distinguish between three different measurements:

    (1) Air changes per hour at 50 Pascals is a measurement made during a blower-door test. This value represents how many air changes per hour under unusual circumstances (namely, depressurization at 50 Pascals). At tight house, according to Canada's R-2000 standards, has 1.5 air changes per hour or less @ 50 Pascals. Most US and Canadian homes are leakier than this. A 1999 study of 299 Wisconsin homes showed a median air leakage of 6.0 ac/h @ 50 Pa. A 2002 study of 24 new Wisconsin homes showed a median air leakage of 3.9 ac/h @ 50 Pa.

    (2) Some people try to convert blower door results into a number called "natural air changes per hour" -- in other words, the number of air changes per hour during "typcial" conditions (whatever that means) due to infiltration and exfiltration. Natural infiltration rate can be roughly estimated as 1/20 of the air leakage @ 50 Pa. The trouble with these estimates is that "typical" conditions vary depending on the weather and wind, which can have a very strong effect on infiltration and exfiltration rates. Although this figure -- "Air changes per hour natural" -- is easy to figure if you know air changes per hour @ 50 Pascals, it can be misleading, since the actual air changes per hour can be much higher or much lower depending on the weather.

    (3) Finally, there is a measurement of the air changes per hour provided by mechanical ventilation equipment. The longstanding ASHRAE 62 standard recommends 0.35 air changes per hour of mechanical ventilation. Whether or not this level is appropriate for residential mechanical ventilation is a matter of some controversy, but it is an often-mentioned standard.

  3. #3
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    Default Re: 2 blower door questions

    Charles,
    A more technical discussion of blower-door units follows.

    A typical blower-door test measures infiltration airflow at a pressure difference of 50 Pascals. The airflow is usually reported in cfm @ 50 Pa (CFM50 or cfm50).

    To calculate air changes per hour at 50 Pa (ACH @ 50 Pa or ACH50), multiply cfm50 by 60 minutes per hour and divide the product by the building volume, including the basement, measured in cubic feet.

    Equivalent leak area (EqLA) is the area of a theoretical sharp-edged hole in the building envelope that would leak as much as all of the building’s actual holes at a pressure difference of 10 Pa. EqLA (in square inches) approximately equals cfm50 divided by 10.

    Effective leak area (ELA) is the area of a theoretical hole (with rounded edges) in the building envelope that would leak as much as all of the building’s actual holes at a pressure difference of 4 Pa. ELA (in square inches) approximately equals cfm50 divided by 18.

    Specific leakage area (SLA) is calculated by dividing ELA by the conditioned floor area; it is usually reported in square inches of leakage per square foot of conditioned floor area. California Title 24 defines SLA differently: SLA is 10,000 times the ELA (in square feet) divided by the conditioned floor area in square feet. The California SLA approximately equals cfm50 multiplied by 3.812, divided by the conditioned floor area.

    Normalized leakage (NL), defined by ASHRAE Standard 119, is a dimensionless number calculated by dividing ELA by the conditioned floor area, multiplied by a correction factor that varies with the height of the building. For most buildings, normalized leakage can be approximated by dividing ACH50 by 20.
    “Natural infiltration” (ACHnat) varies greatly by season and by climate. It is not the same as normalized leakage, although its value may be similar. Rules of thumb for calculating ACHnat vary by climate. In Minnesota, ACHnat equals approximately ACH50 divided by 17, while in Florida, ACHnat equals approximately ACH50 divided by 30.

    Several “leakage ratios” have been proposed. According to one definition, the leakage ratio equals EqLA (in square inches) divided by 1% of the total envelope surface area. The Minneapolis Leakage Ratio equals cfm50 divided by the area of the above-grade envelope in square feet.

  4. #4
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    Oct 2007
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    Default Re: 2 blower door questions

    You can contact the mfrs of the blower doors for a list of contractors but be advised nobody is going to rank them or state who is qualified, honest, accurate or anything else if they are sane. You can also Google it but the bottom line is you must do your own homework and get references. Some companies use unqualified kids who slap the apparatus in the front door, crank the fan, jot down the numbers and pack up not knowing what the numbers mean, how to set up and put a house back, etc. Duct Blaster testing should be considered a separate specialty as is IR thermography even though many blower door operators perform these tests and are often very knowledgeable and capable. Bottom line is you have to do your homework.
    You can start with the links section at www.infiltec.com then go over to www.energyconservatory.org for the Minn. Blower Door. Get the manual for $10, which explains a lot of the ins and outs. Read those articles on the Infiltec site, too. I don't have Retro Tech's url but they have good info., too. Other players include Wohler from Germany.
    HTH,
    Hearthman

  5. #5
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    Jan 2005
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    Kansas City, Mo
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    Default Re: 2 blower door questions

    You can visit the Home Performance with Energy Star sponsor for your area. There are three sponsors in Georgia, I don't know which would be for your specific area. The program requires contractors to be certified Building Performance Analysts. It's a pretty comprehensive training program.

    http://www.energystar.gov/index.cfm?...rovement_hpwes
    Rick Westmoreland, CGB, CGP

  6. #6
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    Default Re: 2 blower door questions

    The following statements seem to be most relevant to my question:

    Quote Originally Posted by Martin Holladay View Post
    Natural infiltration rate can be roughly estimated as 1/20 of the air leakage @ 50 Pa. Although this figure -- "Air changes per hour natural" -- is easy to figure if you know air changes per hour @ 50 Pascals, it can be misleading, since the actual air changes per hour can be much higher or much lower depending on the weather.
    Quote Originally Posted by Martin Holladay View Post
    A typical blower-door test measures infiltration airflow at a pressure difference of 50 Pascals. The airflow is usually reported in cfm @ 50 Pa (CFM50 or cfm50).

