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This Little Hitch Snares ENERGY STAR Homes

ENERGY STAR New Homes Must Pass Performance Testing Of Bathroom Exhaust Fans

Let me tell you a little story about the day that Jeffrey went to test several Habitat for Humanity houses that are going for certification in the ENERGY STAR new homes program (the one in the photo here was not one of them). This was a couple of weeks ago, but I wrote down the numbers he told me because I think you may be somewhat surprised.

This isn’t an issue unique to Habitat, though. If you’ve done the kind of testing required for Version 3 of the ENERGY STAR program, you’ve likely seen similar numbers for the failures I’m about to describe.

The little hitch that snares

One of the requirements in ENERGY STAR V3 is that bath fans have to be able to exhaust at least 50 cubic feet per minute (cfm) of air. It’s got to be measured by a HERS rater as one step among many in the required commissioning process. Many builders going for ENERGY STAR have learned that fans rated at 50 cfm generally don’t cut it. This particular Habitat affiliate, as a result of early failures, made the decision to install fans rated at 110 cfm in all their homes.

Should be pretty easy to hit 50 cfm with a 110 cfm fan, right? No, not necessarily.

Jeffrey tested 9 fans, all rated at 110 cfm. Want to guess how many passed on the first try? Well, the actual number was five. That’s right. Only five out of nine fans passed.

And even their results weren’t stellar. None of them even hit 100 cfm. One barely made it across the finish line with 51 cfm. The other four were between 51 and 85 cfm.

And the four losers? Their exhaust flow rates were 30, 45, 45, and 46 cfm. Did I mention that the fans were rated for 110 cfm? Those four fans couldn’t even get 45% of their rated air flow!

If you’re a builder, HERS rater, or HVAC contractor working on ENERGY STAR homes, what kind of failure rates are you seeing on bath fan exhaust flow rates?

How we measure bath fan flow rates

The photo below shows Andrew Woodruff of Building Performance Engineering in Boone, NC using the exhaust fan flow meter from the Energy Conservatory. We use the same device, as do many HERS raters. You just hold it over the fan with the fan turned on and measure the pressure difference with a manometer. The range of exhaust flow rates it measures is from 10 to 124 cfm with an accuracy of ±10%. They’re quick, easy to use, much less expensive than a flow hood, and way lighter than a powered flow hood.

exhaust fan air flow measurement Andrew Woodruff BPE Boone NC

How to pass on the first attempt

These Habitat builders have modified their processes to ensure they get their homes certified, but they’re still working on some of the details. Here’s the full solution:

  • Use a higher capacity fan. You shouldn’t count on getting 50 cfm out of a 50 cfm fan.
  • Install the housing with the duct port facing the right direction. It’s amazing how often I see them installed so that the duct has to make an immediate 180 degree turn.
  • Shorten the duct runs. Don’t run the duct all the way across the attic. Try to keep the duct length to less than 10 feet.
  • Upsize the duct. Go with a 6″ duct, rather than the 4″.
  • Use hardpipe or install flex perfectly. Poorly installed flex can zap the oomph right out the air.
  • Make sure the wall cap or roof vent is operating properly. Sometimes those dampers stick, or get screwed or painted shut.

bath fan exhaust flow rate energy star version 3 6 inch duct

The photo above shows one that does almost all those things and should have no trouble passing the air flow test. It does have one little issue that may need to be corrected before the final inspection, though. They went to the larger duct, but it’s got a crimp where it goes across the first 2×4.

If you’re doing work on ENERGY STAR homes, it’s good to keep these things in mind. Jeffrey doesn’t like to go back out for reinspections, and your HERS rater probably doesn’t either.


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.


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This Post Has 33 Comments

  1. Excellent article! This is an
    Excellent article! This is an oft overlooked issue when testing isn’t done.  
    BTW, I’m guessing all those fans didn’t fail with a duct installation like the one in the photo, right?

  2. It is good that we are
    It is good that we are learning these things thru the commissioning process. I am glad that Energy Star is requiring this. 
    You have a great blog!

