HVAC Diagnostic Static Pressure Measurements

Link this post to advocacy through ASHRAE, The American Society of Heating, Refrigerating and Air-Conditioning Engineers. At ASHRAE I belong to one Technical Committee, TC 5.2, Duct Design, where I complain against prevailing cheap and poor design of residential HVAC air ducts. As a contractor I tear apart and reimagine some really-awful ductwork hacked in place by "professional" installers seemingly in compliance with the default rule book delegated to ACCA, Air Conditioning Contractors of America. I want superseding, better instruction, to be found at ASHRAE. Absent this improvement I will continue to practice better work as example to others. On this path I must be able to prove the better value of my alternatives, in a growing body of before and after HVAC measurements by simple manometer measurements of system pressures. Collect relevant blog posts by applying Label: HVAC Duct Design

Google the Subject of my proof quest:

HVAC Diagnostic Static Pressure Measurements

Google AI says:

Static pressure measurements in HVAC systems are crucial for diagnostics and ensuring optimal performance. They help identify potential issues like airflow restrictions, undersized ductwork, or dirty filters, which can impact efficiency and comfort. Technicians use manometers, which are pressure-measuring devices, to measure static pressure at various points in the system, such as before and after the air handler, the coil, and the filter. 

This is incorrect only in the mention of detecting a dirty filter.  Any sensible person will replace a filter on-schedule or upon dirtiness observation. The statement is deficient in not mentioning detection of possible air-side fouling of the secondary heat exchanger in a high-efficiency gas furnace.

I have known that a major purpose of the measurements is to evaluate crummy HVAC ducts, and then to celebrate improvements made. Unfortunately I have been slow to accumulate measurement sets in a way that is sensible, recording measurements both before and after improvements made. 

I will quickly learn in a small sampling of customer homes that meaningful measurements identifying good or problematic HVAC duct design are very easy and unambiguous. They are for the purposes I imagine, just confirming my common sense about duct design. Measurements are not for a silly purpose of "measuring" blower flow rate. Rather, they are just design and construction quality documentation. Want least flow resistances rather independent of construction cost.

Find an appropriate graphic by search: 

graphic: a typical HVAC air duct arrangement and gage static pressure measurements

Credit this to energystar.gov

Label the gage pressure sample points as:

P1: Return, before filter box.

P2: Blower suction, after filter box.

P3: Heat Exchanger, wall at exit, if there is separate cooling heat exchange. Else this is at the  Supply Plenum,

P4: (Or P3) Supply Plenum

By themselves, P1 and P3 (or P4) are measurements of builder variables at hand: efficient design of the Return Ducts and Supply Ducts. P1 and P3 (or P4) should be very small numbers., demanded for everyone. Small numbers will correspond to lowest power, quiet operation, best observed and judged directly.

P2 should be a very small nunber, except with fouling of the air filter, not a builder issue. Demand biggest/ best filtration for everyone.

The blower and heat exchanger investment is a builder and buyer choice not measurable by manometer. Running a gas furnace with the secondary heat exchanger fouled by cheap or none filtration, becomes a bad choice.

Customer Rae, Goodman Furnace, Upflow, With Air Conditioning:

Good duct design in a home with already-existing supply registers is simply the use of the largest-size steel pipes that are available in branching to final register-size pipes.  Keep air guided and moving at similar velocity everywhere.  Allocate registers to ducts such that register air flow velocities are equal.

See that overhead space and the furnace profile limit the size of four equal-sized starter collars on the supply plenum. A best-possible air filter is conveniently serviced at the furnace backside. 

Here is my measurements setup for customer Rae with greatly-improved HVAC ducting enabled by demolition of a three-stories chimney. Learn: why should one demolish a condemned, inactive chimney? The chimney chase closed below-attic is now occupied by round ducts mainly the 12" return path from the upper floor here seen atop a collector box, instrumented with tap P1. Here label four static pressure values that are exemplary for most HVAC with separate exchangers for heating and cooling.

