CFM Per Watt – How Do Your Air Handler Systems Perform?

Replies to This Discussion

Permalink Reply by tedkidd on September 1, 2011 at 7:36am

Interesting. 

 

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?  

Permalink Reply by Linda Wigington on September 1, 2011 at 8:02am

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.

Permalink Reply by tedkidd on September 1, 2011 at 8:19am

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.  

Permalink Reply by Linda Wigington on September 2, 2011 at 7:32am

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.