I recently measured disappointing bath fan flows at a project - both being below our program minimum of 50cfm. The builder had the installer come back to have a look and found nothing obvious but he did get a couple of new fan motors and swapped them out and also double checked that the damper flap wasn't stuck. There was a slight improvement to both but it was not enough, still both below 50.

These fans were routed - with a short run of duct - to soffit terminations which allowed us to look up inside and check for blockages but none were seen. 

Then the weirdness: I checked the flow coming from the terminations and both were about 90cfm, yet still only about 45 inside. How can this be? I've never seen this before, but that's because I never test at the outside. 

I hate to bash brands but these were Broan fans which I rarely see used around here. Most all of my contractors use Panasonics.

Views: 184

Reply to This

Replies to This Discussion

I am guessing based on what you posted. I suspect the fan collar to exhaust duct is leaking. If my guess is correct, the fan is acting like an "ejector pump". as the airflow passes the leaking collar it pulls in more air thru the leak. This reduces air flow from the bathroom and increases the air flow at the soffit vent.

I know, I know - high pressure at the exhaust collar should mean any leaks are out, not in. But there are pumps and fans that use the low pressure created to pull more air from outside and add it to the flow leaving the fan. It has to do with the difference between static pressure and total pressure.

Reasonable theory, but if that was the case I'd expect to see some insulation (in which the fan is buried) drawn along with that airflow, no?

Depends on the insulation. Batt insulation I would not expect much to be drawn in. Cellulose insulation would likely restrict the leakage to where my theory does not work. Blown in fiberglass could draw in some insulation but it might be a very small amount.

Anyway - it is just a theory and something to check. 

Just to add on to common problems...

Short - as in straight, pulled tight with minor bends (assuming flex)? Just what is short - 10', 20'?

Most issues I see besides birds nest is hard bends, full 25' worth of material going only 5', and of course my favorite - going out a soffit instead of gable or roof (yeah that is generally a hard bend & bad if sucked back in).

Yes, I should have stated: 6' straight run, all insulated flex insulated duct with tightish bends down to the soffit terminations which did yield a tad more cfm when we pushed up to relax the curve.

But my confusion is how does it end up being more at the termination than the intake? Normal frictional losses will end up doing the reverse: less at the termination than at the fan. This seems like it violates some law of physics.

Was it at all windy - just how did you test / equipment? How did you seal tight on the exterior? I have seen interesting results on ERV's intake, exhausts, etc...

Maybe it is because at the fan, you are measuring negative pressure and at the soffit it is positive pressure. I am guessing that you are using the Energy Conservatory fan flow hood. Check with TEC about that.

I use an Alnor flow hood which I know works both inside and out as I recently measured flows for an ERV installation. 

I will go back to how did you seal up tight / anything else that could impact the flows (just remember that those nice arrows we draw for flows... well nature has a way of screwing with you - especially with wind)

I'd agree with Sean - we get wind effects regularly when measuring ventilation fan intake at the outdoor hood. And there's a notable difference if the capture hood is flush with the wall (eg around foundation wall) or if there are gaps, such as with lapped siding.

Otherwise, I agree with Brad that it could be some measurement error w/ respect to positive v. negative pressure measurements. Unless you're using a powered flow hood (with a fan to compensate for pressure within the hood, not sure if Alnor makes one), the pressure differences could account for at least some of this difference.

Out of curiosity, how did you verify ERV flow? Did you measure supply or exhaust at both indoor and outdoor locations using the flow hood? Did the indoor & outdoor measurements align well?

What size is the exhaust vent? 

I helped with a project in the past where the exhaust was vented out with 3-4" vent (forget which one).  But it wasn't reaching its stated exhaust flow rate.  The contractor had to come back and install 6" vent.  After doing that, the exhaust fan was able to reach the desired flow rate. 

These fans were being vented out of sidewalls - the 6" exhaust vent would not fit in the ceiling.  The contractors had to build soffits in the bathroom to install the 6" vent.

You would have to test the vent at the inside at a location the same diameter as the outside diameter to get the same CFM reading.