  3. Couple things:  &lt
    Couple things: 
    1) Consider going with ECM bath fans such as Panasonic WhisperGreens. They’ll adjust for duct issues, to a point 
    2) Hard pipe wherever possible…it simply isn’t that much more expensive and it tends to enforce installation discipliine 
    3)Beware instrumentation. The full size hood often used for commissioning is rated to 2000 CFM. Its accuracy at 50 CFM may be worth questioning. I haven’t tested many fans but one 2 speed 130 CFM Panasonic I checked with my small flow hood was dead nuts on at both speeds, but I did require 6″ hard pipe. 
    4) Beware an overly restrictive outdoor cap 
    5)Check the 5 cent plastic flap at the fan discharge – it may bind or otherwise fail to swing. Consider removing it if outdoor cap has a flap (as it should)

  4. I see bath fans all the time
    I see bath fans all the time installed so the duct has to do the 180 degree turn. I have always wondered about how that affects the air flow. Looks like I need to be calling this out on my reports! 

  5. This is an issue we see all
    This is an issue we see all of the time in our neck of the woods. Amy in our office actually blogged about this very issue last week, with very similar insights: 
    We agree with both your article and Curt’s comments. Whenever possible, we also recommend to builders that they should vent the bath fans out a vertical surface (wall) versus out a vented soffit. That 270-degree bend really kills the flow. The other BIG issue is coordination between the person running the bath fan flex duct and the person siding the house. If the hole cut in the siding is off at all and happens to be next to a truss or floor joist, the amount of mangling that happens to that piece of flex is unimaginable.

  6. I submit that with a large
    I submit that with a large radius bend, neither the 180- nor the 270-degree turn will be an important factor in killing air flow. People do that with hard pipe all the time, don’t they? 
    And, I submit that if you use 6-inch flex duct and don’t do anything stupid, that will allow for enough air flow. This should be provable with ACCA Manual D and a chart of fan output vs. static pressure. Would like to pursue this more but not enough time this morning. Would anyone feel analytical enough to look into this?

  7. We did have trouble with this
    We did have trouble with this on one of our Energy Star 3.0 houses, even with 80 CFM fans and bathrooms only on exterior walls Two out of three passed, one didn’t. 
    We then opened up the soffit vent for that fan and saw that there was about 2 extra feet flex duct scrunched up in there right at the end. We cut the extra flex duct off and re-attached the duct to soffit vent and got a high enough airflow.  
    Lessons learned: will spec 110 CFM not 80 CFM, will upsize the duct to 6″ instead of 4″, will watch for excess duct at the end of the run, might even consider using only Whispergreens, since as mentioned above they can adjust their airflow somewhat to compensate.

  8. Another part of the problem
    Another part of the problem is fan ratings and industry marketing. The airflow ratings published by the Home Ventilation Institute are based on 0.1″w.c. and 0.25″w.c. static pressure. Fan manufacturers typically promote their fans based only on the 0.1″w.c. value, which is kind of like saying, “Hey, if you install this fan, attach it to less than 5′ of straight sheet metal duct and skip the exterior hood, you’ll get the rated airflow.” The last time I saw this kind of install was….never.  
    Side note to Curt K. – most HERS Raters use either the Exhaust Fan Flow Meter or a “Flowblaster” from Energy Conservatory (+/-10% and +/-5% stated accuracy, respectively), with the ability to measure down to ~10cfm for this kind of testing.

  9. When you have a very tight
    When you have a very tight home lower than 2 ach 50 exhaust fans in these homes have to work very hard to exhaust the air they specify in their literature. I have measured these issues many of times.

  10. Please don’t upsize fans to
    Please don’t upsize fans to 110 CFM models to get your 50. That’s like installing a bigger engine to go faster instead of fixing flat tires.  
    Follow the manufacturer installation instructions carefully. Buy good quality, quiet fans rated for continuous duty like the Panasonic Whispergreens. If you install them like we do (below), they will use ridiculously low Power and deliver an average of 10% more flow than labeled: 
    – use a minimum of a 12″ long piece of 4″ KD ending into a 4″ elbow. Use longer as needed to be plumb beneath a good roof exit point. Slope the elbow end of the pipe to be 1/4″ higher than the fan outlet. Point the elbow straight up toward the roof deck.  
    -install a 6 to 4 reducer at the elbow (or a 4 to 6 increaser, depending on which your shop carries, and whether the glass is half empty or full) 
    -install a RV38 roof vent, after removing the screen mesh.  
    -install a 7 to 6 reducer at the roof vent.  
    -install pulled narly taut, R-8 flexible ductwork between the two opposing ends, using standard duct practices (mastic, bands. Cut back the inner core to permit pulling the insulation over the 4″ section.  
    -Note you will need to slit and resize the outer jacket it to fit the last section, and tape it up with flex fix and some draw bands.  
    – 6″ elbow at the unit, eliminate the 12″ section of 4″ and the 4/6 reducer. No noticeable change in performance. 
    – all 6″ KD with R-8 duct insulation added. More durable and time consuming. Same performance.  
    – 6″ R-8 flex duct into a 6″ sidewall outlet, sloped at a minimum of 1/4″ positive per horizontal foot of run.  
    As noted, all measurements taken with an EC FlowBlaster or Alnor Low Flow (500 CFM max) Balometer only.  

