Duct leakage in cooling loads

[pstu]

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

[second opinion]

Not sure where you got the 3% from but it can and usually is much greater than that.

[lynn comstock]

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/building_america/pdfs/db/34122.pdf
See Page 7 in the PDF file or page 2 in the document.

[HVACTechNC]

3% would be sweet! But no, it is much higher than that.

[pstu]

The source of the 3% number is from this study:

http://www.advancedenergy.org/buildings/knowledge_library/FINAL_Paper_Houston_Energy_Efficiency_Study.pdf

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

[lynn comstock]

The source of the 3% number is from this study:

http://www.advancedenergy.org/buildings/knowledge_library/FINAL_Paper_Houston_Energy_Efficiency_Study.pdf

Right there on page 3 it makes this statement. "the study team found that the REM/Rate projected average cooling load of 5,506 kWh/yr was 3 percent higher than the billing analysis average cooling load of 5,677 kWh/yr." I am very curious how a professional study can say something so contrary to what I have learned over the years... 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.

[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

[lynn comstock]

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?

[pstu]

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

[energy_rater_La]

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
duct leakage.
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
.....
The rated homes duct leakage is % of total load not % of a/c

[lynn comstock]

...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.. So is the total leakage 14% including duct leakage, or 17% including duct leakage? Is this tested at CFM50 for both?

[Shophound]

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

What they may be forgetting is that duct leakage can accelerate building leakage.

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.

[lynn comstock]

Shophound when you say, "Texas flexductomaticflyingspaghettimonster", are you Texans claiming credit for this travesty of ductwork? :D

[energy_rater_La]

LOL! I'd gladly allow that!
I think those remrate guys need to stop 'upgrading' the software
3-4 times a year. by the time some of the houses are finished
with upgrades (6 months average) remrate has upgraded at least once.
this changes preliminary and final hers scores. it can be a real pita.
when I first started back in sept of 1999 it was version 8.43
now its version 12.93 and another upgrade has been released..I just haven't
upgraded yet. (got houses in 12.93 I need to finish first)

Lynn, I think that houses are tested at 50 pascals and ducts at 25pa.
not 100% certain but I know my texas based provider has to be reminded
that we test house and ducts at 50pa. every so often he calls me to tell
me I've entered the info incorrectly. as I recall RESNET and SMACCA (smaca??)
industry standard is 25 pascals for ductwork.

[pstu]

If you look on page 44 of the Adobe document (labelled page 39) there is a paragraph which addresses their statement of duct leakage. I interpret it as consultant bafflegab and wonder if you might agree. Use of a phrase such as "the lower end of expected results", followed by emphasis on GIGO, could that possibly be translated as "outlier which does not make sense"? Here is their paragraph, plus one about radiant barriers:
************************************************** *******************
'...Duct leakage as measured by a duct blaster test appears to be related to
summer/cooling loads, but more weakly than envelope leakage (not unexpected). The
estimated impact is about 2 kWh of summer/cooling load per CFM25 of duct leakage.
This result is on the lower end of duct modeling results, which vary greatly depending on a wide range of assumptions. The 2 kWh/CFM25 impact implies that duct leakage losses only total about 172 kWh for ENERGY STAR homes (86 CFM25 average), which is about 3 percent of summer/cooling loads.

... Radiant Barrier roof sheathing appears to reduce summer/cooling loads by about 0.09
kWh/ft² of sheathing, which equals about 180 kWh/yr per home or 3 percent of
summer/cooling loads.
************************************************** *****************

Separately I wonder if there is grounds to suspect the 3% claim on radiant barrier, as that too is very different from what other sources tell me.

Thank you -- Pstu

[lynn comstock]

...consultant bafflegab... I think you are on to something. :D Obuscation and Mendacity work too.

[Shophound]

"Bafflegab"...I'll need to remember that one...

[pstu]

Note that they spend several dense sentences acknowledging the oddity of their result, and then going further to claim duct leakage of 86 CFM25.

There had to have been some actual testing via duct blaster, and I wonder how many of those houses were down to 86 CFM25. Does it seem plausible that Houston area craftsmanship is far better than we expected?

-- Pstu

[Cool_Air]

LOL! I'd gladly allow that!
I think those remrate guys need to stop 'upgrading' the software
3-4 times a year. by the time some of the houses are finished
with upgrades (6 months average) remrate has upgraded at least once.
this changes preliminary and final hers scores. it can be a real pita.
when I first started back in sept of 1999 it was version 8.43
now its version 12.93 and another upgrade has been released..I just haven't
upgraded yet. (got houses in 12.93 I need to finish first)

Lynn, I think that houses are tested at 50 pascals and ducts at 25pa.
not 100% certain but I know my texas based provider has to be reminded
that we test house and ducts at 50pa. every so often he calls me to tell
me I've entered the info incorrectly. as I recall RESNET and SMACCA (smaca??)
industry standard is 25 pascals for ductwork.:grin2:Ita 25 pascal for the duct blaster and 50 pascals for the blower door test.

