Duct leakage in cooling loads
For construction in the last decade, Houston region, does it seem plausible to professionals that duct leakage would account for 3% of cooling loads? I always thought average duct leakage was higher, even for Energy Star homes built 2002-07. And thought with higher duct leakage, surely would be responsible for more than 3% of cooling loads. This is Houston TX and the vast majority of houses have ducts and equipment in unconditioned attics.
Thanks for your thoughts -- Pstu
Not sure where you got the 3% from but it can and usually is much greater than that.
The default duct heat gain and air leakage for Manual J v7 is 15%. This has worked well. I believe that they also had fudge factors built in to make it work. Thus the real allowance was higher than 15%. My guess is 20% or more.
The DOE says 25 to 40% and may be more if the ducts are in an attic. http://www.eere.energy.gov/buildings...s/db/34122.pdf
See Page 7 in the PDF file or page 2 in the document.
3% would be sweet! But no, it is much higher than that.
It's not rocket-science...
It's electromechanical thermodynamic engineering
The source of the 3% number is from this study:
Right there on page 3 it makes this statement. I am very curious how a professional study can say something so contrary to what I have learned over the years.
As I read it, the study makes a big thing about how Energy Star homes have tight building envelopes, low duct leakage, etc. And that there are many houses built close to ES standards, which do not have the formal label. I never thought ES criteria was exceptional, just definitely above average.
The homes were from Harris, Brazoria and Montgomery counties. For some reason they left out the fast growing Fort Bend county immediately to the west. I was always under the impression that only a few of our municipalities enforced good building practices, and out in the counties there was no enforcement and therefore builders said no rules were applicable. You hear that a lot around here, that you are lucky to get any kind of conscientious craftsmanship.
Hope this helps -- Pstu
I think REM/RATE is a standardized form used to collect the calculated heat load details. Their claim is not about duct leakage but about the accuracy of the projected cooling energy use compared to actual usage for a large group of homes. They are patting themselves on the back and a measure of skepticism is in order.
Originally Posted by pstu
While they do say what you quoted, the one I was dwelling on said "Building envelope leakage appears to be responsible for about 14 percent of summer/cooling loads while duct leakage only appears to account for about 3 percent of summer/cooling loads."
There is a variance between the PDF page number and the one labeled at the bottom of each page, which causes confusion. The above quote is labelled page 3 by the authors, while it is probably page 8 according to the Adobe program. I regret not making that clearer.
Thanks -- Pstu
Duct leakage is a specific kind of envelope leakage that is fan driven. I have no idea how they determined how to split out duct leakage from the rest of the envelope leakage.
The DOE document is more realistic in my opinion, unless Houston code enforces that ductwork be installed inside of the building envelope or enforces exterior duct sealing with mastic during construction. Do they?
Houston is pretty much the "Wild West" of home construction. Only a few Municipalities have rules regarding duct leakage, e.g. Katy which is outside the survey area. In most regions builders rationalize that rules don't really exist because there is no enforcement staff. For the most part dirt is cheap and people expect houses to be also. Ducts inside conditioned space are really exceptional, the vast majority have all ducts and equipment in an unconditioned attic.
Hope this helps -- Pstu
The typical ENERGY STAR home must pass a minimum of two field tests – duct and house envelope leakage testing – to ensure that actual construction performance matches the computer modeling in terms of house envelope leakage and
Building envelope leakage appears to be responsible for about 14 percent of
summer/cooling loads while duct leakage only appears to account for about 3 percent of
The rated homes duct leakage is % of total load not % of a/c
The cure of the part should not be attempted without the cure of the whole. ~Plato
So is the total leakage 14% including duct leakage, or 17% including duct leakage? Is this tested at CFM50 for both?
Originally Posted by energy_rater_La
What they may be forgetting is that duct leakage can accelerate building leakage.
Originally Posted by pstu
I SERIOUSLY DOUBT the average Texas flexductomaticflyingspaghettimonster install leaks on average 3% of all air moved into or out of the ducting. Are you kidding me? Folks who write these articles or conduct their studies need to get out into the field more and actually witness and measure what's going on.
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.
Shophound when you say, "Texas flexductomaticflyingspaghettimonster", are you Texans claiming credit for this travesty of ductwork?