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.

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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.

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