Replies

1. 
   MartinHolladay_GBA | May 04, 2011 09:42am | #1

   Air changes per hour

   DB2521,

   Your blower-door test results are poorly reported, since the units are flawed. There is no such thing as ACH -- you need to explain the pressure difference at which the ACH was measured. The most common units are ACH(nat) and ACH(50), sometimes reported as ACH @ 50 Pascals.

   It sounds like youre 0.57 number represents ACH(nat), but it's hard to tell without more information.

   A 1999 study of 299 Wisconsin homes showed a median air leakage of 6.0 ACH @ 50 Pascals.

1. 
   db2521 | May 06, 2011 08:44am | #3

   Thanks Martin, I appreciate your feedback.  I guess I don't know if the 0.57 ACH number is natural or not, but it sure seems like it is.

2. 
   davidmeiland | May 04, 2011 10:43am | #2

   I wouldn't say there's any typical range that's useful, regardless of the age of the house. I have tested older homes that were fairly tight, and newer homes that were really leaky.

   They should have given you more data, IMO. They are estimating ACHn (natural air changes) at .57 but not saying how they did that. One rule of thumb is to calculate ACH50 and then divide by 20. Assuming you have 8-foot average ceiling height, with 3200 square feet you have 25600 cubic feet, and at 3974 CFM50 you have 9.3 ACH50. Note that they seem to be assuming you have less than 8-foot ceilings if the /20 rule is used. ???

   9.3 ACH50 is fairly leaky. Typical new construction is often 5 ACH50, good is 3 ACH50, and some of the people really paying attention are getting below 1 ACH50. My guess is that your house has some large air leaks that will easily be found and solved, and then you should re-test (actually, the guys doing the work ought to be running a blower door during the work, so they know what level they are getting). I would be looking to cut yours in half or better, then consider some mechanical ventilation. 

1. 
   db2521 | May 06, 2011 08:54am | #4

   An ACH50 number of 9.3 makes a lot more sense (we do have 8ft ceilings), I didn't realize that there was even an ACHn rating. Then again, I'm not very confident in the testers abilities (his solution was caulk, caulk, and then more caulk....and then caulk some more).

   He has offered to come back & retest for free after I have done the work, so hopefully that number will drop somewhat, but probably not by too much. When the test was running, some doors and windows were literally screaming from all the air being sucked by them (so yes, like the tester mentioned, my doors and windows literally suck).

   Thanks for your feedback, it was very helpful. 

1. 
   davidmeiland | May 06, 2011 10:01am | #5

   The test result

   all by itself doesn't help much, at least not if the goal is to do work that improves the house. The ideal thing is for the tester to work with you to identify leakage locations while the fan is running, and then come back to measure again after you have done the work. You could cobble together something with a sheet of plywood and a box fan.... put that in the door and run it while you work, so you can feel the airflow you are trying to stop.

1. 
   db2521 | May 08, 2011 11:50pm | #7

   Understood. We did go through the house with a thermal camera and didn't find too many places where air was leaking (aside from the windows). The tester will come back when I've made improvements and retest.

   Thanks again to everyone for their help & comments.

2. 
   db2521 | May 08, 2011 11:48pm | #6

   Turns out the 0.57 ACH number is ACH50 - not sure if I believe it.

   "That number was at ACH50. I put in 3,000 square feet. I can put it in again at 4,500 square feet but my experience has been that the square footage does not make that much difference until you get up in the 6,000+ range. Will try to do that in a day or so. "

1. 
   junkhound | May 09, 2011 12:56pm | #8

   Here is a perspective

   50 Pa pumping thru 2 normal 2x4 drywall wall cavities on 16"OC will deliver 400 CFM.  400 CFM = 24,000 CFM hour

   24000 CFM hr for a 1 ACH(50) at 3000 sq ft and 8ft ceiling =24,000 cuft.

   re; "will try to do that in a day so"  --  heck I did that in my head just now, 0.57 (round to 0.6 for off top of head)

   0.6*4.5/3 = 0.9 ACH (50),  eh??

   or izzat 0.6* 3/4.5, you be the judge <G>

1. 
   davidmeiland | May 09, 2011 03:58pm | #10

   Since you did it

   in your head, you get the QC job. I had to use my adding machine.

2. 
   davidmeiland | May 09, 2011 03:56pm | #9

   ACH50 or ACHn?

   The number that really matters in this scenario is CFM50, which you have stated was 3974. That's the ONLY thing a blower door tech can hang his hat on, and he can only hang it there if he operates the unit correctly, the manometer is calibrated correctly, the fan is calibrated correctly, and the house is correctly set to winter conditions. You can definitely screw up a BD test and come away with very inaccurate results. The easiest way to do that is to install the wrong flow ring on the fan, which throws the results off big time.

   So again... assuming that your 3200 square foot house tested at 3974 CFM50, and you have 8 foot ceilings as stated... the volume of the house is 25,600 cubic feet, and 3974 CFM = 9.31 ACH50. That part is math, it is not subject to interpretation. For what it's worth, 3974 x 60 minutes = 238,440 cubic feet per HOUR being moved by the fan... divide 238,440 by 25600 = 9.31.

   Getting ACHn from ACH50 IS subject to interpretation. Without knowing the location of your house, the exposure to wind, and the number of stories, I'll say you divide 9.31 by anywhere from 18 to 22. 9.31 divided by 18 = .52 ACHn, divided by 22 = .42. These are entirely likely numbers for a house of your vintage, unless you hired a superinsulation pioneer to build it waay back when. 

   Since your blower door guy can't be clear about this, your skepticism is warranted. Maybe he wants to hire me to QC his work for him. I also fail to understand his statement that square footage doesn't matter, that's ludicrous.