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I would like some info on troubleshooting duct problems with a dual port manometer. thanks Paul S
ASM Mechanical Company
Located in Staten Island NY
Servicing all 5 boroughs of NYC.
[email protected]


  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 1,743

    You can read duct static pressure with a low pressure manometer. If you want to read velocity you need to attach a pito tube to it. I don't know what else you want to know. You could look on Dwyer's site for more information.
  • SpenceSpence Member Posts: 316
    Duct Pressure

    There is an easy way to do this, yet the discussion is too involved for the space we have on this platform. Contact me anytime. In short, you determine your system total pressure by readings directly at the blower inlet and outlet. Then you determine the pressure drops across all external components; evaporator coil, filter, dampers, etc. subtracting the pressure drops from the total pressure leaves you with what the blower "sees" in the duct system. The "early warning system" is if either your supply pressure or return pressure are at .20 IWC or above, or your duct system pressure is at .30 IWC or above.

    Interestingly enough, it is these very principles that will show you that someone who looks at a duct calculator and tells you that an 8x12 duct at .10 IWC moves 480 CFM doesn't have a clue about air flow.
  • TechmanTechman Member Posts: 2,143
    edited December 2013
    I don't have a clue

    about duct work sizing or air flow, so help me please. I am always eager to learn. Are we talking about a "properly sized duct system " from scratch or are we talking about a duct system that is installed and we are trying to figure out if the duct system is adequate for the job or not adequate for the job?
  • SpenceSpence Member Posts: 316

    Paul's original comments were related to an existing system, or at least I assumed as much, since he mentioned troubleshooting. In this case, the procedure is to establish the ESP (external static pressure) from the blower inlet to outlet. This encompasses everything in between; ducts, fittings, registers, grilles, filter, coil, dampers, etc. You then break down the total as to how much pressure is in the SA and the RA so you can see where the problem lies. It's usually easier to relieve pressure on the return side than the supply since the ducts are generally more exposed. Then you determine the pressure drops of the coil and filter and subtract them from the ESP to give you the duct pressure that the blower is trying to overcome. I gave Paul some early warning signals due to the manufacturers rating their blowers at .50 IWC, thinking that amount should be enough pressure for a good duct system and external accessories. However, the national average ESP is a whopping .82! At .90 IWC, the blower is cooked! This means the average duct system is performing at about 57% of its capacity, so you can be a real hero fixing comfort problems with a small effort at overcoming that "I design my ducts at point one" crap.
  • TechmanTechman Member Posts: 2,143
    edited December 2013

    So ,if I understand what you are saying, MOST duct systems are installed wrong, most wrongly installed duct systems are on the verge of burning out the blower motors, and anyone that chooses the very high IWC colume for static press doesn't know what they should know. I'm still lost at the 8x12 duct @.10"wc moving 480cfm is wrong. Must be my goofy Duct -a -lator or me!

    Carrier and RSES talk about using .10 IWC, I guess it is me.

    Installing a duct system properly is one thing, figuring out other peoples method of duct design(6-8 types) is another and making correct adjustments/repairs in yet another.
  • SpenceSpence Member Posts: 316

    It's certainly not you, so don't beat up yourself. It's an industry problem brought on by those lacking understanding of air principles teaching us their lack of understanding. In the 8 x12 duct situation, there are ONLY two situations in which that duct will produce that CFM at that pressure: (1) the duct is exactly the equivalent of 100 feet, and; you have a blower that will PRODUCE .10 IWC of pressure.

    It's the same scenario with that stupid 400 CFM/ton thing. They only tell you that to keep you out of trouble. Reading a duct calculator directly and 400 CFM/ton may work; it's just not the right thing to do. High static pressures nationally prove that. Wonder how 400 CFM came about? If you look at studies based upon years of research, and you look at all of the CFM per ton amounts that those jobs should have had and average them, you come up with 400. Does a customer deserve average? They are certainly getting it because over half of the duct systems nationally are underperforming.

    Believe me, doing it the right way doesn't take that much more time than using some ridiculous formula someone invented based on averages or rules of thumb. And you give customers the comfort and economy they want, need, and flat out deserve.
  • SpenceSpence Member Posts: 316
    Wow Follow-up

    Figuring out design methods is a non-issue, so don't worry about that. This is all about pressure, and the ability of the blower you have to deliver that pressure against the resistances (pressure drops) in the system. This brings out another fallacy of direct-reading a calculator. The calculator gives you a friction rate, which is the total pressure from blower inlet to outlet. You can't confuse this with a pressure drop, which can occur at any point in the system, and your duct calculator doesn't know what pressure drops you have. For example: if you have a blower that delivers 800 CFM at .55 IWC and you have a 100' duct sized accordingly, you're going to get that 800 CFM. If you add a coil with a pressure drop of .25 IWC, your FR has gone down to .30 IWC. With that same duct, how could you possibly get 800 CFM? Yet every day, you have people telling you this doesn't happen, because they've "been doing this for 25-years and never had a problem." OK, I believe you, and I'll bet you put in a Taco 007 in every two-storey house because "it should work."
  • TechmanTechman Member Posts: 2,143
    edited February 2014
    Duct design