Good furnace mechanics are taught that for every well-running furnace, gage static pressure downstream of the furnace heat exchanger shall be less than 0.5 inch H2O. Foolish mechanics conflate this pressure, related to gas combustion conditions vs. atmosphere, to value P3 - P2, sometimes called Total External Static Pressure, a phrase unrelated to science in Bernoulli Principle. Wikipedia lists Static Pressure. but does not allow the unscientific phrase.

Here is the Rae table of gage pressure measurements and some computed differential pressures:

Wonder whether P1 and P4 as duct resistance measurement, are as small as they can be. They are reasonably small numbers, judging by other measurements reported here. Be concerned about Blower/HX Delta high at 0.9 inch H20. Expect readings less than 0.5 in H2O. With larger numbers, suspect fouling of the secondary heat exchanger, caused by operating for many years without a good air filter.  See that for this home with unusual multiple return air paths, interior door positions matter. Don't wish for such variables, but see that variation is small, not to worry about door positions.

Gage static pressure values P1 and P4 by themselves have important meaning about duct flow resistance since they are uncoupled.  P1 is a  measure of return-air  flow resistance from house interior space up to the air filter. P4 is a measure of flow resistance to house interior space, from the final heat exchanger egress. Supply ducts have a bit more flow resistance than return ducts. Good design has the supply and return resistances equal and smallest-possible.

Customer Meyer, Goodman Furnace, Upflow, With Air Conditioning:

Here is a table of static pressures for customer Meyer, reimagined ducts well-documented in this blog post:

See that P3 - P2 is 0.8 inch H2O, exceeding an expected value near 0.5 inch H2O. Wonder if this is due to fouling of the secondary heat exchanger, where the furnace was operated for years without an air filter.

Looking at P1 and P4 as indicators of ducts return resistance and supply resistance respectively, see that supply resistance is double that of return resistance, and both are acceptably small.

My Home Goodman Downflow. Without Air Conditioning:

Here is a table of gage pressures and differentials for my home, 

Looking at P1 and P3 as indicators respectively, of return ducts and supply ducts flow resistance, see that returns are over-sized. The supply ducts are complex and long, with above-average but acceptable flow resistance.

Homes with heat pump HVAC will also have just three pressures sampled. Hope that in retrofit of a heat pump, ducts are reimagined and replaced.

Leet Carrier Upflow, 7/18/2025

An ideal upflow arrangement, with intelligent, biggest-possible flexible ducts. 

This is not the work of the original furnace installer.

10/5/2014 

The homeowner complained of loud furnace noise. I found, and fixed, this. The repair included reimagining all supply and return ducts in the crawl space. Such service is possible because I can rely on the design and build of an excellent sheetmetal shop. Nonsense must be avoided somehow.

Here is the job photo album:

Furnace Plena and Flex Duct Innovation, Multnomah Village

At 11/25/2025 upon finding a nice comment to this post, edit this job photo album, brightening all photos. I wish iPhone photos by default were not so dark. Know that much of the value I share is in a reader taking the time  to learn details of my consistent inventions.

Here mention the security achieved in flexible duct suspension from overhead floor framing using hangers to wyes and other strong steel duct elements.

 

And another invention, insulative coating of the interior of steel duct elements to reduce swung thermal mass.

10/25/2014

See wye trees that replaced an absurd Medusa D-box. Avoid turbulence everywhere, with steady-velocity guided flow.

See P1, understandably tiny return flow resistance from a large floor register directly-above. . See P4, a modest 0.21 supply ducts flow resistance. See suspiciously small air filter delta; perhaps there is something wrong with P2 location vs a turning vane. Someday wish to relocate P2 closer to the air filter. This furnace runs so very quiet. Think it is at lower blower RPM. P3 leaving the furnace is watched to be near 0.5 in H2O, and is much less.

Martinez Heat Pump Furnace Pressures and Error Discovery

Surprise! 

Here is all-electric HVAC, a heat pump furnace with beautiful cabinet simplicity.

And, surprise again to find that lofty heat pump installers can be stupid. A hack-in air filter slot does not support air filtration.

