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 common 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 have been deterred by universally bad internet advice.

The turn-off foolishness is evident in the top Google search result:

Fieldpiece: Measuring HVAC Total External Static Pressure with a Digital Manometer

This writing teaches that sample points of interest are only P2  and P3 in the photo below. P2 is static pressure past the air filter, entering the Blower. P3 of a high-efficiency gas furnace is static pressure leaving the furnace secondary heat exchanger. A manometer with sample ports connected to P2 and P3  displays static head of the blower diminished by furnace heat exchangers, in the presence of all other air flow resistances. This head is a variable dependent on balancing of all flow resistances to capacity of the blower.  The value of this variable is not as a measure of the through-blower air flow rate. We don't care about that flow rate except that it is controlled  by complex ductwork changes beyond the purview of a usual HVAC operator. An excessive value of P3 - P2, beyond 0.4 in H20, may be reduced by drastic changes of duct design.

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

Here is my measurements setup for customer Rae with greatly-improved HVAC ducting enabled by demolition of a three-stories chimney. Learn much better: 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, stupidly called Total External Static Pressure, a phrase unrelated to science in Bernoulli Principle. Wikipedia lists Static Pressure. but does not allow the unscientific phrase.

Delta pressure across the air filter is the best indicator of .air flow rate achieved by the blower. Want that to be a maximum. For comparisons, wish to find that every customer has the same superior air filter, Honeywell 5x25

Note that blower air flow rate is a default maximum in-common for many furnace manufacturers. Manufacturers discourage making non-default settings. Recorded static pressures for many furnaces will be familiar, comparable, not mysterious.

Here is the Rae table of static pressure measurements:

With this, get to know that interior door positions matter although not strongly. Want to apply Item terminology and labeling consistently for all homes. See that my table is a brand-new invention, that makes sense. Wish for simplicity in a standard. Should all interior doors be open. or closed? Can there be a standard set of home conditions? 

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, well-documented in this blog post:

See  commonality in not-anomolous readings for all homes. See that P3 - P2 is not in conformance with some manufacture specification of a value near 0.5 in H2O. Wonder if the value around 0.8 in H2O is due to dust-fouling of the secondary heat exchanger in both cases, where the Meyer furnace was operated for years without an air filter, and Rae furnace was operated for years with a very poor 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 static pressures 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.

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 the absurd 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..

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, in response to the return-path drag (more blower head and less air flow), and is  probably associated with much heat exchanger fouling. 

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. 

Combination of ducts resistance, P1 - P3 is a ridiculous notion, where the ducts are not connected except through house interior air space. Stop doing that.

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 Eddiciency 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.