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Thread: Static Pressure Frustration

  1. #21
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    Quote Originally Posted by WAYNE3298 View Post
    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.
    Actually there were T&B techs around including myself for awhile(2004). Because there wasn't a forum their comments got scattered in other threads. but questions did come up on occasion. Unlike you, T&B was not a big part of my business. Just troubleshooting and some commissioning. I didn't advertise as a T&B company. Too expensive to remain qualified and the costs of instrument calibration to compete with full time T&B companies. Good help is hard to find.

    It looks to me that techs are being taught this method and I wonder if it's NATE because the procedure seemed to shadow the participation in NATE. I know TABB teaches that using static pressure and fan curves is not a way to determine actual CFM. I think it appeals to some techs because it's easy and uses the least expensive instruments. Most T&B reports include static readings because reports require them along with all the other tests.
    In T&B classes I wasn't taught to use static fan readings but I also wasn't taught not to. That was a long time ago. I forgot what I didn't know. Old timers disease I guess.
    Give me a relay with big enough contacts, and I'll run the world!

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  2. #22
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    Quote Originally Posted by WAYNE3298 View Post
    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?
    Wayne, I think a lot of push back is from NCI courses and similar. I have taken several and they answer a lot of questions and seem to be the experts, you learn what they teach and unless you figure out where they may be wrong you continue under the assumption that what you have learned is correct.

    I have struggled with air flow for a long time, especially on a gas furnace. Doing a temp rise on electric elements is relatively easy/straight forward. On a gas furnace it would appear easy but you start adding up all the variables and suddenly you can run down a rabbit hole. So when I was taught that you can use static and the blower curve life was good and easy. Maybe it is not perfect but does seem to get you in the ball park. I don't know if it makes that much difference if you know that you have 1150 or 1275 on a 3 T system but on some issues I suppose it may give the answers.

    i used to do static on most systems but since I got my iConnect it seems to give me just as close of results without the extra time/effort.

    You are right about this site, there is a lot of great info here, thank you for your contributions.

  3. #23
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    BNME8EZ, for what it's worth I'm not sold on certified residential air balance because there isn't much you can do to vary airflows at the blower maximum speed which is where certified air balance is done. The manufacturer gives you the pressure Vs CFM table and to me that encourages using that data to determine CFM. You have the right thinking which is the result is approximate. The sad part to me is if you could get dead on statics the corresponding CFM would be right on.
    Heat gain or loss as you indicated can help but they also have their pitfalls. An engineer challenged my fan powered box heating airflows because the discharge temperature was lower than design. The problem wasn't high airflows but low voltage to the site. The heated air temperatures were on the money for the voltage.
    No man can be both ignorant and free.
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  4. #24
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    Now that I am home, and in front of a real screen and with a real keyboard, maybe I can do a better job of explaining. Keep in mind that these things we are now talking about are beyond the scope of probably 99% of A/C and refer techs out there. I mean the simple question of why are there bubbles the the liquid line sight glass when the fan cycle switch turns back on.

    And, as you well know, words are important. Here is the example I give newbies in the trade that have not learned the proper technical terms, and appear to have no desire to learn (not putting you in that category, just think you might find the following paragraph a bit amusing).

    I'll get a tech that "wants to learn". In reality, he just wants more money, but does not want to do any 'bookwork' to make that more money. So he doesn't use [or know] the proper terminology to use to get ahead. So in order to explain how the same word can mean many different things depending on the occupation, here is my line. Take for example the word Head. In our trade it typically means discharge pressure of the compressor. It may also mean rise or restriction in a pump pumping a fluid. If you are an auto mechanic, there are heads on the engine. If you are in the sex trade, head has a completely different meaning. If you are a doctor, head has an obvious other meaning. I'm sure other occupations have different definitions of the word Head.

    So, let's continue. In answer to the highlighted below, no. I don't think so. Give me a few minutes to try to explain.



    Quote Originally Posted by WAYNE3298 View Post
    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?
    I do a triple evac with nitro to remove non condensables.

    I'm feelin' a little peculiar.

  5. #25
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    That's an interesting phrase.

    Maybe try looking at it this way. Take a liquid and change the pressure. Does the pressure drop always create bubbles?

    The answer is no. The reason is because the boiling point of that liquid at any given pressure.

    Refrigerants are designed to boil at lower temps in order to absorb heat in a cost effective manner. Some refrigerants do work at near ambient, but a much lower suction pressure is required. For example, water can be used as a refrigerant, but the cost of energy would be a lot more. But I digress.

