Dear eQuester:
I want to run a simulation by EEM wizard. I want to replace exiting single pane,
clear windows with double pane clear windows for a school building.
I changed the class type form single pane clear to double pane clear windows.
The energy saving doesn?t look good. I have two following questions need
someone's guidance.
Question 1:
My question is that this EEM will reduce perimeter infiltration rate of the
building. I am not quit sure if I need to reduce the perimeter infiltration rate
in the simulation. If, it does, how many percentage of infiltration rate do I
need to reduce?
Question 2:
Does eQuest programmer has already considered this issue and changed the
perimeter infiltration rate automatically with changing window glass type and
insulation of the windows?
Yingqi Chen
If eQUEST changed the infiltration between the two cases you would be able
to see the change in your .inp deck. I'm not sure what the impact of doing
the change using the EEM Wizard vs. simply copying and renaming your input
deck and making the change in it's own .inp deck would be. You could
experiment and see if there's a difference. If it doesn't change your
infiltration I would lower the perimeter infiltration rate conservatively by
a fixed percent. You might Google infiltration rates on line and find some
information related to how much to expect for a single pane window to a
double pane window. Also, be sure to account for your frames, too. Are they
thermally broken? Does the frame material change, etc.
Carol
Hi Yingqi,
I'm pretty sure eQUEST does not automatically adjust infiltration rates.
If you search through the archives and look up "infiltration windows" you can find some relevant posts. I've attached one email that was sent around earlier this year describing one way to change the infiltration between current and proposed cases.
I don't think you can apply a percentage reduction without some more information. I believe that the requirement to meet the DOE's VPP for windows is a maximum infiltration of 0.3 CFM/SF - this leads me to believe that some new windows have more infiltration than this. My company manufacturers windows with infiltrations as low as 0.01 CFM/SF. I'm having a hard time finding a source, but I'm pretty sure I've heard of modeling existing windows with infiltration as high as 1 CFM/SF. A building with a large WWR and very leaky windows could substantially reduce whole-building infiltration values.
Good luck!
Alex Krickx
Wow! It seems like 1 cfm/sf for infiltration would be a lot. The people's
hair next to the windows would be blowing all around! That's the average
number I use for HVAC ventilation.
Carol
Yeah, it does sound high, doesn't it? But when compared to the maximum level for new windows of 0.3, then it doesn't sound as high... new windows must be 3x less leaky than old ones - that sounds like the right reduction to me.
The only link I could find on performance of old windows was this one: http://www.bfrl.nist.gov/IAQanalysis/CONTAM/table00_arld.htm
It gives leakage areas in "area per linear meter of seam". A colleague converted this to leakage area in CFM/SF at 75 pa. His number was 1.4 CFM/SF based on the table (not sure which value he used).
As a thought: perhaps 1 CFM/SF seems too high because we imagine it as the infiltration for a whole wall, when in actuality it only applies to the window? Assuming the wall has little infiltration, a 25% WWR would bring 1 CFM/SF (window) to around 0.25 CFM/SF (whole fa?ade) - probably a little higher if accounting for the wall leakage, but not significantly so. Does that number still raise alarm bells? Not to me, but I don't have as good a feel for this as Carol does.
I've definitely seen some leaky windows - unfortunately I've never tested how leaky they are so I'm stuck with looking for industry papers and best practice. I definitely would like to learn more about this though. Typical infiltration values of old windows seems to be a big question mark with people I've asked....
Regards,
Alex
ASHRAE 90.1-1989 prescribed the amount of
infiltration to use in the design and budget
cases as 0.038 cfm/sq. ft. of gross exterior wall
area. I believe that subsequent versions of 90.1
dropped any reference to modelling infiltration
rates. The infiltration is "on" when the fans are off.
The MNECB/CBIP in Canada uses 0.05 cfm/sq. ft of
gross exterior wall area - with a schedule on all of the time.
Who knows where those numbers came from and it
doesn't really answer the question of how much
infiltration will be saved with new windows but it gives a starting point.
new windows
.
So maybe we can use what Alex said: new windows must be 3x less leaky than
old ones. Since that is just the window, not the whole exposure leakage, I'd
go ahead and pull a number from the table Alex sent that matched my window,
reduce it by 3 and then do an area weighted average for the entire wall. As
a guide you could use .038 cfm/sf or 0.05 cfm/sf for a wall of the same
exposure as yours and with the same glass area as yours to figure out what
the wall cfm/sf is, unless you happen to know it. As an example xx sf wall x
wall cfm/sf + yy sf window x yy cfm/sf for your window = xx+yy sf x .038
cfm/sf, you know your xx and yy sf, you can pick a reasonable cfm/sf for
your window, or ask Alex for one, and then solve for your wall cfm/sf.
Assuming you aren't doing anything to improve your walls you then have your
xx cfm/sf. Then reduce your yy cfm/sf by a factor of 3, plug all the numbers
in and solve for your new xx+yy, or overall wall cfm/sf. Put that number in
eQUEST, document your calcs for the LEED, or whoever, reviewer and you are
good to go.
Carol
I think Carol's approach is spot-on in terms of calculating infiltration. I always figure out the wall infiltration first (usually an estimate, but it stays constant) and then figure out my "before" and "after" window infiltration values, area weight them with the wall values.
My only caveat is that I'm not sure 3x is always the right number. I threw that out as an example. 1 CFM/SF is pretty high infiltration, so the actual reduction may be less. But 0.3 CFM/SF is the maximum allowable - if you substitute a low-leakage window that has an infiltration of 0.1 CFM/SF then you could get a 10x improvement. Again, that 3x or 10x is just for windows, not the whole fa?ade - you need to know Window-Wall Ratios to figure out the total infiltration reduction.
Unless I've been told that the windows are "the leakiest things ever" I'll typically assume 0.3 CFM/SF for existing windows. Even though this corresponds to a pretty tight new window, I don't like to overstate savings - especially as I work for a manufacturer, I don't want to unfairly manipulate variables to make a product look better than it is. And without more public information available, it's hard to assign a performance to existing windows.
Alex
I have a question along the line of infiltration through windows.
I know that there are methods of calculating infiltration and as an
architect we usually go with the crack method calculating the linear length
of all seams. I have read that air infiltration is not listed on NFRC labels
but needs to be obtained by manufacturers catalog.
My question is, how important is the infiltration when your building is
positively pressurized? Most of our buildings are designed that way, do you
still need to calculate that in your air exchange, since infiltration
through windows is typically an air exchange. My engineer told me not to
worry about it.
We also use metal panels on our PEMB buildings which our manufacturer says
has a lower infiltration rate than other metal panels. It?s how they set up
their gasket. So in the opinion of our engineer you would have to calculate
all the infiltration at all the seams. It?s like going down the rabbit hole.
Does your architect call for taped seams on tyvek? Does the architect call
for butyl at penetrations? Are you accounting for infiltration at doors?
ETC
and believe me the list goes on because I almost went down it.
Thanks,
PETER HILLERMANN