Static Pressure Frustration

[WAYNE3298]

Until recently I couldn't understand why there was such a misunderstanding concerning measured static pressures in an air system. I did some research and found that there are a lot of "tutorials" explaining how to measure the pressures and how to interpret what they mean. There is a lot of misinformation given by so called experts and that is too bad.
Not one article I read explained that almost all duct systems have system effect and system effect can't be measured yet they explain how to measure the statics and calculate airflow from the readings. Not one article explained that if you measure the static pressure in the fan blast area that the pressure includes velocity pressure some of which downstream is converted to static pressure. That is called static regain.
For these reasons and there are other reasons there is no point in me pissing people off here trying to explain the pitfalls with using measured static pressure readings as accurate information.

[kdean1]

At least by measuring we’re trying to get a handle on the performance of the equipment. Measuring static pressure is more practical than using a flow hood for most residential installations. Only the basket cases merit that much effort.

What do you recommend for a reasonably accurate measurement of residential air flow?

[WAYNE3298]

Pitot traverse if you have a spot. The NEBB manual is accessible on line and it tells you static pressure is the worst way to try to determine airflow.
If you need to measure airflow you need to get reasonably accurate information.

[kdean1]

Pitot traverse if you have a spot. The NEBB manual is accessible on line and it tells you static pressure is the worst way to try to determine airflow.
If you need to measure airflow you need to get reasonably accurate information.

I agree, but many residential systems don’t have a trunk with 8 or 10 diameters of straight run. And how many technicians are capable of doing a good traverse? I’m happy to get my guys to measure static pressure so we have any idea of air flow. IMO, we’re miles ahead of the companies that don’t even do that.

[WAYNE3298]

Please understand kdean as a certified balancer I couldn't simply take a single reading no matter what it was and draw a conclusion about system performance. The problem I see is most HVAC guys have been taught that you can get an accurate airflow by measuring statics but you can't. NEBB advises that static pressure determination of airflow is the least accurate. On residential I would be a lot more satisfied that a system was performing well by measuring temperatures. In most systems if you don't have a place to do a traverse the static pressure measurements aren't going to be worth much.
Everyone should read what NEBB says about measuring air systems then decide what you want to use.
What does complicate things is CFM is totally dependent on static pressure and is so by design.

[kdean1]

Wayne, I have downloaded the Procedural Standards for Testing and Balancing of Environmental Systems. Section 6.2.2 is what I think you're referring to concerning "field measurement of static pressures is not a reliable tool for analyzing fan performance. Accurate assessments of fan performance in the installed condition require rpm, airflow, power data, and an evaluation of System Effect."
I don't have the qualifications to disagree, not that I am inclined to disagree in any way. Residential jobs don't have the budget for doing all that testing, even if I knew how. The best I can do is to use static pressure along with the blower chart to get a SWAG (scientific wild ass guess) for air flow. If we have a system that doesn't perform in line with our expectations, then I'll resort to the flow grid and flow hood. As it is, we take longer than our competitors to commission and adjust our equipment and our prices reflect that.
I'm just stating my case. I respect your experience and your contributions here. I just can't see those of us in the residential field going to the extent of what's in the TAB manual. If you can suggest a practical method for us to do better, I'm all ears.
-Ken

[BBeerme]

I used to do a lot of measuring static pressures of air flow. Almost never used those readings for calculating things like CFM; only on a couple of critical up-sizing of blower motors, and looking at the fan curve chart provided.

Can't remember what made me go out and buy a U-tube manometer. Probably just trying to get a handle on what was going on. Seems like there were large discrepancies in what I was taught in school, what 'senior' techs in the field were telling me, and just what my own intuition was telling me. So I started checking whole bunches of units.

Keep in mind, these were 'smaller' RTU's in the 3-10 ton range. I did know that velocity would come into play, so I would often move the tube around, seeing how much difference there was depending on the direction or angle it was facing. Surprisingly enough, there was often little or no difference. I'm sure as the velocities increase, so would the differences in the static I was reading.

