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January 04, 2011


Al D'Ambola

Would it not be easier to test the duct system the way it functions? Without hooking up a secondary device (duct blaster). Why not verify fan speed, test actual static pressure and use the manufacturers fan chart to determine the fan flow. Once this is done use a flow hood (commercial type) for your readings at the register and grilles. The difference is the real duct leakage in CFM, not sq inches of leakage.

Example, the fan curve tells you the fan can move 1000cfm, your flow hood tells you that all the supply register air is 850cfm and the return air is 500cfm. I now know that the supply ducting is leaking 150 cfm and the return is leaking 500cfm. Once I have a cfm total and if you knew temperature you actually could calculate the btu loss/gain through the ducting.

What does an HVAC contractor do with sq inched of leakage? A duct blaster is a crude test at best. I have seen where we have moved the pressure sensor a few feet and got totally different leakage results from the duct blaster. Hard to get repeatable readings with it.

Temperature diagnostics determining the percentage of BTU loss/gain through a duct system is a phenomenal test that only takes about 10 minutes.

Alistair Jackson

Al - thanks for your comment. There is definitely great merit to your approach - and I agree the duct blaster test has its limitations. Understanding the real word performance of the system is obviously important when looking at upgrades. I wish there were more qualified, experienced balancing contractors around interested in working in the residential sector.

One attraction of the duct blaster and blower door approach is the ability to isolate the leakage to exterior. This focuses sealing efforts on getting ducts inside the thermal envelope as much as feasible and then sealing up those left outside.

I'd be very interested to know if there is any way to make this differentiation with your approach?

Danny Gough, Energy Solutions, Inc.

Why do you have such confidence in the accuracy of a flow capture hood? I have 3 Alnors and 1 TSI and can tell you they are much more finicky than a calibrated fan. We have simply rotated the hood 90 degrees and get 10% to 15% variation in the readout. LBL also published a study showing their accuracy to be highly suspect. I agree that flow capture hoods are a great tool for air balance. But measuring airflow accurately has never been their strong suit.

Al D'Ambola

Danny, the LBL study 7 or 8 years ago has a lot of flaws. TSI had one of there engineers watching how LBL performed there test, he was not impressed with how they used the hoods. They did say if my memory is correct using a garbage bag with a stop watch to calculate the flow is more accurate. No way would that ever happen on a balancing project. When it comes to research who is paying for it? And who benefits from the report?

As professional commercial balancer's we are often asked by engineers to prove our readings in the field, using hoods and pitot tube traverses or hot wire anemometers. We are always within a few percentage points using different methods (repeatability)is what we strive for, and achieve.

My calibrated hood matches my traverse readings, so I do not no why your hoods are giving you different readings. If you are using the Low Balaometer from Alnor it is of 30-40% off it is not a product that a professional balancing firm would use. Saying that hoods are not accurate I guess all the systems that have been tested by commercial balancers for hospitals, industrial facility's and office buildings and commercial kitchens, we are all wrong...is it time for garbage bags and stop watches?

Alistar, I believe the fan curve method of determining flow is much more dependable and less labor involved in achieving repeatable results. Professional airflow training is most critical.

You wrote a very good article, glad you're helping people!!!

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