    So here is my example, as complicated as this seems to be getting:

    If pressure drop alone created the void, then how come a liquid refrigerant [in a dynamic system] with a subcooling of 100* will not create bubbles in the liquid line with a sudden drop of pressure resulting in only 2* subcooling? Spoken from the other side of the fence . . . How come that same system will create bubbles in the liquid line as soon as there is no subcooling?



    Quote Originally Posted by WAYNE3298 View Post
    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?
    I do a triple evac with nitro to remove non condensables.

    I'm feelin' a little peculiar.

  6. #26
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    Your point about words is a good one. I'm convinced I'm using the wrong words but haven't thought of a better way to say it.
    No man can be both ignorant and free.
    Thomas Jefferson

  7. #27
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    Well, you got a couple of things going on here. Subcooling is desireable, it gives us a solid column of liquid to our metering device.

    The reason we stop or slow the condenser has little to do with subcooling. It has to do with the head pressure and the flow through the metering device to the evaporator. The metering device is designed to operate under certain pressure drops. Head pressure gets to low, then the metering device won't meter proper. And, typically, the evap freezes up.


    Quote Originally Posted by WAYNE3298 View Post
    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?
    I do a triple evac with nitro to remove non condensables.

    I'm feelin' a little peculiar.

  8. #28
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    Don't worry too much about it. Meaning, telling seasoned techs what they are actually looking at is something that is very difficult for most of them to wrap their head around.

    They just cannot accept that they are witnessing liquid refrigerant boiling in the high side of the system.

    Like I think I said prior, it probably took me a few years to answer my question, AKA . . . Figuring out what I was looking at.


    Quote Originally Posted by WAYNE3298 View Post
    Your point about words is a good one. I'm convinced I'm using the wrong words but haven't thought of a better way to say it.
    I do a triple evac with nitro to remove non condensables.

    I'm feelin' a little peculiar.

  9. #29
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    I publicly challenged a guy on YouTube one time for his use of statics to determine cfm. He really got steamed. I dropped it, because he was an expert......

    When I learned T&B, I was taught that in the laboratory, where it was possible to control every aspect of the installation, that it was sometimes acceptable to use statics for CFM calculation. But here's the kicker.....

    In the field, the actual installation has so many possible variables that it's a crapshoot to try to determine CFM from statics. The same piece of equipment might be used in various applications, and a properly designed system might be installed by good installers, or lousy installers. The system design will also vary from Engineer to Engineer.

    Basically, out in the field, statics for determining CFM is a waste. For troubleshooting though, it's another matter. There's nothing like a good static profile to troubleshoot system performance issues.

    Sorry, if you can't spend an hour or two's worth of labor using a flow hood to verify that the design flows are at least in the ballpark, (and fix it if they're not) that's a bit cheap in my book. It doesn't take that much more effort to verify that you got it right.

    The savings grace for a lot of the fly by nighters is that, especially in heating applications, you can really blow it, and still get by with it.

    It does make me angry when I see some of the garbage some of these outfits are getting away with. That's why a few years back, our State started a licensing program with hefty fines associated with shoddy work. It has had some positive effect, but there's still a lot of junk out there. Last month a friend sent me some photos of a residential gas furnace installation that was downright scary. And the home had recently been inspected, and passed.

  10. #30
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    An AHU manufacturer's engineer said in a speech he made "my AHU fan will do exactly what I say until you hook your duct to it". That is a true statement.
    No man can be both ignorant and free.
    Thomas Jefferson

  11. Likes BBeerme, CHAINIK, icy78 liked this post.
  12. #31
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    Quote Originally Posted by WAYNE3298 View Post
    An AHU manufacturer's engineer said in a speech he made "my AHU fan will do exactly what I say until you hook your duct to it". That is a true statement.
    that sometimes still needs proven in the field ie bench test before spending too much time looking at ductwork , on a similar job recently
    Keep it simple to keep it cool!

  13. #32
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    Quote Originally Posted by WAYNE3298 View Post
    An AHU manufacturer's engineer said in a speech he made "my AHU fan will do exactly what I say until you hook your duct to it". That is a true statement.
    And when it's learned that the tests are made with no return restrictions (ducts) it's easier to see where the tests could be flawed. I don't see the value of resi T&B either. Most commercial systems are run out with some logic where many resi system are made to fit the structure. Often no way to make adjustments except at the outlet and that might not survive the HO.
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

  14. #33
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    Put the Air back in Air Conditioning ....Delivered air = Delivered Capacity ....i think duct assessment knowledge should be a requirement for all techs ...its the pink elephant in to room and the root of most system failures rivaled by install and maintenance

  15. #34
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    Are you saying that taking the static pressure drop across a cooling coil is useless information and does not correlate to the manufactures CFM to pressure drop information?