    To calculate air changes per hour at 50 Pa (ACH @ 50 Pa or ACH50), multiply cfm50 by 60 minutes per hour and divide the product by the building volume, including the basement, measured in cubic feet.

    Rules of thumb for calculating ACHnat vary by climate. In Minnesota, ACHnat equals approximately ACH50 divided by 17, while in Florida, ACHnat equals approximately ACH50 divided by 30.
    Alright, so let me see if I understand this correctly:

    1. Natural infiltration in Florida tends to be almost half that in Minnesota, because the stack effect is greater in Minnesota.

    2. To be conservative in a calculation, I should choose the Florida factor.

    3. There is no need to convert the CFM50 result of the blower door test to ACH50. Simply divide CFM50 by 30 (the Florida factor) to get natural CFM. If this result comes out lower than 60 CFM (what a family of 4 needs), then mechanical ventilation is required.

    How am I doin'?

  7. #7
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    Default Re: 2 blower door questions

    You're doing good, Charles. Here's the rub: natural air changes per hour is a rough-and-ready estimation that has nothing to do with the real world. When the wind is blowing and it's cold outside, your house will have more air changes per hour. When it's not so cold and the air is still, you'll have very few air changes per hour.

    Bottom line: Build a very, very, tight house. Then provide a good, energy-efficient mechanical ventilation system.

    Martin Holladay, senior editor
    www.greenbuildingadvisor.com

  8. #8
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    Default Re: 2 blower door questions

    "Natural" infiltration is also dependent on equipment on the premises. Example: I have a gas water heater in my house, which happens to be induced draft and does not have an outside air supply. It sucks air out of the house, and runs quite a bit. So it is increasing my "natural" infiltration compared to the same house with an electric water heater. This would not show up on a blower door test.

  9. #9
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    Oct 2008
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    Default Re: 2 blower door questions

    Actually, it would.

    If your house is leaky enough that the water heater has enough air to breathe without backdrafting, then the blower door test would show that your house is leaky. The "natural" air changes per hour that we discuss as a result of blower door testing is an artificial number that we use to compare houses, that's all. It doesn't mean that you actually get that amount of air exchage at any given point in time. The estimate is good enough that you probably get reasonably close to that number on average over time. Enough fudging in that statement?

    The bottom line is that the water heater doesn't suck hard enough to really change your natural air exchange much, if at all. For natural draft appliances to work, there needs to be enough air naturally leaking into the house to supply them. Whether they are running or not, the air is leaking in and leaking out. For an appliance to increase infiltration, it must create a pressure drop. No pressure drop = no air flow. Natural draft appliances operate on such a small amount of pressure drop in their vents that even a small change in house pressure causes them to backdraft.

    This is why the Codes now require makeup air for naturally drafted appliances. It's easy using modern building techniques to make a house that is too tight for them to work right.
    All complex problems have a simple solution. That solution is invariably wrong.

    Peter Engle, PE
    Almost Home, Inc.
    www.almosthome.com

  10. #10
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    Default Re: 2 blower door questions

    Quote Originally Posted by Pete Engle View Post
    It's easy using modern building techniques to make a house that is too tight for them to work right.
    Pete, I think the days of natural draft appliances should be over...
    You can not have a Low Energy Home with natural draft appliances within the conditioned envelope.
    The answer is not to Avoid modern building techniques or to provide make-up air to the old fashioned appliances.
    The home should be as airtight as possible with electric or sealed combustion appliances only.
    We must provide healthy ventilation... that means the opportunity for natural ventilation and mandatory mechanical ventilation.

  11. #11
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    Default Re: 2 blower door questions

    I have a 3rd blower door question....

    Negative (Depressurized) blower door tests seem to be the standard.
    How do Pressurized Blower door tests compare to Negative Tests?
    Shouldn't we be doing both?
    Our homes are sometimes pressurized and sometimes depressurized.
    Sometimes parts of the home are in opposite conditions.
    Some airtightness construction techniques would perform differently under the opposite pressure conditions.

  12. #12
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    Oct 2008
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    Default Re: 2 blower door questions

    I've only run the tests pressurized once or twice, just for fun. I got very similar leakage rates both times. Certainly not scientific, but I think the general belief is that either way the results will be similar.

    The reason we do it most often depressurized, is that it is much easier to see and feel the air leaks, both directly and in IR. This way, we get both the absolute leakage numbers and some idea of where the leaks are occurring. It is entirely possible to do it all with the house pressurized, but it's harder. Things are hard enough already.
    All complex problems have a simple solution. That solution is invariably wrong.

    Peter Engle, PE
    Almost Home, Inc.
    www.almosthome.com

  13. #13
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    Default Re: 2 blower door questions

    There is one type of blower-door test that uses pressurization rather than depressurization: a test with a theatrical fog machine. The fog machine is indoors, producing fog. The blower door is pressurizing the house. The builders, subcontractors, and consultants are outdoors, walking around the house and pointing at all the places the fog is escaping.

  14. #14
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    Default Re: 2 blower door questions

    I hadn't heard of that one before. Does it actually work? Theatrical fog is oil-based, and in relatively large particles. I'd think a lot of it would be filtered out by the wall filtration system, er, fiberglass insulation....
    All complex problems have a simple solution. That solution is invariably wrong.

    Peter Engle, PE
    Almost Home, Inc.
    www.almosthome.com

  15. #15
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    Default Re: 2 blower door questions

    According to Marc Rosenbaum, Henri Fennell, and others who use the test all the time, it works very well, and the fog does not get hung up on insulation.

    One main advantage of this trick: subcontractors rarely talk back when the energy consultant points to the fog. There isn't any arguing. The usual response is something like, "Holy Cow! Look at the fog pouring out of the soffit! Let's find the problem and fix it."

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