Seven Basic Concepts Of Plumbing Design

by

H. Kent Craig
© 1999




Within the premise of wherever you have human beings occupying a habitable structure you must in some way furnish them potable water for drinking and sanitary needs and then make provisions of evacuating wastewater once contaminated by human touch from the building, plumbing design concepts have remained untouched since Roman times, and won't change appreciably until physics or human beings do.


Concept #1: Before putting a single thought to paper, locate the appropriate manhole or other sewer outfall on the site plan.


For those of you who are experienced designers, this might seem to be a too obvious statement, but this a primer on plumbing design basics. I've caught more than one really dumb error from senior plumbing designers who tried to have the outfall invert inside the manhole a couple of feet deeper than the best-case manhole floor scenario. They didn't bother to properly calculate the needed fall from the maximum point away on the underground horizontal lines to the bottom manhole ring.


While sewer lines between manholes are typically run nearly flat, often with a 1'-100' pitch, you can't do that on runs from inside the building's footprint to the manhole. Wherever possible, have the main horizontal run(s) under the slab pitched at 1/8" per foot fall (this may or may not be a code requirement in your area), and absolutely not less than 1/16" per foot fall. Any pitch less than 1/16"/ft fall is asking for continual maintenance headaches after the C/O has been issued, if it works at all because of creating multiple traps in the piping runs after the inevitable settling of the backfill under the groundslab.


Concept #2: As much as possible, design around central mechanical cores.


I've had some senior designers and engineers argue with me that this design principle is unnecessary at best and increases costs at worst. As long as the architect allows for mechanical chases wide enough to fit the closet carriers and such in, those arguments are specious. Basic, common sense will tell you that by grouping all the public bathrooms together, you save the owner money by minimizing initial material costs in installation and save further money by maximizing maintenance productivity after the building is occupied.


Anytime you can shorten horizontal runs to the vertical soil and waste stacks, you design a better system that's cheaper to install, pipe hangers being a substantial part of any system's cost. Vertical stacks using gravity to accelerate system flow are less likely if not never likely to stop up and cause damage than are long wastepiping runs. Using one 2-1/2" valve as a main water cutoff for one central bathgroup is cheaper than two 1-1/2" cutoffs for two separate bathgroups, as one 40 gallon water heater would be cheaper than two 10 gallon ones, or one 52 gallon one with a circulating pump.


Concept #3: Gravity piping always takes precedent in the coordinated space above any ceiling, but don't be a jerk about it.


In the boilerplate in any set of standard preconstruction specifications is always the proviso that "gravity piping in the space between the dropped ceiling and floor above takes precedence over all other systems". Common sense dictates this, of course, since forced warm and cool air and forced hot electricity systems can be more worked around a gravity sewer line than vice' versa, but don't abuse the privilege.


If you design around central mechanical cores as much as possible, this won't be much of an issue. But on every job you'll have orphan fixtures such as water coolers or washing machine hook-up boxes or an executive's bar sink that will need to have a long horizontal drain run to a vertical stack some distance away. If the HVAC and electrical drawings have been done previously, or at least the main AHU's or rooftop units and central high-pressure duct columns located, then do some mental coordination drawings and try to save the owner a little bit of money by being nice and not having those runs run willy-nilly across multiple horizontal branches of the other trades, when and wherever possible.


Concept #4: Always remember to be productive to your company and design plans that will save the owner money too, but ultimately, your first responsibility is the safety and health of the public which will occupy the building you're designing the system for.


Code requirements or not, this principle involves ultimately saving the owner his or her shirt by designing and specifying pieces and parts which protect the health of the general public. Such things as specifying vacuum breakers on outside faucets or wall hydrants with built-in vacuum breakers, whether the code requires them or not. Anti-scald tempering valves on the hot water supply to bathgroup lavatories, to prevent a "Macdonald's"-type lawsuit. Spec'ing a cheap, 1/16th horsepower circulating pump on the hot water supply, especially if the h/w/h is not in the same room as the fixtures it supplies, and especially if the building is a restaurant where all employees are supposed to wash their hands after each trip to the bathroom but sometimes won't if they wait forever for hot water to come out of the faucet. Mandating trap primers for all floor drains in janitor's closets and similar places where trap seal evaporation and negligible water seal replenishment via area use might be a problem. The list is endless; think ahead, think worst-case scenario, think possible lawsuit against your firm by the owner if they're sued by someone because of a design omission.


Concept #5: A design/build narrative is a license to save your client money, not bust them with blue-sky wishlists.


This principle is meant more for those who work as designers for design/build contractors, not A/E firms.