  16. #35
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    I agree Ikapigian that duct problems are the elephant. What bothers me the most about duct systems is that nothing has been done for years about determining system effect factors that are not known. Because of this there are some duct configurations that pressure losses cannot be determined.
    Steve static pressures are approximate. The value of measuring and recording component pressure drops is to assist in troubleshooting future problems.
    No man can be both ignorant and free.
    Thomas Jefferson

  17. #36
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    Wayne,

    I am a refrigeration guy but am a student of air flow in systems. I agree there is a lot of misinformation out there. The best stuff I see is from fan manufactures, product manufactures like Titus, and AMCA. I wish there was something that put all the pieces together, some have tried but failed.

    I have moments of clarity but it’s not second nature to me yet.

    This is an important part of delivering performance in HVAC system.

    Like the thread

  18. #37
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    The most accurate method of determining CFM is by pitot traverse. The problem with a pitot is finding a place to do one. Most of the time on commercial jobs there will be a good place but on resi jobs very often there is no acceptable place.
    No man can be both ignorant and free.
    Thomas Jefferson

  19. #38
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    Quote Originally Posted by WAYNE3298 View Post
    The most accurate method of determining CFM is by pitot traverse. The problem with a pitot is finding a place to do one. Most of the time on commercial jobs there will be a good place but on resi jobs very often there is no acceptable place.
    Satisfactory locations for traverses are directly related to the design and quality of the installation. We know how to solve these issues. Unfortunately, it's rooted in the pursuit of the almighty buck, but that's a topic for another discussion.

    If some dingleberry outfit thinks it's proper to install some randomly selected sheet metal box on top of an upflow furnace, then run a randomly selected piece of flex off the box out to each room in the structure, then we have an issue with finding an acceptable traverse location. You see this crap quite often, and they continue to get away with it.

  20. #39
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    Quote Originally Posted by lkapigian View Post
    Put the Air back in Air Conditioning ....Delivered air = Delivered Capacity ....i think duct assessment knowledge should be a requirement for all techs ...its the pink elephant in to room and the root of most system failures rivaled by install and maintenance
    This is a huge problem and it didn't just start last year.

    I went through trade school in the 70's. We learned Man J and D but then we learned that we did not need to be that specific with the "D". Sure we learned how to size duct correctly as to the trunk line but then rule of thumb popped it's ugly head up and all of a sudden we were being taught that if the house requires 3T of air that a 6" run would carry 100 CFM so we needed 12 - 6" runs. The extra air from the 6" oversized run to the bathroom would magically find it's way to the bedroom that needs 135 CFM and the living room that needs 215 will be fine with 2 6" runs.

    As I worked in the field building and installing duct work I learned that some fittings moved air better than others and it was better to do things one way rather than the other. I took some classes, relearned some things and went from all 6" runs like my competitors to to having as many 4,5,&7 runs as 6's. The systems started to balance themselves, were quieter and the customers happier. I had a contractor that on the first house we did for him say that he had been building houses for 40 years and that one was the first one that when he was putting the trim down he could not tell a temp difference from on room to another. I learned that the fitting within 3' of the equipment could make a huge difference in airflow through out the system, which just made my replacements better.

    I had one particular installation in a dome house that was problematic. It started with the first installer getting thrown off the job so the owner was already upset. Add to that it was a half hour away, during winter, because of the floor plan getting duct measurements for the house at one time was not possible and I got sick which all added up to things getting tense again on the job to the point the owner was not happy. Toward the end of the job I was getting measurements for fitting to bring a register out the front of the kitchen cabinets and the owner comes in. My first thought was Oh No. Then I noticed he was happy and smiling. The builder was there also. He told me that a friend of the owner does air balancing and that the friend and owner walked through the house just showing his friend his new house. The balancer looked at my duct and was happy. Then he added he is never happy about anything. All of a l sudden the owner could see the why of what I was doing and became a great customer.

    It is stories like these that make me want to do better with my air flow systems and at the same time make me frustrated when I see what others are passing as a good duct system. Homeowners do not understand and why should they, it is not their job to know. It is also hard for them to understand why someone like me says X,Y,Z needs done but no one else does. Even explaining why doesn't hold much water when you have 1 guy saying this and 3 or more not even mentioning it. I have tried to get a few of them to think a bit by saying either they don't know or don't care and if that is the case do you really want them in your house. That didn't work either. It is hard to sell something you can't see and have never experienced.

    Sorry rant over.

  21. #40
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    The "rant" was good BNME8EZ because you gave us an example of how well static regain design can work.
    No man can be both ignorant and free.
    Thomas Jefferson

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