Sample question about psi

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    Brandon Estes (Edited )

    1) It's Kaplan - you should already be skeptical.  But more importantly, 2) it assumes a bottom-feed (up-feed) system instead of a top-feed (down-feed or gravity-feed) system, although there is no mention of that in the question.  You have to convert the height of the building to psi which is seems like you already know how to do.  This is obviously an easy task/calculation so count on NCRAB to pull some type of trickery and overly-complicate the question.  Also, a great tid-bit to remember is that 1' = 0.433 psi so keep your units in check.  2.3' = 1 psi whereas 1' = 0.433 psi. 

    Some systems provide pressure from the public water main such as residential and low-rise commercial; other systems use pumps separate from the public main to pump water to a rooftop storage tank where the weight of the water/gravity provides the pressure.  (Other systems simply use a pump in the basement to provide pressure beyond what the municipality provides, but I have not encountered that in my studies or on PPD/PDD.)

    From my experience and hearing from others, Kaplan (now Brightwood) isn't the best material from which to study.  However, the typos, grammatical errors, and  questions that you will never encounter in practice offered by Kaplan are perfect preparation for the ARE exams questions.  Be sure to read about up-feed and down-feed systems.  Also be sure to know your stack diagrams (vent stack and stack vent are different - stack vents penetrate the roof) and difference between building drain, lateral, and public sewer.  Good luck.

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    Michael Ermann

    This concept trips everyone up. Know that while every 2.3 higher you store the water in a down-feed system (like a tank on the roof) creates one extra psi at the fixture (sounds like this part is intuitive to you). . . it is also true that every extra psi in an up-feed system (like from an underground municipal system) is able to push the column of water 2.3 extra feet higher into the building (sounds like this part is less intuitive). So, in this case, your fixture is 120 feet in the air, so you need 120/2.3=52.2 psi to push the water to the height of the fixture. But the fixture itself requires 15 psi to work properly (just pushing the water up to that height might not allow the tank to fill properly; we need extra pressure for that). So the line into the building requires the following pressure: 52.2 (to lift the water 120 feet) plus 15 (to operate the fixture) equals 67.2psi.—Michael Ermann, Amber Book creator

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    Robin Tannenbaum

    Thank you both!  I agree that the question is worded poorly and implies it wants an answer for the bottom floor and in fact wants one for the top.  I have a plethora of 4.0 materials that I like to intersperse with the 5.0 primary sources for studying but I do need to be careful.

    Brandon - Luckily I think that NCARB gives at least part of that formula so I am set to do the inverse math. I'm all over the vents and get the difference between the big two but once we get into branch v. circuit v. back v. loop it all gets a little hazy.  Also on tonight's plumbing focus - valves!

    Michael - I did your videos last year for PPD (and my first attempt at PDD) - they were awesome. Just found two practice questions on this topic in the book.  Got 'em both!

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    Nicolle Gitlin

    Michael Ermann in your Amber Book for the topic of Water and Wastewater question 37 says the highest fixture requires a minimum of 12 psi to operate. In the solution you subtract 12 psi, I would've thought to add 12 psi.

    Can you please clarify? Thanks!

    Ps all your videos and questions are extremely helpful thank you!!

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    Michael Ermann

    Every time you go out to lunch you lose money from your bank account. Lets say you have $50 coming into your bank account this month from your salary.  For every time you go out to lunch, your bank account balance drops a bit. How often can you go out to a restaurant for lunch and still have enough at the end of the month to pay your $12 rent, which is due on the 31st? You can go out to lunch enough to lose $38 so you still have the $12 left for the rent. (50 minus 12 equals 38).

    Likewise. . .

    Every foot you move up in the building, you lose water pressure in the pipe. Let's say you have 50 psi coming into the building. For every foot of height in the building you move up, the water pressure drops a bit. How high can you go and still have enough water pressure to operate a fixture that requires that there still be 12psi in the pipe at the fixture on the umpteenth floor? You can go up high enough feet to lose 38 psi so you still have the 12 left for the fixture. (50 minus 12 equals 38).--Michael Ermann, Amber Book creator

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    Nicolle Gitlin (Edited )

    Thank you Michael for getting back to me! Can I please ask you to explain why in the above Kaplan question you would add the 15 psi instead of subtract? It seems to me that the two questions are asking the same thing. What am I missing?

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    Michael Ermann

    The Kaplan question is asking you how much water pressure you need in the building's feed to make the system work, so you must ADD together what is lost as you move up the building, PLUS what pressure is required to be left in the pipe when you get to the fixture at some upper floor. The Amber Book problem, by contrast, is asking you how high up in the air you can go with a given building feed pressure, so you need to SUBTRACT what you have to keep at the end to see what you can lose as you move up (you lose pressure as you move up the column of pipe).

    Keeping with the analogy, Kaplan is asking you how much money you need to make this month in salary to go out to lunch (add the cost of lunch to the price of rent) and the other is asking you how much money you'll have available for lunch this month (subtract the rent from your monthly salary).--Michael Ermann, Amber Book creator.

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    Nicolle Gitlin

    Totally understand! Thank you so much again Michael! 

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    Nicolle Gitlin

    Someone just shared with me another practice question that instead of multiplying stories x floor to floor heights, it would be floor plates x floor to floor heights.

    So, going back to the Kaplan question - instead of doing 10 stories x 12' they say you do 9 floor plates x 12' feet.

    I need confirmation. That's not right, right?

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    Gang Chen

    Nicole,

    Good catch.

    If you draw a simple diagram you will see, it should be 9x12=108, not 10x12=120.

    The rest of the answers should be adjusted accordingly.

    Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)

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    Nicolle Gitlin (Edited )

    Gang Chen I still think it is 10 x 12 though, if you count the bottom floor plate? 

    This should be so simple but I am getting so confused. *I understand for >6 stories you should use downfeed, but it would be the same opposite - this is just for visual. Thanks in advance!!

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    Michael Ermann

    Yes, Nicolle, you're correct, 10x12 is a better answer than 9x12.

    9x12 gets you the correct MINIMUM water pressure were the toilet in the floor

    like this, but the height of the fixture, not the height of the floor determines the minimum water pressure so maybe 9.5 floors would be a best MINIMUM answer. . . but rounding up is a better option.

    It’s important to note that the 15psi would be the minimum required pressure (the maximum would likely be 80psi) so anything above 15psi and below 80psi should work. And even if we got the highest fixture to 15psi, remember that the 9th, 8th, 7th, and other floors will have higher psi values in their toilets because they are lower in elevation. The game for an upfeed system is: find the highest fixture(s) and make sure you have met the minimum psi criteria for them; then find the lowest fixtures and make sure you are under the maximum psi for those. Pressure-reducing valves, downfeed systems, and pumps can help throttle or amplify the psi if needed.

    Email me at ermann@amber-book.com and I’ll give you my phone number. We can talk this out if you’d like.  

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    Gang Chen (Edited )

    This is one of the question that demonstrates the need for beta questions in ARE exams. The intent of the exam writer was probably for the exam takers to use 10x12, but how the exam takers read the question is another story. The bottom of a toilet water tank is probably 2 feet max above the floor, when compared with the 12 feet floor height, it is a very small number.

    2/12=0.167. So, the correct answer is not 10x12, but may be 9.167x12 if you want to be precise.

    That is another reason I always advise people to focus on the fundamentals because every ARE exam question has to be beta test before they can be counted..

    Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)

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    Gang Chen

    In the real ARE exams, if NCARB wants you to use top feed, it would have a criteria in the exam question stating use top feed above 6 floors or something like that.

    Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)

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