HVAC Diagnostic Static Pressure Measurements

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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 that of 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  measures 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 docts and supply ducts, see that returns are over-sized. The supply ducts should be a bit bigger, but do have modest 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.