So, here's what I ended up learning. On your smallish RTU's, you were looking for a total static of about 0.5" water column. Supply might be 0.3" and return might be -0.2" for a total of 0.5". The numbers can vary quite a bit, but when I'd see number something like supply 0.1" and return -0.4", that would tell me there was a problem with the return. Conversely, if my supply static was really high, then there was a problem there.

Like I said, only maybe twice in my career did I ever use the static readings for determining CFM. It was just a tool I ended up using to figure out other problems.

Been where I am now five years. Shortly after I started there was this heat pump that was causing a lot of problems. So they sent me out. Found the static too high. My boss dude says those unit can go up to (something like) 0.8", I replied maybe, but you'll get all of the problems you are getting now.

Then I sent him a photo of the nomenclature plate that stated Max Static 0.61"

Boo Yah!

Older system could muscle it's way through the undersized ducting, but the newer units are like wussie boys, just couldn't pull it off.

[WAYNE3298]

Your points are valid kdean. My frustration with this subject on this site is there are folks that swear by static pressure measurements and get really angry when I tried to warn them about the accuracy. I don't see the need to balance a residence because the owners can and probably will change it anyway.
With resi units I would measure static pressure in the best spot I could find (supply & return) and write the pressures on the duct. If the readings are repeatable they can be used for trouble shooting when something goes wrong with the system. I would compare the TESP to the fan curve to get some idea of CFM. I would want operating temperatures of the air supply and return and motor current draw. I would not list static pressures or CFM on the report to the owner. The reason is if someone comes in behind you and measures SP's you don't know how they will do it or with what type of probe or meter.

BBeerme you learned about static pressures a lot like I did. In the beginning my thinking was that since every fan I designed the thing I calculated my ass off to end up with was CFM at a specific static pressure there had to be a way to measure statics accurately. I'm not saying static pressure measurements are useless because they are very useful. I've tried to warn all of you that they are not dead on accurate.

[kdean1]

Wayne, I agree that some are entrenched in their thinking. Timebuilder tried without success to explain the difference between MCA and MOP, and why it is correct to use cable that has a lower rating than the breaker.

The lengthy discussions are one of the best aspects of this site. Though it can be time consuming to read all the comments, I don't mind learning from the experience of others.

[lkapigian]

for all intents and purposes, the typical scenario discussed here is for equipment performance and the static the equipment sees .....System balance is another..another reason I do not do Block LOads on my calculations as I want to see and design Room by Room ....its nice to have the air from the equipment , but it can't be distributed WIlly Nilly .........

[WAYNE3298]

This discussion has been a pleasure Ken because we are discussing the subject. We don't always have to agree with each other on this site because we all learn more by not agreeing than if we always agree.
I really like this site and have been impressed from the beginning with the overall amount of knowledge here.

[jbhenergy]

I take static on all my units I service and repair. For a gas furnace one probe in blower compartment and one in HX compartment.

Not so much for an airflow perspective but rather how much extra work the fan motor is doing.

If there are airflow issues then I will use my statics and performance data to see how much air my furnace is putting out.

But I can’t say I immediately jump to static as an airflow number.


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[hvacker]

The question I have is who is teaching techs to test systems this way.
This never came up in posts years back and some of us, you included I believe, asked Dad for a T&B forum hoping maybe someone would read it and learn procedures. Techs are still clinging to the blower charts as gospel. Lab conditions and no inlet restrictions and the tests might not be referring to the exact blower but might have interpolated math.

[WAYNE3298]

I may have been the first T&B guy here hvacker and the T&B forum was started not long after I arrived. When I first started posting T&B related things here there was a lot of push back because what I was saying was foreign to most. It was difficult for a lot to believe that statics could be unreliable and that it took experience to know how much the readings could be trusted. Part of the problem is the instruments are represented as being accurate and the limitations are not talked about.
A lot of design engineers think you can hang your hat on statics and if you have accurate ones you can. When I did TAB I included in my report the design and actual system curves hoping the engineers would get a better grasp on TAB. Don't know how well it worked but they didn't argue about it.

