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Graywater Collection Plumbing and Stub Outs

Homeowners: How to construct them correctly
Regulators: How to inspect them


Orientation to graywater collection plumbing & stub outs

In states which follow the Arizona greywater regulatory model the collection plumbing may be the only part of the system which needs inspection, however there isn't much guidance on how this should be done.

Stubbing out the graywater collection part of the system without the graywater distribution part of system has several advantages:

Since they are a subset of builder's considerations, we'll look at graywater collection/ stub out requirements from the inspector's viewpoint first, then we'll look at the additional considerations for builders.

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For Regulators: How to inspect graywater collection plumbing/ stub outs

The AZ graywater rules has only this to say about collection plumbing:

6. The gray water system is constructed so that if blockage, plugging, or backup of the system occurs, gray water can be directed into the sewage collection system or onsite wastewater treatment and disposal system, as applicable.

In the California Plumbing Code there is only one sentence on stub outs, reproduced here in context (it's in bold at the end):

G-5 Inspection and Testing.
(a) Inspection
1. All applicable provisions of this Appendix and of Section 103.5 of the UPC shall be complied with.
2. System components shall be properly identified as to manufacturer.
3. Surge tanks shall be installed on dry, level, well-compacted soil if in a drywell, or on a level, three inch concrete slab or equivalent, if above ground.
4. Surge tanks shall be anchored against overturning.
5. If the irrigation design is predicated on soil tests, the irrigation field shall be installed at the same location and depth as the tested area.
6. Installation shall conform with the equipment and installation methods identified in the approved plans.
7. Graywater stub-out plumbing may be allowed for future connection prior to the installation of irrigation lines and landscaping. Stub-out shall be permanently marked "GRAYWATER STUB-OUT, DANGER - UNSAFE WATER."

Not much to go on, is it? (the UPC is totally silent on graywater stub outs).

Suggested checklist for inspection of collection plumbing/ stub outs to meet the California Plumbing Code:

Stub out is permanently marked “GRAYWATER STUB-OUT, DANGER - UNSAFE WATER" as per appendix G, section G-5 (a)-7 (above)

Required elsewhere in plumbing codes:

Pipes slope 1/4" per foot minimum in all flow directions entering and leaving the diversion (the only way to do this with currently available 3 way valves is to tweak the pipes in the hubs, which do not provide for slope).

Cleanouts are present every 270° of aggregate bend.

Not mentioned in code but should be required in inspection:

Diversion is downstream from vents and traps, so they will perform their function in either graywater or septic/sewer modes.

In the case of a stub out terminating at a valve, valve is in sewer position and stub out pipe to future graywater distribution system is securely capped.

Other considerations

Three way valves which have a removable cover plate can function as cleanouts in all three directions.

Three way valves for pools, the most common type, can work equally well in all orientations, however check that the builder has positioned the movable "inlet" designation on the valve cover to the port which is receiving the inlet water.

Not to belabor the obvious, but confirm that no toilet is connected upstream of the graywater diversion (downstream is OK, upstream connection through vent pipe connected 12" above spill point of highest fixture is allowed).


Figure: Example of Collection Plumbing for House with Crawlspace


Figure: Example of Dual Waste Collection Plumbing for New Slab

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For Builders: How to design & construct collection plumbing and graywater stub outs

There is much more on collection plumbing in Create an Oasis with Grey Water and the  Builder's GW Guide-book 

One outlet or many?

Your first critical decision is to (A) bring all the graywater together to one point, then divert it through one valve and distribute it from there, or (B) divert graywater at multiple points with multiple valves and start with it already somewhat distributed. Once you plumb it one way or the other, you are committed. All the considerations for making this decision are covered in Create an Oasis with Grey Water.

Outlet(s) as high as possible

The next critical design issue is to get the outlet(s) from the house as high as possible. While this can involve extra work, the value of having the outlets high can't be stressed enough. This is also covered in Create an Oasis with Grey Water.

Valve handle(s) accessible

Ideally the position of a diversion valve can be seen while using or on the way to use a fixture, its position can be changed while using the fixture or without going far from the fixture, and it's position can be locked against meddling by children and curious guests. Sometimes this can be achieved with valve handle extensions, but often there isn't any alternative to slithering through a crawl space to change the position.

Valves serviceable

I suggest installing the valves with no-hub connectors (which use removable clamps instead of glue) so that you can remove the expensive valve for service or replacement without sawing up any pipes. If there is no space (e.g., street angles plugging right into the valve), you can use silicone sealer in place of ABS glue. This makes a watertight seal, but the fitting can easily be removed.

Valve sources

I prefer using Jandy-type three way diverter valves, not least because the inlet can be moved to any of the three ports. Ortega and other three way valves also work fine. Using a tee or wye with two ball valves is commonly done, but less advisable. If the valves are seldom operated, crud can accumulate in the short dead end before the shut valve and congeal the passage shut. If you must do two ball valves, provide access for cleaning out the dead ends.

Professional installation

I counsel greywater system do-it-yourselfers to hire a plumber to either do the collection plumbing, or check the design and your installation. There are several reasons for this.

  1. Apart from the special considerations above, plumbers already know how to do collection plumbing, so you might as well take advantage of their expertise (they don't have the right skill set for distribution plumbing and will generally do more harm than good).
  2. Much drain/ waste/ vent plumbing behavior is counterintuitive and hard to anticipate from common sense alone. Furthermore, there are real health issues with cross-connections and waste flowing in unexpected directions, more so indoors than out.
  3. Collection plumbing is less apt to be changed in the future and is a longer-term investment than distribution plumbing.
  4. Seeing an impeccable installation of the collection plumbing (which they can understand) will reassure inspectors that the rest of your system (which they probably won't understand) is well-thought out and executed. The converse is even more true; forget passing inspection if you've done a schlock job of the collection plumbing.

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