Open Letter To ASHRAE and ACCA, Better HVAC Air Ducts In Our Homes

 The following is a re-posting of a Google Docs document that will be subject to constant correction and update from reader comments.

https://docs.google.com/document/d/1o3j4ayBkhDhq8U9HVDwYujwKQCOQLpANUYvNFKFWfco/edit?usp=sharing

Better HVAC Air Ducts In Our Homes

The following message is sent to the Duct Design leadership at ACCA and to ASHRAE Ducts Design Committee colleagues, TC 5.2.

From my PhD-level work forty to fifty years ago, with hydraulics of pressurized water nuclear power generators, I have an understanding of HVAC duct design with direct application of Bernoulli Principle.  This understanding contradicts ACCA Manual D design of residential HVAC ducts, where there is no usage even of the word Bernoulli. By ACCA methods, nothing is seen as wrong in silly long too-small ducts that sprout Medusa-like from beloved D-boxes and assorted Supply Trunks. Absent Bernoulli math, air is somehow transported as if it were no-energy toothpaste. Static pressures change and diminish for reasons little-known, not controllable. Manual D pros say: Just squeeze enough at the blower,to push the needed flow. Flow violence and attendant inefficiencies are only a noise problem.  Ignorance is revealed in non-scientific phrases total static pressure and total external static pressure.

With Bernoulli energy balances, we have spectral local values of static pressure and kinetic head, additive to  total pressure..Energy losses attendant to poor design are more-usefully computed, than measured. Better than computing penalties, use common sense to avoid them. Let the controlling flow resistance in any path be at the register discharge to ambient. Find folly always in trying to fix a mess with use of dampers. Resize any register and its approaching duct, to deal with complaints.

Please see a best demonstration of my duct construction and  analysis methods in one home:

Duct Hydraulic Analysis, Koempel

Koempel Job Photo Album

Blog Post An Attic Ladder Installed Diagonal to Attic Floor Framing

Blog Post Flawed Measurement Of R-Value With A Certainteed InsulSafe4 Gage

I have several similar achievements to share:

Chamberlain-Mann Crawl Space Ducts Hydraulic Analysis

Chamberlain-Mann Crawl Space Job Photos

Waters HVAC Ducts Plan

Waters Attics All Photos

Duct Hydraulic Analysis, Leet Rental

Blog Post, Leet Rental Crawl Space

Leet Rental CS Overhaul

Leet Home Better Furnace Ducts

HVAC Ducts Hydraulic Analysis_Meyer

Job Album, Meyer Attic

Find more duct redesign from found cheap industry expectations. Employ shop-built fittings, often lined with insulation. No leaky, inefficient on-the-job hacks to just make connections. Ducts are buried as much as possible, never placed carelessly, obstructing access.

Blog Post: Best Placement of an In-Attic Gas Furnace, Among Roof Trusses

Blog Post: More Furnace Plena and Flex Duct Quality Work

Blog Post: Following ACEEE Blog, Furnace Fans As Energy Hog

Blog Post: Better In-Attic Furnace Ducting

Blog Post: Steel Ducts Heat Capacity, Forced Hot Air Heat

My insight in all of this is with confidence from my work in the nuclear power industry where a key achievement was in the reversal of a management decision that the cost of reactor pressure vessels should be reduced by making the inlet and outlet nozzle forgings abrupt, sharp-edged, no chamfering. A million dollar scale-model flow test under my direction proved that flow is then with damaging violence. Such silliness.

Please see that these contributions are extraordinary. All measures taken for energy efficiency are affordable. The wonderful sheet metal shop that enables my work shall wish for more customers like me. At present the cleverness I offer to customers has insufficient demand.

ACCA Manual D does not offer scientific best methods. Together ASHRAE and ACCA must offer better guidance. I want to help.

Phillip Norman <pjnorman@gmail.com>

1764 Bonniebrae Drive

Lake Oswego, Oregon 97034

503-255-4350

My Customer Meyer:

Please visualize the defect of D-box work imagined as no-cost squeezing of toothpaste. The discharge of a blower over a plenum area is quite directional. Noisy turbulent eddies form if flow does not remain guided in consistent direction. In a D-box, the orderly kinetic energy fan-generated,  is not converted in efficient buildup of static pressure to drive lateral flows through the resistive and leaky sharp-edged hacks. Chaos has a cost. energy-wasting large eddies uselessly elevate the blower head.

Here see guided concentrating flow continue, steadily, quietly  and with best-possible efficiency. Flow is quite turbulent, but with very small energy-blasting  eddies. Regardless of frenzied froth, flow wants to be guided, and absent guidance behaves wildly and noisily. All needed turns must be efficient, at lowest-economical velocity.

Where flow may divide in wyes, each path to register discharge may be individually summed in a Bernoulli balancing analysis. Where the dominant resistance is at each register, register velocities will be equal as obvious good design. The ONLY place to throttle flow is in a register. Control the service to a room in the sizing of its register and approaching duct. Keep duct sizes as large as possible except in a consistent final length, say of six feet, at each register.

Here see effort to reduce steel-duct swung thermal mass by insulative coating that does matter..

Let the planned flows be charted for analysis, in diagrams such as this for the plenum above:

Here is the completed ductwork at the furnace:

None of this simple common sense is the Medusa-teaching of ACCA Manual D.

The convenient found storage space in this beautiful attic now super-insulated, is quite valuable.

At 11/16/2024 I am inspired by yet-another project eliminating a leaky, thoughtless hack-in D-box, to address this topic:

loss coefficient for unguided turn through an abrupt sharp take-off. I know this is costly of energy and produces chaotic, noisy turbulence. I find a loss coefficient as high as 1.8, probably greater than the total of all other losses in a path. An ASHRAE Handbook Table 10.5 of 1981 is presented in a mark-up at the University of Texas, Austin.

Now, make that take-off ragged. Say, like this:

Projecting tabs and leakage burn energy.

Support a campaign to eliminate construction with D-boxes. 

Here is the shop-built replacement supply ducts top-box of this furnace:

10/29/2024

Let developed flow remain guided and silent. Here nine home registers are geographically collected to four equal outlet paths. Nothing in ACCA Manual D recommends this. This 21" square 12" tall and with 9" outlets might become a common standard supply box. Flow progresses through sequences of wyes, always maximizing pipe size such that the discharge register is the main flow resistance. Want the total loss coefficient in each path at final duct velocity, to be little more than 2. It is easy to do.