[lynn comstock]

:grin2:Ita 25 pascal for the duct blaster and 50 pascals for the blower door test.That is my understanding, also.

However, when the duct leakage test is performed with a blower door using the subtraction method, isn't that done at cfm50?

When duct ESP in the field is often .6"wc to .8"wc, I believe that testing at cfm50 would give a more realistic estimate of duct leakage. I don't recall that duct leakage is ever tied to duct ESP of the site tested. I think it should be.

[jimj]

However, when the duct leakage test is performed with a blower door using the subtraction method, isn't that done at cfm50?

be.

Yes it is, cfm50 when doing the subtraction method with the blower door.

[energy_rater_La]

cool air,
my point was that here in Louisiana
our program tests both house and ducts @50Pa.
with ductblaster.
this is one of the rules for our program.

if subtraction method was
allowed, it would be..for us..still 50pa.

I haven't had time to read the post pstu..
maybe over the weekend.

[jimj]

Will the duct blaster measure duct leakage properly at 50pa? Not saying it won't just never done it.

[catmanacman]

performed duct leak test with flow hood on many duct systems mostly duct board and flex and found plenty of them leaking 25 percent and more but most were in the 15 to 20 percent range after repairs less than3 percent, also found sheet metal duct systems that leaked in the 15 percent range . think about it paying to cool the air than through away 1/5 of it or 1/4 of it also when your duct system does not leak your filters stay clean longer duct sealing can and will save money

[energy_rater_La]

jim..beats me. I think that the state just wanted to make processing the ratings
standardized. like setting rate statewide for utility costs @ 9 cents per kwh when it actually fluctuates. some small towns have their own utilities and cost is .18 per kwh,
lowest is my co-op @ .08 and the two largest utility providers are .14 cents per kwh.
our software (remrate) allows us to change these rates to provide actual savings,
but to submit to the state we have to use their numbers.

for me..it isn't a big deal if the duct leakage is off a little.
I think folks get too distracted by that minimal percentage.
the fact is if the system provides 2000 cfm and only 1500
is actually delivered then what does it matter if its 1550
or 1450? you still have unacceptable duct leakage.

in my world its more important to find the leakage and fix it.
our program mandates 5% Energy Star 10%.
it is difficult to seal to 5% in existing homes.
I've taken duct systems and returns apart and put them
back together sealing everything..including equipment leakage
and never gotten less than 5%.

an average is 25-30% for flex. sheetmetal ducts higher due to more
seams & joints. harder to seal. but with a lot of time and effort
it can be done. just delays wrapping ducts until mastic dries.

keep in mind that probably 80% of our duct systems are in the attic.
15% in crawlspaces and 5% in conditioned space.
the % of ducts in conditioned space is changing with the advent of
foam insulating the rooflines.

[jimj]

for me..it isn't a big deal if the duct leakage is off a little.
I think folks get too distracted by that minimal percentage.
the fact is if the system provides 2000 cfm and only 1500
is actually delivered then what does it matter if its 1550
or 1450? you still have unacceptable duct leakage.

:ditto::ditto: Yup you got it!


keep in mind that probably 80% of our duct systems are in the attic.
15% in crawlspaces and 5% in conditioned space.
the % of ducts in conditioned space is changing with the advent of
foam insulating the rooflines.

People are truly becoming educated and concerned about energy costs ( at least some are) and are asking questions. Unfortunately most in the HVAC industry have no clue!

We started up two Energy Star Home systems ( 3.0) today, both foam homes. I know the homeowners will not believe how quiet and comfortable there new homes will be.:grin2:

[Cool_Air]

cool air,
my point was that here in Louisiana
our program tests both house and ducts @50Pa.
with ductblaster.
this is one of the rules for our program.

if subtraction method was
allowed, it would be..for us..still 50pa.

I haven't had time to read the post pstu..
maybe over the weekend.:whistle:Thanks I understand.

[adamwhatley]

I have always used Remrate to size homes out over the years. I have tried all load programs and all are pretty much the same. With that said Manual J and wright soft are up there with Remrate to me. Wright soft software in my opion over sizes by 1/2 ton unless everything is perfect. Manual J is spot on when it is done right but very time comsuming. Remrate to me is easy to use but I was taught on it through the same Energy Program that LA Energy Rater has and is working with.


As far a duct leakage goes I do belive that uses good Margins but need to watch your blower door results and factor into the load along with air filtration along with all the other important sizing things that our homes have.

[Cool_Air]

cool air,
my point was that here in Louisiana
our program tests both house and ducts @50Pa.
with ductblaster.
this is one of the rules for our program.

if subtraction method was
allowed, it would be..for us..still 50pa.

I haven't had time to read the post pstu..
maybe over the weekend.:gah: thanks

[r-290]

FWIW here in California we can have up to 15% duct leakage with a system change out. If new ducts were put in then that number drops to 6%.
So duct work in the attic and loses of 50% total energy is not far fetched.
as Lynn pointed out in the DOE doc.

