The following is a re-posting of a Google Docs document that will be subject to constant correction and update from reader comments.
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:
I have several similar achievements to share:
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: Better In-Attic Furnace Ducting
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 <firstname.lastname@example.org>
1764 Bonniebrae Drive
Lake Oswego, Oregon 97034