The main reason any client goes the design/build route is because they ultimately want to save money. They want to save money by cutting out as many of the percentage A/E fees as possible, they want the job to be as self-managed as possible, and they want the highest quality product in terms of system material and fixtures for the lowest number of dollars possible. In the end, a design/build job is simply a process of never-ending value-engineering.


Take advantage of the more intimate relationship your firm will have with the owner's representatives. Explain why spending the extra money for Fixture A will enhance the overall image of the firm and intrinsic value of the building over using Fixture B. Chalk-talk the salability/leaseability/rentability probabilities and increased square-foot margins of spaces when extra halfbaths are added adjacent to every executive office suite. Speak common sense when you mention how an irrigation system could potentially save the thousands of dollars' worth of new plantings out in the green areas.


Concept #6: Except for a handful of specific exceptions, never show any DWV (drain, waste, vent) piping less than 2" or any water supply piping less than 1/2" on any job.


I've seen countless drawings that specify no-hub cast iron pipe only for all DWV above slab, then go off on some common sense-less tangent and show 1-1/2" or 1-1/4" vent piping or horizontal branch arms to fixtures. They don't make 1-1/4" CI pipe, and 1-1/2" CI pipe and fittings, when you can find them, are more expensive than 2". In the specifications, there's always boilerplate that states that the specs are a minimum, not maximum, standard of performance and design. So what do you think is actually installed in the field 99.9999% of the time? Yep, 2" pipe and fittings.


I'm not saying you won't run across a highly unusual design circumstance where designing and mandating a minimally-sized vent is appropriate, such as a lone floor drain which is way-away from any easy source of trap seal replenishment via use and which can't be trap-primered without great aggravation because of it being in an unheated space. In an extraordinarily rare instance like that, specifying as small a vent cross-section as possible that will still work as a vent in order to minimize trap evaporation, is appropriate. Vents and waste arms in bathgroups and the like, on the other hand, will always end up being installed as 2" anyway, so go ahead and design them as such. Even if you don't show and tell the actual fitting, if a plumber sees on the fixture table that a lavatory uses an 1-1/4" waste and it goes into a 2" vertical arm, they'll know to use a 2x~1-1/2" tap tee with a 1-1/2"x1-1/4" desansco at the fitting.


Similarly, don't calculate flows on water supply piping down so precisely that you show 3/8" or smaller piping. Again, it's a matter of economics and common sense. 3/8" copper pipe costs more than 1/2", let alone the 3/8"cxWhatever fittings, and the plumber in the field will install 1/2" anyway. 1/4" soft copper for ice makers or similar is different, that's cheaper than 1/2", but 3/8" water supply piping for a fixture as a mandate should never be so.


Concept #7: Take pity on the poor SOB's that have to install what you design and service it afterwards.


Within reason, make life easier for those who actually have to make the crap you design work in the real world.


Let me mention a small handful out of an encyclopedia of possible examples. Have cuttoff valves from the main runs to each and every bathgroup and/or orphan fixture, not just for the entire main runs on each floor or on each area; yes, it might cost the owner .034% more initially, but it will save them 1.75% in increased maintenance efficiency over the life of the building. Show c/o tees and access doors at the beginning of each horizontal run; don't use removable fixture traps as excuses for DWV access. Keep in mind that dropped-ceiling tiles are perfectly acceptable access panels, but try to to have the combination cleanouts at least 24"-36" away from anything else so the maintenance person can have access to them with a c/o snake. Don't ever specify hard tubing supplies just for aesthetics, flexible brass ones are always just as pretty. Poured hot lead-and-oakum CI pipe and fittings have their place, in a museum somewhere. Don't stick top-accessible, non-blow-off grease interceptors in a corner under a potsink, where some poor person will have to stand on their head to dip the accumulated grease out. Every supply line to a wall hydrant should have its own easily accessible cutoff valve inside; a stop-waste cutoff valve outside in a valve box is even better. Depending on your local code, use of a French Drain to take care of an isolated, little-used fixture like a water cooler in a warehouse space might be acceptable, and will save a decent pocketful of change over running a 100'-200' flat run back to the main building sewer. Just because janitors use some pretty strong chemicals in cleaning doesn't mean that spec'ing acid-waste piping from the mop sinks to the building sewer is necessary; CI pipe can resist virtually any non-lab-strength chemical corrosive thrown at it. Acid-waste neutralization tanks without a constant, ever-flowing stream of dilution water to back them up are a waste of time and money and won't work without constant maintenance anyway. Drip-legs on natural gas lines that you can't reach and clean out from time to time might comply with code but become less than useless over time.


More than a concept of mere common sense and courtesy to those who will have to deal with your legacy, thinking and planning ahead for the future is more of a principle of taking moments of your time now, so that others won't waste days of their time later.


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