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Thanks for all the input. I have just re-read all the posts and all you guys made my day. I've said it before but it's worth saying again "THIS SITE IS GREAT".
I started a post on water where I have a problem job but not much interest so far. It may be the holidays or not much interest in water or maybe not enough information there. If not interested in water say so. That isn't a problem.

Thanks Wayne I'm going to put that in my brain and let my three brain cells grind on it for a while. I think I'll move downstream and estimate losses for the intermediate transition. OH YEAH !!! (just killed a mosquito)

I have measured supply side SP'S that readout negative. I have also got readings in AHU'S where the SP after a coil was higher than entering the coil.
If you have a pitot tube disconnect the low side and use it to measure the SP'S. You can use it for SP traverse. I doubt you will like the result but worth a try.

I just measured the ESP of my home unit and got 0.06 wc pressure supply side and - 0.56 return side for a total of 0.62"wc TESP

I don't believe the supply has only 0.06 wc pressure drop but it is remotely possible. I carefully chose each measurement point so that (in my mind) it wouldn't be in a bad part of the duct. Gotta find a way to traverse but I don't know an easy way yet. I just want a good TESP measurement but am not sure if I can ever get one without making it a science project.

I can just drill more holes ... my probes are only 6" long but I need foot long versions.

If you traverse the duct measuring both dynamic pressure (velocity) and static pressure then you will find variations of both across the duct. Near the wall there may be a thick boundary layer downstream of a turn where the velocity may even be going backwards or "upstream" if measured downstream of a bad corner.

If the pressure tap on the business end of your manometer probe happens to have airflow directed towards the measurement point then the "static" measurement will read high because the kinetic energy contained in the velocity of the flow will be converted into static pressure when it comes to rest at the location of the pressure tap. If there happens to be a steady vortex from an upstream bend whipping right by the measurement point and if it causes an "above average" velocity in the vicinity of the pressure tap then the total pressure, or total energy, might be the same but the static pressure will read lower than expected.

What we really want to measure is the total pressure which includes static pressure plus dynamic pressure. I don't understand why our standard hvac probes are these weird looking static probes instead of a total/static pitot probe that gives both static pressure and velocity of the air. If they just added another hole on the front and another tap on the back we could get both or either.

Do they even sell total/static probes in the hvac industry? But without averaging measurements across the area of the duct (traversing), even the extra data of velocity head plus static head does not reduce the primary uncertainty of the calculation.

The term static pressure in a dynamic system in my opinion is a contradiction in terms. I have always taken static pressures at the equipment because it is required but I don't have much trust in them. If you can get a good run of duct near the fan inlet that is your most reliable reading.

I find the most reliable static pressure readings are obtained by using a pitot tube and pulling it up against the inside of the duct or unit casing . In my opinion the most reliable discharge static is obtained by taking a reading on each side of the duct ( 4 readings ) and averaging them.

Because your readings are not purely static pressure - there will be also velocity pressure due to turbulence and/or incorrect use of instrument.

One reason static pressure measurements in an HVAC system are unreliable is the air is not static. The measured pressures are dynamic and Pascal's law does not apply. Pascal's law that states the pressure is the same in all directions is only true for fluids (air) at rest. In a system where the air is pressurized but is not moving the air molecules bang into each other and fill the entire space until all pressures are the same. That happens almost instantaneously. In a duct since air is compressible the molecules don't reach steady state position because they are moving down the duct and their position changes with the effects of duct friction, elbows and etc. and the air is being compressed and decompresses constantly.
More later.

Originally Posted by heatingman

I would call that a Venturi effect.


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The same effect. very related. Like a carburetor or that air powered cool/heat suit. The vortex something or other. I usually think of a venturi as a device and vena contracta as an effect. Often something not planned or sometimes intentional like in a diffuser design.
The name just means a vane ( or jet) contracting on itself.
A lot of staitc measurements go south because this effect is common. Common in resi ducts when extended plenums are used. No mechanical way to raise pressure after takeoffs and velocity takeoffs on the end of the duct.

Originally Posted by WAYNE3298

Can anyone give a technical explanation of why measured static pressures in air systems are unreliable?

Measured static pressures are reliable. In that, what you measure is an accurate reading of the pressure at that point in the system, and at that time. And when you see it change, that reading is also true.


