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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The simplest way is probably with a pitot tube. I know that's not exactly a "common" home performance tool. But they are cheap ($50 -$60 from Dwyer) and supported by TEC digital gauges.

Field Controls make a damper that goes inside the duct that can be adjusted for the amount of cfm that you want to bring into the return.  The damper opens to the set amount when the A/H engages.

Jim, that's a good idea!! Don't even bother measuring -- with a 12 inch duct, it's a pretty safe guess that the intake flow is too high! The only question is by how much.

I am not usually a "why measure?" sort of guy. But, I don't know that detailed info adds a whole lot to the picture here. It might matter if you were doing a full-blown energy model. But, it seems to me an intake flow measurement only helps if you need to know PRECISELY how badly the system is mis-designed. 

Just figure out what the make-up air needs to be, and get the system fixed so it will do that. Simplify, simplify...

I've use my energy conservatory exhaust flow pan (on the intake grill), flow hoods can work too.

I would not recommend using a pitot tube, as the accuracy of your flow measurement will depend upon the average velocity pressure across the flow field of the duct, which may or may not be symmetrical.  Either way, a pitot tube traverse will not be worth your time, nor will it likely be accurate.  Potentially you could use the garbage bag method, depending upon the type of obstruction, though that is quite hard to do from the intake end of a duct.  I agree with Don, probably not necessary to measure...though a powered flow hood using custom cardboard box and your duct blaster can work in some situations where the exact design of the manufactured flow hood will not work.  You could also build a custom box sized to fit over the intake, but whose size is compatible with the true flow meter from the energy conservatory (14x20, for example).  These are all terribly troublesome ideas, but they will work (depending on the intake obstructions) if you MUST measure the flow.  Good luck!

This makes me think of the "5 why's" you should ask when someone asks a question.  Here's the first:

Why are you wondering how much airflow occurs at this intake?  

Why are you wondering how much airflow occurs at this intake?  


1) Required for Energy Star 3.0 certification

2) Code compliance

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

5) others?...

There are in-duct measurement techniques that are accurate enough for this purpose. Here's what I use - http://www.kele.com/flow/fxp-series.aspx It's essentially an "averaging" pitot tube to take care of the likely asymmetric velocity pressure in the duct.

The RESNET Technical Committee will soon be reviewing an amendment request to allow this kind of ventilation airflow measurement technique in Chapter 8 of the RESNET Standard.   

Not inexpensive!

Do you install it and leave it, or do you drill a hole, and have to patch?

HVAC contractor (ideally) drills the 1" hole, then tapes over afterward. 

I invested about $250 for 4", 5", 6", 7" and 8" versions of the device. 

By the way...not sure what climate you're in, but someone should probably should insulate that fresh air duct!

Set up your blower door and duct pressure testing. Get a duct leakage to the outside, with the vent open. Then repeat with the vent taped off.  That would be the amount of air flow in CFM at 25pa through the duct.  If you are wanting the number for Energy Star or other program purposes, you already have the set up near the final.  Whatever amount of air is allowed in with an un-dampered duct must be considered as part of the duct leakage for Energy Star. That said, I am wondering why you want this for Energy Star?

How would you utilize a number in any other of the 4 uses mentioned?

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