I'm sure in the next few years foil flex will be the only thing allowed in attic's

[energy_rater_La]

so even in a change out you address duct leakage? thats pretty good.
wish we had that kind of enforcement here.

[pstu]

The original paper sure seemed to say 86 CFM was the average duct leakage for the Houston area homes. Houston homes built in the last decade are not small, and it could be reasonably estimated the average AC size is about 5 tons -- that would be 3-5% duct leakage would it not? I find it hard to believe Houston area homes are constructed with this degree of craftsmanship.

If this is not the straight up honest truth, then I would be interested to understand what data would allow a professional organization to say this. Scuttlebutt about the Centerpoint Energy marketing department is they are hardly committed to the truth, but the partner Advanced Energy looks like it has a reputation to uphold.

-- Pstu

[r-290]

Makes those mini splits better and better all the time

[energy star]

From what a read California has a third party inspection on almost all HVAC work (title 24 and whatever else) that is being done in a home. I like it.

[lynn comstock]

From what a read California has a third party inspection on almost all HVAC work (title 24 and whatever else) that is being done in a home. I like it.New construction gets inspected when there is a water and electrical hookup.
In our area (AZ) most replacement equipment is not inspected because contractors do not get permits. I imagine that this is true in most jurisdictions. Code doesn't mean anything when it is not enforced.

[energy_rater_La]

Code doesn't mean anything when it is not enforced.
which is often the case in my area. in the cities there is
enforcement, but in the rural areas it is spotty at best.

often the code inspectors pov don't agree.
code is dependent upon their interpertation of what it says.
all those shalls and musts don't always make it to the job site.

pstu..I find it hard to believe that 6% duct leakage is average anywhere.
but I'm a skeptic until I see it/ test it for myself.
I do know that it would have to be installers trained by same trainer
or same installers doing the sealing work on all projects.

and
"If this is not the straight up honest truth, then I would be interested to understand what data would allow a professional organization to say this. Scuttlebutt about the Centerpoint Energy marketing department is they are hardly committed to the truth, but the partner Advanced Energy looks like it has a reputation to uphold."

scuttlebut I get is that everyone a hedge factor. as utility programs begin to
influence the auditing business people need to be realistic about just how
much utility cost they want the average homeowner to save.
50% lol..more like 15-20% this is what I get from my co-op and one of the
two large utility provider's energy managers in my area.

[lynn comstock]

....people need to be realistic about just how much utility cost they want the average homeowner to save.
50% lol..more like 15-20% this is what I get from my co-op and one of the
two large utility provider's energy managers in my area. It really depends on the peak load capacity of the utility. Cutting AC load reduces peak load and makes the utilities more profitable. Why? Because peak loading consumes the most expensive electricity as most of the time the equipment needed is on standby. Also, if peak demand grows, new power plants must be planned and constructed.

[r-290]

From what a read California has a third party inspection on almost all HVAC work (title 24 and whatever else) that is being done in a home. I like it.

Correct, the name Title 24 Building Energy Efficiency Standards is called title24 for short. It covers all type of home energy use from HVAC to windows to water heaters. Its updated every few years.

If you take out a permit it will be inspected by the building dept ( code compliant) and then you are subject to a HERS testing. Can't remember what HERS stands for, but just think Energy Rater. They will check the subcool level by connecting up there gauges to the condenser and check the liquid line temp. They check the fan watt draw and CFM flow. They run a duct blaster and check for leaks.

Like anything there are lot of rules and exceptions which is nice and a pain at the same time.

http://www.energy.ca.gov/maps/renewable/building_climate_zones.gif

[r-290]

It really depends on the peak load capacity of the utility. Cutting AC load reduces peak load and makes the utilities more profitable. Why? Because peak loading consumes the most expensive electricity as most of the time the equipment needed is on standby. Also, if peak demand grows, new power plants must be planned and constructed.


So the power company's answer is Smart meters that charge you twice as much during peak time. Which is 10am to 8pm and they charge you double when you go over the base line of 700 kWh too.

http://hvac-talk.com/vbb/attachment.php?attachmentid=217352&stc=1&d=1318357176

[pstu]

It really depends on the peak load capacity of the utility. Cutting AC load reduces peak load and makes the utilities more profitable. Why? Because peak loading consumes the most expensive electricity as most of the time the equipment needed is on standby. Also, if peak demand grows, new power plants must be planned and constructed.
Another thing about the utility point of view, is usually a cut in residential demand (peak KW) goes hand in hand with a cut in energy (KWH). Since most utility tariffs are based simply on KWH, they have trouble wanting that. A very specific program to cut demand, with repayment from the utility commission for their "negawatts", would be a clear exception.

I know the Houston area utility invested in some programs to manage demand, and their request for repayment was rejected by the utility commission. So they may view conservation with some suspicion, at the top management level.

Regards -- Pstu