See a half-size return air path at the P2 probe half the height of the air filter slot. All return-path cabinetry cut into the furnace LHS wall must be replaced, including a horizontal proper air filter 20x25x5 and a big turning box with much taller side cutout of the furnace wall..

Think about the presence of a condensate pump. Condensation does occur  at the furnace evaporator in cooling mode.

A 12" x 20" x 1" filter propped upright, is freely bypassed.

See that furnace and heat exchanger delta-P, P3 -P2, is quite large,  due to foolish construction of the return air path of the furnace.

 

HVAC ducts are very efficient:

P1 is the resistance in return ducts, -0.20 in H2O. 

P3 is the resistance in supply ducts, -0.15 in H2O. 

Here is my plan for improving Martinez return air ducts:

(Wait to do this in September 2025, when A/C won't be missed for a few days.)

Return air flow resistance will be reduced by a factor of four, a decrease of about 0.5 in H2O of static pressure measurement. The furnace blower will run much quieter and at less cost. The new  filter will at last be protective of the heat exchanger. Inquire whether the heat exchanger should and could be cleaned.

Reject cost/ benefit of a rounded, lined turning plenum with turning vanes (Leet). No other ducts are lined. This follows the example of Rae. The found return path area is doubled, and there is a proper filter box. The side opening into the furnace is doubled in size. Inquire with Rheem as to recommended furnace side cutout markings and size.

The turning box and filter box will be 3/4” above the floor. vs. found down box 66” tall and 2 3/4” above the floor.Combined height of the down box and filter box will be 68”.

Customer Menezes, Bryant High Efficiency Gas Furnace, Downflow. With Air Conditioning:

Here is a first instance of observation in both cooling and heating modes. Shifts in location of pressure drop are surprising, rather small, and real. More energy is needed to move cold air into supply ducts in cooling mode. Like the Leet excellent Carrier gas furnace, operation is surprisingly quiet, probably with less energy applied at the blower and longer cycle run times. Get what you pay for.

On-site, we talked about a hostile crawl space. There is some evidence in the static pressure measurements that supply ducts were installed with care. Still, I wish to inspect the crawl space someday.

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

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 primary loop hydraulics of commercial 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

Waters North Attic Ladder With short-lived flex ducts, access is mandatory.

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. Attic ducts are buried under insulation 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>

1234 NE 118th Ave.

Portland, OR 97220

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.

Instead, use common sense to create great value in a very tall attic space.

(11/24/2017)

Let the furnace and ducting become non-obstructive. With very little energy loss, silently turn the developed air flow 180°. Bury all ducts under a maximum of batt insulation covered and protected air-tight. Retain one awful feature of the queer found installation: There is no return air filter!

When, later, an air conditioning heat exchanger was added, there still was no way to add a filter box.

Here is a composite photo of the dark and daunting found-conditions.

All attic access is obstructed. Even the small OSB furnace floor is blocked. One must dangerously duck under 2x4 truss bracing. All of this is of odd and wasteful USA attitudes toward attics. Don't go there. Let them be booby-trapped. Let all attic work be delegated to newby unfortunates, without inspection and thoughtful supervision, never to be improved.

Now talk about the new possibilities.

Look down upon  efficiently-dividing paths below the 180° turning plenum. See guided concentrating flow continue at steady velocity, quietly and efficiently. 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, all the way to a controlling flow resistance at discharge in registers.

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 internal insulative ceramic coating, that does matter.

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

Loss coefficients are dimensionless multipliers upon velocity head (air pressure of motion), at bulk velocity in the pipe approaching the register. A bulk velocity is the ratio of a measurable flow rate, and pipe area. An ASHRAE Handbook Table 10.5 of 1981 is presented in a mark-up at the University of Texas, Austin.

In the awful found D-boxes then demolished and improved-upon, see that Medusa streams at register velocity, V = Vr, with side-stream K = 1.8, see that pressure loss exceeds that in the register discharge, and the resistance at the register is not controlling of room air flow.

Here is the completed ductwork at the furnace:

Simple common sense is in beautiful contrast to the Medusa-teaching of those claiming to adhere to guidance of ACCA Manual D, Chapter 6. There is safe, inviting passage throughout the attic.

