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Is there a limit to the amount of ductwork you can put on a low-static pressure ducted mini-split heat pump?
Posted by Allison Bailes on December 9, 2019
I like mini-split heat pumps, especially of the ducted variety.  And because I don't like to pay for a lot of excess power, I prefer to use ducted mini-splits with smaller blower motors operating at a low total external static pressure.  The Mitsubishi* units we have in our office and that I have...
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An older house with ducts located in the third worst place you can put them
Posted by Allison Bailes on August 14, 2019
Ah, so many choices.  If you want to design and install a duct system to create problems, the possibilities are endless.  I've certainly written about a lot of them (as well as how to do it properly).  Today, though, let's focus on one particularly bad place to put a duct.  In fact, I think it's...
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What is the best velocity for air moving in ducts?
Posted by Allison Bailes on June 12, 2019
It's obvious that moving air too quickly through ducts can be a problem.  Faster air means more turbulence, more resistance, and more noise.  But I run into people who think that low velocity also can be a problem in ducts.  Just recently I heard someone talking about how low velocity causes "...
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Aprilaire high-MERV filter with carbon
Posted by Allison Bailes on November 9, 2018
What is a hair's breadth?  A strand of the finest human hair is about 20 micrometers (also called microns) in diameter.  Take a piece and slice it in half.  Then take one of those halves and slice it in half again...and then one more time.  Now you've got a sliver of hair that's 2.5 microns wide. ...
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What happens to the air flow in ducts when the size changes?
Posted by Allison Bailes on October 29, 2018
As we continue our study of indoor air quality and filtration, we now come back to duct design.  Today's lesson is on an interesting bit of physics that applies to anything that flows.  It could be heat or particles or electromagnetic energy.  In our case, it's air, a fluid, and the physics we're...
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Friction rate is the wronge name
Posted by Allison Bailes on October 12, 2018
Last year I wrote some articles on duct design and one of the topics I covered was this thing called friction rate.  At the end of that article, I discussed "How to think about friction rate."  It confuses a lot of people because it's the wrong name.  First I'll tell you why and then I'll tell you...
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