[BBeerme]

Glad you are here Wayne. Sometimes your post's are over my head, I have to read them over and over again. I either get it, or I don't. Somehow, it usually slowly sinks in.

Regarding another point you made, about the push back. Same thing happened to me when I first got on this forum. I stood my ground and others that were respected on here slowly backed up what I said was fact.

It took quite a while, but I think the regulars on here have seen I have forgotten more than I remember.

I may have been the first T&B guy here hvacker and the T&B forum was started not long after I arrived. When I first started posting T&B related things here there was a lot of push back because what I was saying was foreign to most. It was difficult for a lot to believe that statics could be unreliable and that it took experience to know how much the readings could be trusted. Part of the problem is the instruments are represented as being accurate and the limitations are not talked about.
A lot of design engineers think you can hang your hat on statics and if you have accurate ones you can. When I did TAB I included in my report the design and actual system curves hoping the engineers would get a better grasp on TAB. Don't know how well it worked but they didn't argue about it.

[WAYNE3298]

There are a lot of guys like you here BBeerme that have earned the respect of all here. I don't know how much you have forgot but your memory looks good to me. I'm addicted to this site because the broad base of all around knowledge still amazes me. Do you think maybe the original push back is to find out if you're confident in what you said and have the courage to push back?

[BBeerme]

No. It's because they didn't know or realize what they were looking at. Here's one example [that still crops up on occasion] . . .

Some HVAC equipment like for a server room runs all year long. So you have to control the head pressure when it gets cold outside. Often with an on/off pressure switch. During this process you will often see bubbles in the liquid line sight glass. Probably took years for me to figure out why. The reason is due to the rapid drop in head pressure past the point of subcooling. The refrigerant is literally boiling to cool itself to meet the now new current pressure. (I like to call this 'soldiers sacrificing themselves so the others can continue on and get the work done'.)

On top of that, due to the rapid expansion, and since the metering device is only so big, the refrigerant is often going backwards, being pushed back into the condenser to release heat so it can turn back into a liquid.

There are a lot of guys like you here BBeerme that have earned the respect of all here. I don't know how much you have forgot but your memory looks good to me. I'm addicted to this site because the broad base of all around knowledge still amazes me. Do you think maybe the original push back is to find out if you're confident in what you said and have the courage to push back?

[WAYNE3298]

Doing what I do that has never been something I ran into. Correct me if I'm wrong but the bubbles appear because the liquid is colder than normal and has contracted to the point that there is room for a gas pocket. If that's what happens it is in one respect the opposite of the norm and easily misunderstood. It never occurred to me that would happen but it makes perfect sense.

[BBeerme]

Doing what I do that has never been something I ran into. Correct me if I'm wrong but the bubbles appear because the liquid is colder than normal and has contracted to the point that there is room for a gas pocket. If that's what happens it is in one respect the opposite of the norm and easily misunderstood. It never occurred to me that would happen but it makes perfect sense.

What happens in a refrigeration system, At least the vast majority of them, Is hot vapor is sent to the condenser and comes out as a liquid. In the middle is a foamy frothy mixture where the vapor is giving up its heat and turning into liquid.

If the pressure drops suddenly then the liquid can be at a higher temp than what the liquid pressure says it should be. That is when the liquid refrigerant has to boil in order to cool itself down. As the refrigerant is boiling, those are the bubbles that you see in the sight glass. You typically do not get that if you have a control that modulates the outdoor fan motor, this typically only happens when a fan is turned on and off and especially during cold weather.

The whole concept comes back to the temperature pressure relationship that the refrigerant must maintain.

[WAYNE3298]

I think we are saying the same thing in different words. The pressure dropping is what creates the "void" (For the lack of a better word) that allows the refrigerant to form gas pockets. If the condenser fan is slowed down the unwanted sub-cooling will stop thus preventing the formation of the gas pockets.
Have I got it?