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Drain Performance Investigation

(Note:  the following case study is fairly common;  an unqualified person installs a “fix” to the system which actually causes more damage.  This is especially tricky where real estate disclosure laws for water/mold/dirty water damage could impact the value of property or the fiduciary responsibility of the board.  The engineering report must be factual and effective without necessarily implicating the community for liabilities beyond the scope of the analysis.  The wording of this relatively simple report is appropriate. It is essential to hire seasoned professionals for this type of problem).  

[PNW] Condominiums

Edmonds, WA 98020

June 25, 2013

Mechanical Engineers: 

David Coles, PE
Dan Robles, PE


The Home Owner Association for a condominium association in the Pacific Northwest has experienced numerous problems with their domestic water drainage system during the time period from 2008 to the present.  The total cost of plumbing and restoration during this time has exceeded 25,000 dollars.  Failure data collected during this time period has been summarized below.  This community seeks an assessment of system performance

Failure History:

A review of service calls revealed a typical pattern; stacks 01 and 02 cause the majority of the problems for this community.  In later reports it was determined that stack 02 may be the cause of the problems in stack 01 (no mention of such causation was identified prior to 2013).

Drain Chart

(Note that the naming convention for this condominium identifies four above ground floors 1-4 and ground floor is 5.  There are 8 identified drain stacks.  As such, unit 501 sits directly below 101, 201, etc.  The notation: 501 <- 102 means that the failure in 501 was found to be caused by a problem in 102)


Site Review:


As-built plumbing drawings were not provided and presumed to be non-existent.  This is a typical situation for structures of this era (c. 1974).  A visual inspection was performed on June 21, 2013 noting a predominantly cast iron system with a minor amount of sections replaced with ABS plastic.  Many visible sections were joined with no-hub band clamps, which is adequate and has been industry standard since the mid 1970’s.

It is assumed the drainage system was installed to meet the code being enforced at that time when the building was constructed.

The root cause of a problem with this system cannot be immediately identified, however, there are areas of suspicion that may be tested in an effort to discern a cause; or a next test.  An observation was made of a non-typical modification to the drainage system located at the bottom of stack 02.  Since this stack 02 is implicated with a majority of the most expensive problems, corresponding to lower floors, we consider this point first.



A non-standard piping arrangement was identified at the base of the 02 and the 01 drainage stack.  A pair of 2-inch pipes converges to a 3-inch drain.  This work appears to be more recent than the rest of the system, although it is not known when the modifications were made.  A check valve has been introduced possibly for the purpose of blocking flow of drain water from the adjacent pipe.

Layout Drain 1


Check Valve:  This particular check valve appears similar (but not exactly) to Zoeller part number 300151 and also resembles the Little Giant™ brand of water back flow and sewage gas preventer, each of which could be purchased at Home improvement store for about $36.00 dollars.  The valve contains a weighted flapper and may be used in the vertical or horizontal positions. A threshold differential pressure (hydraulic head) would be required to open it.  Minor leakage in the no-hub band coupling suggests that water is sitting up stream from the valve.

 Leakage Observed:

Leakage in Drain

Further, across the garage space from the check valve arrangement was a capped and abandoned 2-inch pipe that appeared to be about the same age as the check valve installation.  This pipe appears to flow into an assumed to be under-utilized drain through the floor.  Little else is known about this modification.

Reroute Drain Pipe

 Test Method:

By making one change at a time and observing the behavior of the system, we are able to determine of the change had a positive, negative, or null impact on the problem.  If the impact is negative, we can revert to the prior condition and test again.  If the impact is positive, we can move on to the next test or recommend further remediation.



For the first test phase, we suggest removing the back flow valve in order to eliminate this component as one of the causes.  However, in order to eliminate the need for the back flow prevention, one of the two 2-inch lines that are currently connected to the 3-inch line may be connected to the abandoned 2-inch line at the far end of the garage space.

It does not make a difference which of the 2-inch pipes is removed from the 3- inch drain, it is however important that each pipe drains independent of the other.  It is likely that the plumber would connect the pipe that currently does not have a check valve to the abandoned 2-inch across the room.  An equivalent length of through pipe could then replace the check valve.  The Y-pipe should be capped.

It would be prudent to inspect, and possibly run a maintenance check on this abandoned line to make certain that it is not blocked or constrained in any way.

This relatively simple repair appears to be at least as safe and sanitary to the occupants as the existing configuration.   Since the frequency of failures at [PNW] Condominium is less than 3 per year, the suggested observation time should be approximately 9 months without incident.


Behaviors and Community Notification

Other conditions that can degrade system performance may include poor maceration by kitchen disposal, use of powdered detergents, hair, coffee grounds, grease, and deferred maintenance of the overall system.  It would be prudent to investigate engaging the community in a system performance awareness program.

It is further recommended that [PNW] institute a proactive maintenance schedule of all other stacks, perhaps a yearly brush and flush by a qualified plumbing contractor.