Measuring fresh air intake ducted to a return and no way of measuring from outside.

I am sure this is a challenge that some of you have faced before:

You have an air handling system with fresh air intake that is ducted into the return and is terminated on the outside in a way that would not allow you to measure it with a flow pan. 

My thinking is that there should be a way to measure the air flow by measuring pressure difference between outside and the fresh air duct and perhaps some more variables, like the size of the pipe and it's lenght.

I found some formulas and tried to do calculations using them but did not get the most realistic results. 

Or maybe there are some clever ways of measuring it using some common home performance tools?

Your suggestions? 

Attached is a picture of the particular system I am testing that has a huge (12" pipe) fresh air intake. 

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Others have given good ideas for roughly measuring the actual airflow. If I had a bit of time to kill, and the freedom to experiment with the house, I would probably seal the exterior inlet of the fresh air duct and then measure any changes in external static pressure and the RH% of the house. At 12" it's a lot larger than any I've ever seen on a residential system, and I've also seen a lot of them on houses that didn't seem tight enough to need them. 

Wow, just looked at the picture!  

Jack, you sure that's a fresh air intake?  Looks like 16" duct - and it's got a Carrier Infinity Zone Control damper on it.  I guess the Infinity might be managing fresh air, the thing is really complex.  

Since Jack hasn't responded to my "why" question, I suspect John Semmelhack's suspicions around 1 or 2 are correct.

1) Required for Energy Star 3.0 certification

2) Code compliance

Measuring airflow while the unit is operating will tell next to nothing unless you know at what static that fan will operate, and for how many hours.  Since it's an ECM unit likely with variable flow rates in both heating and cooling cycles, I think John Nichols answer is the only one that will tell you much. 

JS, be cautious committing too fervently to these fabrications:

3) Ensure there's enough airflow to help ensure healthy indoor air quality

4) Ensure there's not too much airflow to provide good energy efficiency 

"enough" and "too much" are complete abstractions attempting to tie to an unmeasured metric using "universal averages" that have a tremendous amount of deviation.  We need to make the word "ignorant" not have such bad connotations.  

Have you ever measured indoor air quality?  I have.  It make me realize that without CONTINUOUS measurement I am completely ignorant of it's "good"ness or "bad"ness.  ACH may be the best we can do for now, but let's not drink the cool aid and ascribe words like "Ensure" to IAQ.  That implies certainty we do not have, provides assurances that may prove fraudulent  and in my eyes is a path to appearing stupid and incompetent.     

The more we kid ourselves that we can "know" indoor air quality without actually measuring indoor air quality, the more we move from from science to religion.  From believers in data to dogma.  From ignorance to stupidity.  From ethical to unethical.  This bad information will eventually become persistent belief that causes harm.  Do no harm.  

Might it not be better to be honest about our ignorance than to make assurances we have no intention of measuring or proving?  Or are we just used car salesmen, saying what it takes to close the deal? 

Ted,

That is a sealed combustion furnace. We don't know what the hot water heater is. 

If that is someones idea of mechanical ventilation to meet ES 3.0, I believe they missed a class somewhere. Code compliance for fresh air in this area is required to be ducted into the return. This is not.

We need more info.

John N, the huge duct on the left with the blue zone control apparently ducted into a "bottom" return (bottom of the picture) 

"...fresh air intake that is ducted into the return..."

I don't think anyone was referring to combustion make up air.  I think this is about meeting building airflow standards with mechanical ventilation. 

Ted,

Thanks.  I had to pull the image of and lighten it up to make out the details down there.  It looked like a duct with no bottom in the original.

Sure John.  

Have you ever seen such a huge intake?  And so close to the unit.  LOT of air!  Not worried about shocking the heat exchanger?  Unlike an ERV, I would think you absolutely would NOT insulate that duct.

Where is this house? 

Any chance that duct is configured as some sort of "economizer"? A/C is not common here so I'm not familiar with all the permutations. What I usually see is a 4" or 6" duct from the exterior, with no means of controlling how much outside air is introduced into the airflow. Sometimes there's a mechanical damper that closes the duct when the fan is off, or a barometric damper.

Use a duct blaster as apowered flow hood, look in duct blaster manual.

That duct is an economizer. It's purpose is ventilation, and possibly cooling (rather than using the AC). Night time cooling works great in the western states in many places, where it's dry, and the temperatures drop at night to below house temps.

12" is bigger that needed for ASHRAE 62.2 Ventilation, maybe they wanted more for "better" IAQ, at a conditioning cost! It's sized more like a small commercial project that I was at last week.

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Rick Karg has instructions for using a pitot tube to assess airflow in ducts.

His website Residential Energy Dynamics includes a calculator to determine duct CFM.  It's fairly accurate, provided you have plenty of straight duct upstream from the point of measurement.

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