RSS

Forum Discussions

Blower door test for small apartment building

Started by Jerry Needham in General Forum. Last reply by John White on Friday. 9 Replies

ENERGY STAR Version 3

Started by Allison A. Bailes III in General Forum. Last reply by Franco Oyuela on Thursday. 9 Replies

Radiant Barrier Southeast

Started by Gary Wright in Best Practices. Last reply by yassin chahdi on Thursday. 6 Replies

Latest Activity

Brett Little posted a discussion

How to become a Certified GreenHome Professional - Free Webinar!

The Certified GreenHome Professional (CGHP) training is a way to learn about the basics of…See More
16 hours ago
Corbett Lunsford's video was featured

Testing Air Quality with Your Nose: Control Your HOMEChem

Charlie Weschler sits down with Grace and Corbett and gives fascinating examples of the complexity of indoor air chemistry. He is an air quality legend, a professor on three continents, and a super nice guy.
Friday
Profile Iconwallace van, Tony kroos and Sunmar Construction, Inc joined Home Energy Pros Forum
Friday
John White replied to Jerry Needham's discussion Blower door test for small apartment building
"Well, I think it completely depends on the purpose and requirement of the test. Testing one unit in…"
Friday
John Nicholas replied to Jerry Needham's discussion Blower door test for small apartment building
"I can use my Duct Blaster on any house that I expect to be less than 1300 CFM through the…"
Friday
Sean Lintow Sr replied to Jerry Needham's discussion Blower door test for small apartment building
"FYI, he is in Illinois where code is 5 (well up until March as noted when it drops to 4)  Heh…"
Friday
John Nicholas replied to Jerry Needham's discussion Blower door test for small apartment building
"Here are two pictures of the duct blaster fan as a blower door.  1 in a door and 1 in a…"
Thursday
John Nicholas replied to Jerry Needham's discussion Blower door test for small apartment building
"What standard are you testing to? Energy Star requires 5 ACH @ 50.   Code is 3.0 etc…"
Thursday
John Nicholas replied to Jerry Needham's discussion Blower door test for small apartment building
"What standard are you testing to? Energy Star requires 5 ACH @ 50.   Code is 3.0 etc…"
Thursday
Franco Oyuela replied to Allison A. Bailes III's discussion ENERGY STAR Version 3
"The ENERGY STARnew homes program has been around since the mid-1990s and is currently undergoing…"
Thursday
Blake Reid commented on Blake Reid's blog post Managing Mold & Humidity (in Hawaii)
"Thank you very much! They're actually the same, though - the one with the dehu cover photo has…"
Thursday
John Proctor commented on Blake Reid's blog post Managing Mold & Humidity (in Hawaii)
"Nice post -- much better than the one that only showed the dehumidifier."
Thursday
Brett Little updated an event
Thumbnail

Full Circle Urban Forestry: Introducing Urban Wood Opportunities - Free CE Webinar at Webinar Online

December 5, 2018 from 12pm to 1:15pm
Many common urban tree removal practices in the United States view felled urban trees as costly…See More
Thursday
Brett Little replied to Brett Little's discussion Virtual CEU Tour of the Pembroke Passive Solar Zero Energy Ready Home
"You know utilities are pretty complex, especially in IL, however it looks like just about 6.8 cents…"
Thursday
yassin chahdi replied to Gary Wright's discussion Radiant Barrier Southeast
"It’s hard to choose on of these as most of the time the reviews and buying guides found on…"
Thursday
Bruce Fillmore replied to Mark Furst's discussion Bath fan weirdness
"In my opinion venting into any soffit is a bad idea, it usually requires a sharp bend to get into…"
Thursday
Tray Biasiolli replied to Mark Furst's discussion Bath fan weirdness
"I'd agree with Sean - we get wind effects regularly when measuring ventilation fan intake at…"
Thursday
Charles Buell replied to Mark Furst's discussion Bath fan weirdness
"You would have to test the vent at the inside at a location the same diameter as the outside…"
Thursday
Blake Reid's blog post was featured

Managing Mold & Humidity (in Hawaii)

A blower door measures how tight a house isAir sealing done as part of the install, and after…See More
Thursday
John White replied to Cameron Home Insulation's discussion Why is Air Sealing Important?
"Most attics have insulation which helps reduce heat loss. But it won't stop the air flow.…"
Wednesday

Photos

  • Add Photos
  • View All

© 2018   Created by Home Performance Coalition (HPC)   Powered by

Badges  |  Report an Issue  |  Terms of Service