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Tuesday, April 14, 2015

More Furnace Plena and Flex Duct Quality Work

This story began with call to fix insulation in a crawl space, 2007 construction, 2x12@16"oc framing. Code required simply R30, or at minimum R19, to fill the framing. The proper requirement is that insulation shall be in full contact with with flooring. Well-placed insulation almost never "fills" the framing, and as such, must not rely on twining of framing bottom faces.










During the height-of Summer insulation repair, I found that the crawl space was noticeably cooled by leaks of conditioned air, mainly in a very restricted space over the furnace. Where I am not invested in cheap, flawed methods and am known for integrity and professional engineering experience with hydraulics and more, the customer trusted me, to fix the HVAC ducts in the crawl space. I would start over, in a further demonstration of highly efficient ducting that challenges common practice with crudely-hacked D-boxes and octopus ducting.


The story is drawn from this complete photo album:
Furnace Plena And Flex Duct Innovation Multnomah Village October 2014 

A sampling of photos and captions is posted to Picasa for public sharing.

https://picasaweb.google.com/108533770292578040917/MultnomahVillageBetterHVACFlexDucts 

Here are some of the photos and captions of the Picasa web album:


A 10” flex duct to a 90° elbow atop the warm-air D-box serves floor registers of the main floor. The elbow lower end is not sealed in any way and loses at least 20% of its flow to the crawl space. It was just too hard to reach. What could the installer do? Simply grab the elbow with a couple of straps, with no tab-ins.

The topside 10” elbow with its two loose straps and a sector of hacked sheet metal concealed but did not seal a badly-nipped D-box opening. No goop here. No tape.












Awful return at bottom. A plate closing the filter box couples with a 16” elbow. Most furnace noise is here. The jet from the 16” elbow has no space to adjust to the shape of the filter. The filter is misaligned with the furnace opening. Velocities far exceed that in developed flow of a 16” duct. More than half of the blower head is simply wasted, right here.
























Imagine a better duct system with no D-boxes and with efficient furnace plena, instead.



























Use turning vanes and development length in the lined plena. Insulate and seal all metal fittings with two coats of applied flexible grout, where most volume in the grout is insulative ceramic microballoons.

















I believe in turning vanes. Here they are in the air-return plenum. Try to avoid air churning. It does expend energy.



















Survey the completed crawl space. An intelligent return plenum with excellent filter, is prettier too.

It is still difficult to view the warm ducts attachment atop the furnace. Now, please be assured, they are leak-free and efficient.
























Crawl space headroom is very much improved.












The found crawl space was barely navigable, with octopus supply ducts from a crude hub. All D-boxes are highly resistive to air flow and should be eliminated. No flow junction should be with full losses of velocity head in re-forming into sharp take-off ducts. Employ steady onward flow in well-formed wyes, or with chamfered lateral take-offs for a single register.











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