    Hi Spence.My most common duct design type is the "Static Regain Method", I've used some "Equal Friction Method" systems. Both are out of Carrier's vast knowledgeable books. What type of duct design do you use? Running across so many different types of "designed duct systems" I find it quiet involved to make "the proper" repair/alteration/change and FIX the complaint. I enjoy the challenge that ducted system's bring.

    I thought that 400cfm/ton had enough btu's in it to boil off the Freon at the proper rate? Rule of thumb , no? How lo/hi of a cfm/ton  ratio can you go? HeatPump cfm/ton is 450, no?

    I like Rules of Thumb's. I am not saying "The Thumb" is the ANSWER to the problem or the #'s in a mathematical calculation.The Thumb gets you in the ballpark, getting to the pitchers mound is something else. I'll keep my "Rules of Thumb"..

    Putting an AC coil in the picture should mean a slightly higher duct press, and a slightly larger duct. If the coil is an addon then you live with what you have.

    Glad I'm not one of those 25yearer's w/o problems, I'm pushing 35 years w/o problems. YEA!!!!. I find problem ,lots of problems, on other people improperly put in duct jobs. NONE on my tinknocker's work!  Sure, there have been a few goofy mistakes, so what!
  • SpenceSpence Member Posts: 316
    Duct Design

    Using either static regain or equal friction is purely what you prefer, and I applaud your use of either. Here is the more important issue: before even thinking about ducts, you have to choose a blower that will provide the correct pressure to get the CFM required to the conditioned spaces, less the pressure drops of all components in the system. CFM is NOT 400 per ton; rather, it is the air needed to satisfy the sensible load gathered from your load calculations. For example, a low sensible load (high latent) requires a colder evaporator (low CFM), while a high sensible (low latent) needs higher air flow and you will have a warmer coil. This can be anywhere from 300 CFM to over 600 CFM per ton. Oddly enough, if you average CFM research from years of studies, the average is 400, so the manufacturers believe if you base air flow on this number it will "work" but it is rarely correct. Once you calculate the load-required CFM, you then pick a blower that will give you that amount at a decent pressure; try to stay within .50 to .70 IWC if you can. Once you back out your pressure drops (dampers, coil, filter, etc.) you have the pressure left for your duct design.
  • TechmanTechman Member Posts: 2,143

    I agree that in different area's of the country the latent/sensible heat loads vary as does the CFM/ton. But, here in L.I. N.Y. and in N.J. the CFM/ton is damn near steady at 400cfm/ton.

    I believe the duct system that you prefer is the ACCA manual D type. Am I correct?
  • SpenceSpence Member Posts: 316
    edited February 2014

    Oh yes, my fellow duct guru. I'm an ACCA blue blood. When I finally got the secret of comfort from them, I have never looked back.

    In Jersey, I have found the housing stock to be reasonably well-constructed, so my latent loads are on the small side. This is great, because I can take a credit from the latent capacity of the machine I have selected and apply it to the sensible load. I love this because I can use smaller units and still neutralize both of my loads. With a light latent my CFM can hit 500+, yet I can still guarantee 75/50 on a design day.
  • TechmanTechman Member Posts: 2,143
    edited February 2014

    I've repaired ductwork on systems installed using the ACCA method,also. Its the "installed "system that I diagnose. Someone designed it, someone was told to install it and someone  did install it,then ,someone else gets to troubleshoot it and fix it. I'm the last someone !

    The cfm/ton,latent /sensible  ratio is a result of the bldg. construction, not as a result of the duct.    Now, a 3t @ 400cfm = 1,200 cfm ; a 2 1/2t @ 500cfm=1,250cfm?
  • SpenceSpence Member Posts: 316
    edited February 2014

    Existing ducts are a different animal, since you weren't in the driver's seat. So now you determine your ESP and subtract individual pressure drops. The pressure left is what the blower "sees" as the distribution system. In most cases this will be sky high because the installer does not understand that you choose a blower FIRST, not that .10 IWC thing. The good thing is the system doesn't care where you relieve pressure. It is easier to do RA mods than SA in most cases.

    CFM/ton is a RESULT of your calculations, NOT the basis for them. If your load is 22,000 BTUH sensible and 30,000 BTUH total, your target CFM is 852. You now have to find a blower that will give you that at hopefully not over .72 IWC when you add in your duct pressure from above and other components. Try to get your target on medium speed so you have room to adjust up or down.
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