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

Climbing up is with a conveniently-stowed telescoping ladder, and that is ok.

Where my invention is focused on attic access, do provide thoughtful, safe climbing from the ladder, setting aside a lightweight insulated hatch cover.

My resources supporting continued HVAC invention inlude many happy new friends in metro Portland, Oregon. We shall wish to revisit these job sites to verify superior HVAC duct performance. This home is upon a buried powerful stream-flow to the Willamette River. Think: experiments with community geothermal heat pumps. A two-block complex of decrepit colonial homes should have become a square of Copenhagen-like three-stories and more row homes with more-responsible prevention of ground-settling.

At November 2024 I share news of yet-another project eliminating a leaky, thoughtless hack-in D-box.

One Before photo:

9/30/2024

See common shabby dirtiness in a Medusa of pipes register-sized full-length, blown air tripping over and leaking past barely-capturing tabs. Nothing stably guides the flow into a pipe. 

Look up:  loss coefficient for unguided turn through an abrupt sharp take-off . 

Know there is costly waste of energy in chaotic, noisy turbulence. Unguided fow out the side of a plenum has a  loss coefficient that with trip-edges, might exceed 2.0! 

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

10/29/2024

Let developed flow remain guided and silent. Here eleven 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, K in each path adjusted to  final duct velocity, to be little more than 2. It is easy to do. From my cited hydraulic analysis examples, see that path K with careless design is commonly more than double this. Undersized portions of any flow path add to troubles.

Well-guided flows are equally divided in the four 9" diameter supply outlets, branching to individual registers through wyes and straight pipes of maximum size. The goal always is that the dominant flow resistance in each register path is at that register and a minimum of approach pipe length at register size. Where most pipe length is upsized by at least one inch of diameter, any register might be upsized as-needed, with a minimum of expense. An over-size register may be throttled without added noise.

Every pipe junction and elbow segment is thoroughly taped with Nashua 557 tape. I will not use super-adhesive Nashua 367-17 Foilmastic tape; where someday disassembly and elements reuse is unbearably difficult. Here difficult taping in the chimney chase would be followed by impossible disassembly. I appeal to manufacturer Berry Global, that a removable tape more durably adhesive than 557 should be offered and recommended for my need.

Sensible ducts are as large as economically possible, reducing  progressively beyond each wye branch as in any living circulatory system, down to a controlling squeeze through a final capillary at each register.

Ducts assembly with rigid elements is challenging, but with patience, workable element orientations are found.

Graphics of duct planning with photos of the found ducts and improved ducts are presented in album New Ducts Hydraulic Analysis.pdf.
 

There is real, substantial cost of all inefficiency. We can afford engineered and shop-built best components. Reject determination by fabrication cost and pennies pocketed by a builder.. Lifetime operating cost is the concern. 

Now, how do I inspire others to follow my example of thoughtful HVAC duct design?. Look for an evident likely ally, a respected believer, to weigh in. Conduct this Google search: blower energy cost savings if I reduce flow resistances by half . Find this excellent simple and scientific post by Donaldson Filtration Solutions, Donaldson Company, a global manufacturer based in Minneapolis:

FAN-tastic: Cutting Energy Cost Out of Thin Air

 

See that piping flow resistance reductions are simply additive to that of a new money-saving and heat exchangers-preserving much bigger air filter. Furnace filters and heat exchangers are all additive resistances. There is a penalty in having two in-series heat exchangers in a forced-air gas furnace with air conditioning.

There should be much opportunity to demolish HVAC ducts, starting over, better, in adopting air-ducted heat pump HVAC. As we do this, acquire a big history of Diagnostic Pressure Measurements . The link is to a blog post that will present measurements that I take, along with interpretation, and defense of the appeal to ASHRAE and ACCA. In time, I will be heard and respected. Despite frustration over inaction, I will continue my membership in ASHRAE, for service in TC 5.2, Duct Design.  In advocacy of ground-source heat pumps rather than air-source, there shall be much reimagining of HVAC air ducts.

.