  11. I cannot tell you how many
    I cannot tell you how many different ways we’ve failed the bath fan test. Every time it’s a different way. Follow the above guidelines and you’ll be well on your way to avoiding nearly every problem we’ve had. 
    I will say we regularly have problems with installation with screws blocking the flap (as Curt Kinder mentioned in #5). We’ve corrected that specific problem several times. Its now specifically written in our HVAC contract that bath fans must be installed with tape and no screws as a friendly reminder. 
    We also do a simple pre-(or immediately post) drywall test (if possible). If the fan can draw up a piece of toilet paper with a good tug then we have a good sense of if we’re drawing over 50cfm.  
    There’s nothing sadder than a 80 cfm fan in a finished bathroom that tests at 14cfm.  

  12. David B.:
    David B.: No, the photo above wasn’t from one of the homes tested in that batch. 
    Curt K.: All good points. I added a section to the article about how we measure flow rates for bath fans. 
    Matthew V.: Nice article! Here’s the direct link for those who want to click: What to do when your bath fan fails
    M. Johnson: True, but as Matthew Vande said, it’s often impossible to get that large radius bend. 
    Leigha D.: I didn’t say it in the article, but you don’t need to go all the way to 110 cfm rated fans. Stick with 80. 
    John S.: Thanks for bringing up how bath fans are rated. As you say, the static pressure in real applications can’t get you to the 0.1″ w.c. rating. 
    Anthony G.: That’s something I’ve wondered about with tight homes we’ve worked on, but we haven’t been able to confirm that it made a difference. We’ll have to talk more about this. 
    Charles L.: I’ll have to ask Jeffrey, or get him to comment. 
    Mike M.: As usual, you provide a wealth of knowledge and experience with a lot more detail than I did in the article. I didn’t mention it in the article, but I totally agree that going all the way to 110 cfm rated fans to get 50 is overkill. You should be able to do it with 80. 

  13. Heads up, we found out that
    Heads up, we found out that our box was under by 20%. So we called Advanced Energy and they checked theirs for us. It was also under by 20%.  
    In other words, we were seeing, I think, 41 cfm when the fan was actually pulling 49 cfm (according to the duct blaster powered flow hood). 
    So we got EC to send us a new one and it was under by 10% – brand new. We did look and fiddle with ours but couldn’t figure out any reason it would be off by another 10%. 
    Not trying to negate the valid points of this story but it’s worth checking yours if your making people replace things based on the numbers you’re getting.

  14. Skye: Good
    Skye: Good point. I’ve wondered about the true accuracy of the exhaust fan flow meter but we haven’t tested it yet. Still, even though 3 more of those houses technically would have passed if the flow meter were 20% too low, when you’re getting only ~50 cfm from a 110 cfm fan, you’ve got problems.

  15. Yes, very true. There are
    Yes, very true. There are definitely good points there. Same things we see all the time. As I said, I wasn’t trying to negate the thrust of the argument. It just reminded me of what we had found (the hard way). And I found it interesting that they weren’t just that far off, but all low. 
    And then theres the problem that Andrew is doing the testing in that picture. Probably 30% off on that one!

  16. Skye: No,
    Skye: No, I knew what you meant, and it’s a good point. Now you’ve give me some incentive to go out and test the EFFM for accuracy. Have you found that it’s always low, no matter which aperture you use?  
    But as for Andrew’s accuracy, I think the two of you should settle that the good old fashioned North Carolina way – with a blower door duel at sunrise. ;~)

  17. Yes as I recall we tested
    Yes as I recall we tested with each setting & got consistent 20% under.

  18. Allison: If you want to talk
    Allison: If you want to talk more about this let me know. We did a lot of testing on low load, very tight homes to understand pressures in a home caused by fans in regards to ventilation and back drafting issues.

