Tom Székely, P.E., LEED AP

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November 1, 2005

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The Shareholder/Unit Owner Board Member Survival Manual, or, Engineering for Dummies – Part 1


Early in the life of this publication (which has always been published under the name of Explanations & Examples), it was referred to by one of my Contractor friends as Engineering for Dummies, and in keeping with the philosophy first expounded by that series of books, this is to commence a series of newsletters based upon the same thinking.   That is, there is no reason stuff having to do with how the systems in multi-tenant/multifunction buildings work has to be explained in terms understandable only to those involved in their design and construction.


The series will discuss all systems except structural in some depth, but will also discuss structural to the extent a building’s structure impacts upon its MEP/FP (Mechanical Electrical Plumbing/Fire Protection) systems.  With that, let’s get right on into it.


To start with structural issues, buildings in any urban environment are divided into two major categories thus:  While townhouses and other buildings no wider than a standard building lot (about 25 feet) nor taller than six stories might have interior walls which serve some structural purpose, larger buildings are comprised by structural skeletons, interior partitions, and exterior walls, and the notion of a “bearing wall” does not apply in such buildings. 


This leads to the immediate realization that one could knock out every interior wall within (or between) any dwelling unit(s) in a typical apartment building, without any structural effect on the apartments above and below.  Although this could be extended to knocking out the walls between dwelling units and public corridors, fire rating issues preclude such.  Furthermore, the interstitial space between floors in prewar buildings usually has a cinder fill “deefening” (deafening, as in noise reduction) layer between the floor and a supporting tile arch above the finished ceiling of the level below.  This second thing leads to the realization that the relocation of kitchens and bathrooms is not necessarily constrained by the pitch required to be applied to waste lines which need to be connected to existing risers, which in turn leads to the understanding that if anything in an apartment building pretty much has to stay where it is, it’s the risers.


This is not to say that it’s a good idea to allow a bathroom or kitchen to be located above a living room or bedroom or other such “non-wet” room.  On the other hand, there are ways to allow for such arrangements at minimal risk to rooms below, which is to say that rejecting certain arrangements out of hand may be an overreaction.


OK, since all this has been related to plumbing more than electrical or mechanical, I may as well continue with plumbing.


Gotcha #1: While penthouses may be cool, the Federal Clean Water Act of 1974 threw a monkey wrench into the works.


Gotcha #2: Remedies for Gotcha #1 have to be carefully thought through to be effective.


Gotcha #3: Existing Building Code requirements regarding domestic hot water availability are not infrequently discovered to have been ignored or otherwise not to be operative when new work is proposed which connects to the existing domestic hot water system.


Gotcha #1 has to do with water pressure available to upper level dwelling units, which has to do with water seeking its own level.  That is, water pressure is directly related to how much water weighs, which has to do with how much water is above any given outlet.  Remember the illustration in elementary school science textbooks with holes in the side of a can full of water?


The higher the column of water above a hole, the more of it presses down, forcing the water out of the hole, which is why water coming out of the hole at the bottom is jets away from the can nearly horizontally, while that coming out of the hole near the top of the can only dribbles out. 


So what does all this have to do with penthouses?  To answer a question with a question, why do high rise buildings have water tanks on their roofs?  Right.  The water tank is the can, applying water pressure to outlets below.  Why do you not see roof tanks on most townhouses?  Because water seeks its own level, and the pressure available from the upstate reservoir system is a function of the elevation of those reservoirs minus the pressure losses in the aqueducts carrying water to the City, which gives us a net water level (pressure) of about six stories,  with a residual pressure adequate to force water out of a tap. 


It turns out that every foot of water depth weighs enough to generate a pressure of .433 pounds per square inch (psi), so, if the top of the water level in a roof tank is 60 feet above a roof, the water pressure available at the roof is 60 x .433, or about 26 psi.  In any building with a roof tank, water is pumped from the water service entrance in the basement or cellar, up to the tank to ensure water pressure above the sixth story, but this leads to a complication.


In a 50-story building, figuring about a 10 foot floor-to-floor height, the roof tank water level is 560 feet above grade. With street pressure not being able to get water past the sixth floor, the roof tank that supplies all floors above the sixth is 490 feet above the seventh floor, which translates to a water pressure of about 212 psi.  This is enough pressure to blow valve components to pieces and throw debris around like missiles. This is why engineers select, and building codes require, pressure-reducing valves in water supply risers.


So what has the Clean Water Act got to do with all this? Well, one of the Act’s spin-offs was low flush toilets, now down to 1.6 gallons per flush.  These things work by using water pressure to drive  this small amount of water around the inside face of the bowl at high velocity.  This high water pressure is generated by hardware within the toilet, and the “gotcha” is that such hardware requires a minimum of 25 psi at the supply connection to the toilet in order to work properly.  Roof water tanks can be drawn down by 10 feet or more before the basement pump control systems have the pumps start to refill the tanks.  This 10 feet translates to 4.33 psi, which when subtracted from the 26 psi available from a full roof tank leaves 21.67 psi at a penthouse toilet supply connection, not enough for the toilet to operate properly. 




We can always put in a local pressure booster system for the unit(s) so affected.  How complicated can it be? 


Well, actually, it can be complicated enough to lead to Gotcha #2, which will be discussed in the next issue.  As for Gotcha #3, why should it take so long to get hot water out of faucets some distance away from the hot water heating source?


Building codes say it shouldn’t. Again, stay tuned.

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