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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