More HVAC Circuitry, In A Crawl Space

The above diagram of a crawl space warm air duct system is a 1:120 scale drawing, with overlay of structure details upon a Google satellite photo. Please note similarity to planning graphics and involved ducting elements for similar achievement in an attic.

Photos and discussion parallel a post in a web album, More HVAC Circuitry, In A Crawl Space. The really curious may examine too, a full photo album CS Insulation Repair and Heat Ducts, including found conditions of the failed UltraTouch batt installation by "twining" method, and of impassable and leaking conditions in the found system of steel warm air ducts. A separate blog post will address the cotton batt insulation repair. The involved home in Hillsboro, Oregon is also the site of a publicly-shared full photo album documenting attic floor weatherization with UltraTouch cotton batts: Real Sealing Fixes HPwES Sealing In An Oregon Attic. Full pdf photo albums always tell a story best, but please know they are poorly rendered when viewed in GoogleDocs. If you want to examine such albums, please bother to download them.


Elements of a new warm air duct system are in two nearly-equal branches, called North and South. The branches include a well-considered set of plenum components below the furnace. A 14x19 turning plenum replaces a crude D-box.

The furnace plena have a 1" insulation liner and end in a triangular head with two 12" starter collars serving a system of flexible ducts.

Two 6" registers near the furnace are served by the first of four lateral take-offs, circuit label T12x6. These and all steel couplings of flexible ducts, are hung from the overhead via eye bolts. These two, hanging low, are pulled up by wire loops. Never support steel couplings by strapping upon the flexible ducts. Don't rely on screws and tape for suspension via metal couplings.

Here note the 1" insulation lining, and turning vanes. Getting prepared with such well-conceived and well-built elements is a small step of installer professionalism. Expect you may need a couple of weeks lead time for fabrication by a superior sheet metal fabricator. My fabricator in Portland, Oregon is Vinje & Son.

Don't accept dissing of flexible ducts, that they have extraordinary friction. Where each duct section is fully supported by straps and by steel couplings, draw small tension in the liner, taking up coils as needed, to align the duct and coupling. High-resistance problems are always the result of careless alignment, with tripping over steel edges. Plan excess of insulation jackets, to draw them fully-overlapping, over couplings.

This is coupling component R8x6, used two places at branch ends, where 6" ducts to registers are of length not more than six feet. I wish for simpler one-piece reducers, but want ample starter lengths for liner take-up and secure taping. Here and in the previous photo, see that the eye bolt hanger of a steel fitting passes through a hole in plywood overhead. A wood dowel cut from nimble tree branch passes through the bolt eye to releasably hold everything up. The release can allow better fit of insulation jacket overlap.

Here is the completed end of the North branch at a 6" register. The hanger of the R8x6 is out if sight above the jacketed water pipe. See only tough black liner, strapping and some Nashua 557 tape. Leave no opening for mice. Wherever possible, draw ducting tight against plywood guides, as barrier to mice.

This is the new heart of the duct system under the furnace, with a turning plenum.

This is the well-strapped head of the North branch 12" duct, tightly in contact with overhead 1/2"  plywood strips. One can readily crawl anywhere in this crawl space, sliding under the smooth plastic covering if necessary.










The following is an exercise in defense against anticipated criticism there are no balancing dampers and ACCA Manual D consideration, in new invention of furnace warm air ducts.

Please see that residential HVAC should be naturally balanced with good flow dynamics in orientation and more, of plena, take-offs and reducer or wye fittings. Air flow in thoughtless ducting can be extremely chaotic, and not amenable to tweaking with manometer measurements not-at-all understood. With smart ducting, simply rely on the already-present register grills.

The practical concerns in duct design are to allow overall free flow, limiting air handler power draw, and getting the heat where a deficiency is noted. The complaint to be resolved will be of too little heat in a favored room. Don't respond by dampering flow to other rooms. Just tackle the duct path to that room. Find an obstruction, perhaps a crushed or pinched duct, or misalignment in a coupling. Failing that, increase the size of ducting for that room, perhaps just in a more-intelligent register. Look for missing insulation as the cause of discomfort.

At Pinterest , please find a growing collection of affordable and suitable HVAC registers, that include easy damper adjustment.