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Originally Posted by hvacker

For those not familiar one way to explain vena contracta works is how some paint sprayers work. They direct a jet of air over a small pipe that's in the paint. The jet draws in paint into the air stream and also air.
Only reason I say this is because I have had trouble a times having some to grasp the concept. The dirt sometimes seen next to a diffuser isn't from dirty filters, it's from dirty air in the room being drawn into the jet from the diffuser. Usually in a space with a lot of paper of fabrics.

I would call that a Venturi effect.


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For those not familiar one way to explain vena contracta works is how some paint sprayers work. They direct a jet of air over a small pipe that's in the paint. The jet draws in paint into the air stream and also air.
Only reason I say this is because I have had trouble a times having some to grasp the concept. The dirt sometimes seen next to a diffuser isn't from dirty filters, it's from dirty air in the room being drawn into the jet from the diffuser. Usually in a space with a lot of paper of fabrics.

Originally Posted by Scoobie

And I get what bad fittings and undersized ducts do.

But, if I install the fan with the correct inlet and outlet conditions...it should still perform on the curve. The bad duct will just equal more static, and move on the curve.

It's interesting to read how fans are tested in the lab.
First is the tests are made with no inlet restrictions (no ducts on the return).
Second is the test data might not be for the exact fan your testing. Similar fans have curves developed by interpreting data. Only thing I can think of is if the fan is similar enough the actual tested fan data is regurgitated.
Third: the fan curves are developed under ideal conditions not field conditions.
Forth: System effect can't be predicted for a fan because no one knows to what extent it will have. On resi systems, system effect can be huge. So often the fan discharges into a plenum that bullheads then turns with no turning vanes and into leaky ducts that are sized by what seems like what ever was in the shop. A single return with no way for air to leave the spaces that were pressurized. Flex run like an octopus and drooping like a wet noodle.

I would believe 95% of resi installs have system effect issues. That % was only in my head. At one time in my incarnation I had been in over 10000 homes. I can't hardly believe it my self but I quit counting at that number.
Anyway scoobie, those are some reasons static fan curves might not be reliable. It's not that they aren't a means to troubleshoot a system. In a T&B report, statics are taken along with the other stuff because it's expected.
It can create a question of the report numbers when the other measured data like the traverse and the terminal readings don't match the fan curve though.

The vast majority of testing I did was commercial. I mention that simply because even though I tested a lot of residential type units the install was in a commercial setting and more space for the install was available which reduces the odds there is system effect. The accuracy of the readout varied a lot but very few were on the money. I tested 36 of these units on one job and two readouts were very close. Airflow was within design on all the AHU'S which says a lot because the ESP mistakenly did not include the filter pressure drop. Strictly from a balancer's standpoint I like this type of control. The 36 units had a central control panel and the airflow readout could be calibrated from there. In my opinion if you read 1200 CFM you have enough airflow for the system to work properly with few exceptions and that to me is the bottom line.
I hope that answers your question.

Wayne what is the accuracy of true variable speed blower cfm settings?

Sure is nice to see that board saying “1200 CFM” but how accurate is it?

Trying to get valid information from a lab guy should be appreciated and he should not have thought you were messing with him. I don't think you are messing with me and I actually understand your concern. If you get accurate statics the fan performance when plotted on the curve will make sense. When you get an install where you can traverse the duct and know actual airflow within 2 or 3% expected accuracy take statics and KW along with the pitot and compare all the data then plot on the curve. If anything doesn't fall where it should it will be the measured statics. Static pressure measurements almost always help troubleshoot because they will point you to the problem. An example with a residential unit if the return static measures high the duct is probably too small or a restriction such as a dirty filter is indicated. It doesn't have to be dead on to help determine where a problem exists but has to be very close to determine airflow and where to plot on the curve.

And I get what bad fittings and undersized ducts do.

But, if I install the fan with the correct inlet and outlet conditions...it should still perform on the curve. The bad duct will just equal more static, and move on the curve.

Thanks Wayne. I pressed a lab guy in a factory about this and just ended up looking like I was messing with him. I’m not messing with you. I really just don’t get this. Pressure is an easy measurement. I think.

Controlling VFD speed using duct static pressure is common and works well. This measured static pressure thus control is adjustable and doesn't necessarily have to be accurate but should be repeatable. At a seminar I went to a factory design engineer addressed us and the first thing he said was "My fan will do exactly what I say it will do until you hook your duct to it". I don't remember him or his company but of all the seminars I have attended his opening statement is the only one I remember word for word. The fan and motor have to do the work therefore see the real load. I promise to get into this deeper later on and hope to be able to make things more clear.