Slides 38 - 42 show data from 55 duct systems - Air handler performance varies from 6.5 to 2 CFM Per Watt


Forced air distribution systems–ducts–can contribute to unanticipated energy use in a variety of ways: through duct leakage, pressure effects, conductive and radiant heat gain, and insufficient airflow. Ducts provide a great opportunity to demonstrate system effects.

Often, when one part of the system is addressed, unanticipated problems may occur. ECM motors appear to offer significant efficiency and performance improvements over the old PCS motors that have been the workhorses for heating and cooling equipment for years.

 As part of routine inspections through the Performance Tested Comfort System program,  ( Mark Jerome (KAM Energy, Roseburg, Oregon) collected measured airflow and amp draw data on 55 systems that he was installing or inspecting. The attached PDF is his presentation of that data, given this July at Data, Not Dogma, in Goldendale, Washington. One thing to note is that these systems were installed by trained contractors as part of a program that has a focus on performance. One may suspect wider diversity of results in a sample of installations outside of the program’s influence.

It takes energy to move air. What are the implications for the design and installation of new space conditioning systems? What happens when we install central cooling and ECM motors into pre-existing duct systems? What are potential unexpected consequences of furnace filter replacement and duct sealing in systems with ECM motors?

In Thousand Home Challenge projects, the energy it takes to move air in combustion venting systems, spot and central ventilation, as well as heating and cooling can be a significant portion of a home’s threshold allowance. It is critical to reexamine our assumptions: how can we achieve exceptional air quality and comfort at a fraction of the energy used in most homes today?


Share your data so we can better understand the opportunity to reduce the energy use of blowers and fans.


Tags: CFM, CFM Per Watt, ECM Blowers, ESP, Forced Air Distribution, Thousand Home Challenge, external static pressure, field data, performance, residential

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Replies to This Discussion



Unfortunately this is a slideshow that requires a presenter, some issues are not clear in silent mode.  One issue that did not make itself clear to me is the purported "problem of low static / low airflow issues".  This is something I've never seen cause problems, so I'm unfamiliar with the problems it causes.  What should I be looking out for?  

Here is Bruce Manclark's response:


Low static seems to confuse the infinty system into delivering low cfm


And Bob Davis's response to Bruce:


Bruce is right but Mark [Jerome] can give a more complete description.  The very short answer is that if there isn't enough resistance for the fan controller to work against, it can end up supplying not enough air.

Thanks Linda!  That's really helpful!  


Bob/Bruce/Mark, I haven't had a problem with that.  Can you tell me the circumstances you've seen that occur?  All my systems are operating well under .5, particularly at lower stages, and I've had only great reports from homeowners.  

Response from Bob Davis, Ecotope:


In the NW, we've seen this happen several times with newer heat pumps outfitted with ECM air handlers.  If the duct system is sized so that external static ends up being below .2 or .3" WC, the ECM controller ends up "hunting" for the right RPM and settles (typically) on a very low flow (generally about 250 CFM/ton or even lower).  Installers have had to change out control boards or even add more restriction to the duct system to get desired flow rates.


Response from Mark Jerome, KAM Energy:


The issue with low airflow low static is not very common, approx. 20 sites in the Pacific Northwest. I have personally seen it around 12 of these sites with 8 others I have discussed with contractors contacting me by phone.
Specifically low return static and or low supply static with total external static pressure at or lower than .3" WC has normally been the issue. Short supply plenums or nonexistence return air plenums, and a few return air of a mobile/modular home plenums, seem to be the biggest culprit. This issue has been across all manufactures which leads me to believe this is certainly a motor problem combined with a duct sizing problem. Homes that have been weatherized or right sized with a new heat pump to match the load of the home (heat pumps sized smaller from a larger heat pump or other forced air system) would be the probable cause.



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