  19. Don’t forgot that some Broan
    Don’t forgot that some Broan fans come w/ the flap taped down before it is boxed. The installer doesn’t always catch it. Found this at least 3 times. Found the attic insulater has stepped on the exhaust line a couple of times too. 
    Several times I’ve matched up my $150 exhaust fan flow meter w/ my $1300 Alnor flowhood. They’ve always been w/ in 5 CFM. Yes the flowhood needs to go in for calibration every couple of years.

  20. I apologize for chiming in so
    I apologize for chiming in so late. Great discussion. 
    David B. – No, the install in the photo actually moved 119cfm when I tested it two weeks ago. It was rated for 110cfm. This shows what many others pointed out in the comments; there’s no need to spec a 110cfm rated fan, just install the one you get correctly. 
    Charles L – As background, the 9 bath fans were in three adjacent homes and were measured on the same day with the same equipment. Regarding conditions, there were other workers on site that day and it’s most likely that an exterior door may have been open during some of the testing. I don’t believe these homes were tight enough to impact the ratings though. Tightness results (cfm50/ach50) were 643/3.6, 696/3.9, and 711/2.9 (larger floorplan). I’ve done a quick comparison in similarly constructed homes by this builder and did not see an impact on the bath fans with exterior doors open versus in “tight” mode. I’ll test both ways going forward to see the impact. 
    As Allison mentioned, we’ve often wondered about the impact of super-tight homes on bath fan performance. We had that hindsight revelation/question on a 0.77ach50 (<400cfm50) house I measured last year. I would be very interested in seeing Anthony’s G testing results. 
    Not to be overlooked, John S made a great point (as always) about the fan ratings being marketed at 0.1″wc static pressure. This has come to the forefront recently with ES Rev 7 and the prescriptive duct sizing compliance paths for kitchen exhausts, where their prescriptive requirements reference fans rated at 0.25″wc.

  21. Wow, there is an in-depth
    Wow, there is an in-depth conversation about bath vent CFM! This is hilarious. Nothing against all of the E-STAR chrony contractors, but just a few thoughts to try to bring common sense back.  
    A: Will the occupant even use the fans? 
    B: Will the occupant leave the fans on so that they get dirty in a few years and produce a fraction of the original CFM? 
    C: Think the occupant will ever clean that vent? 
    D: Is your real priority the occupant, or filling out your gubment checklist? 
    E: If the desired results of using a bath vent are to remove the steam, and smells, don’t you think the homeowner, no matter the CFM will use the fan until the steam or smell is gone? If minutes are a variable decided by the occupant, exactly what does the CFM matter? A 1cfm fan running all day will move more air than a 1400cfm fan running for a minute.  
    You all seem to question the fans, the installation, the testing methods, where are the comments on how unreliable, and flawed is Energy Star? Are you really going to delay the building process and have to do return audits over 5cfm in a bath vent even though your equipment is only w/in 10% accurate at best? 

  22. Greg:
    Greg: Thanks for helping us get some common sense! All of us here are just book smart energy geniuses who never go into real houses and measure things or have to deal with unhappy clients, so I’m really glad you’ve taken time out of your busy energy auditing schedule to bestow your great wisdom upon us. 
    Seriously now, Greg, this is the second article you’ve left one of these sanctimonious comments in. You’re not likely to make any friends—or even get people to consider your ideas—if you come in here with that kind of holier-than-thou attitude. You sound like someone who failed the HERS rater test and now carries a grudge against anyone in the industry.

  23. I am not trying to be morally
    I am not trying to be morally superior, I am trying to make sense of these bath vent concerns, notice my comment is mostly questions of which you couldn’t seem to answer one. Energy Star is not an authority in home building, but HERS/BPI contractors just follow their lead, why? I am just picturing the Hab for Hum house that had it’s 45CFM fan replaced with a 50CFM fan, and the occupant standing under it blowing their meth smoke through it, thinkin, gee thanks for that extra 5CFM.

  24. Greg: Well
    Greg: Well apparently your moral superiority happens without your even trying then. This article is about homes going for certification in the ENERGY STAR program, which is voluntary. If you create a program, you can require whatever you want, and actually the bath fan exhaust flow requirement is a reasonable one. Your list of questions is irrelevant in the context of this article, and that’s one reason I didn’t bother to address any of them. 
    Actually, ENERGY STAR is an authority in home building, and I find your stereotype about families who live in Habitat for Humanity homes quite offensive. 
    M. Johnson: Indeed.

  25. Greg said: 

    Greg said: 
    >>>A 1cfm fan running all day will move more air than a 1400cfm fan running for a minute. <<< 
    Yeah, but when I stink up a 11.2 x 11.2 x 11.2 bathroom, I don’t want to wait 24 hours to clear the air…and gain 40 more CFM to boot :p 
    Seriously, Greg, I’m a homeowner, also. I’ll run the fart fan until the fart smell is gone. I’ll run the steam fan until the steam is gone. It’s even better when one fan can do both tasks.

  26. How many homes has energy
    How many homes has energy star built? “Energy Star program had accepted 15 out of 20 bogus products submitted for approval” including a diesel powered alarm clock. <—that energy star is an authority on home building? How is it not relevant that a 1CFM bath fan that gets used will move more air than a 1 Trillion CFM bath vent that doesn’t get used. That is extremely relevant. As a builder it just gets old seeing these attempts at “one size fits all” and the common mistaken mentality that homes use energy. Homes are not responsible for fresh air, occupants are. Homes don’t use a drop of energy, occupants do. We continue to take responsibility from the occupants for energy and health, and we continue to have greater irresponsibility from occupants for both, and we sit back and wonder “why?” I fell for the energy auditor dream, but realized very quickly that if I drop the duct blaster and pick up some mastic I might be able to do some actual good in reducing energy usage.

  27. Cameron: As a homeowner, if
    Cameron: As a homeowner, if the fan doesn’t move the air I want, I can spend 30 minutes and 40$ replacing it. That is the responsibility and prerogative of the homeowner. I doubt very much you are there to turn the fans off at the moment they remove the smell or steam. Say you get 100cfm fan and leave it on too long every day, you have to find a balance for energy and health and that balance can only be defined by the occupant, not any third party tester or standards.

  28. Yeah, I never could figure
    Yeah, I never could figure out why someone would bother reading a blog like EV if they’re not on board with the program. I can think of only two legitimate reasons for reading and participating in online discussions like this… to teach and to learn. Apparently some people like to destroy. 
    @Greg, if you get a kick out of being negative, rather than constructive in how you frame your arguments, then I hope I never have to see your name again.

  29. Cameron T.
    Cameron T.: Excellent point. 
    Matthew: Well, it’s easy to see now that I should have just let Greg’s first comment sit there and stink, unventilated, but I jumped in anyway. This may get to the point where I have to do some source control if things get out of hand. It’s certainly clear now that this Greg fellow is a troll with little real value to contribute here.  
    David B.: Hear, hear.

  30. Fine. Ignore the HVAC pro’s,
    Fine. Ignore the HVAC pro’s, and builders, surround yourself with one week class HERS raters and stay happy. Bet those underperforming exhaust fans came with an Energy Star stamp, yet for 35$ still do more for the home owner than any HERS rater ever could.

  31. 2 little hitches for ENERGY
    2 little hitches for ENERGY STAR builders in the Pacific Northwest (We have our own version of ENERGY STAR) are: 1) HVAC Contractor’s Checklist (is the contractor “approved”, do they have the right tools to do the room by room flow testing, did they submit the paperwork?) 2) Zonal Pressure Relief (if they failed at any of the listed items in 1) above they certainly won’t meet the ZPR requirements of <3 pa pressure difference between bedrooms and hallway.

  32. @Ryan, those HVAC checklists
    @Ryan, those HVAC checklists (rater and contractor) and all the rest you mentioned are national requirements under ESv3.

  33. Thanks David. I knew there
    Thanks David. I knew there slight differences in our programs but I wasn’t sure which ones. I more or less wanted to make the point that I think there are bigger fish to fry when it comes to builder’s meeting the ENERGY STAR specs than the exhaust fan requirements.

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