Why Do Gray Water Tanks Have Small Drain Lines?

[00Buck]

I'm on my 6th RV.  All of them, save one, have had a 1 1/2 inch drain line coming from the gray water tank.  The exception was my last one, a 05 Winnebago Vista.  It had a wonderful 3 inch drain.  Woooshh and done!!  The 1 1/2 inch lines drive me nuts, not to mention the poor soles waiting behind me.  Is there are reason for using 1 1/2 inch pipe instead of 3?  I know the smaller pipe is cheaper but there's not that much of it.  I hope it's not the handiwork of some clever MBA!  ???

 

[muskoka guy]

the plumbing in a tt or mh is no different than standard house plumbing. the only thing that requires a 3 inch line is the toilet and possibly the main vent stack.

 

[Frizlefrak]

How do I put this tactfully...???  ;D

The contents of the gray tank generally require less pipe diameter for egress than those of the black tank. 

 

[Lou Schneider]

The grey tank only has a 1 1/2" vent line.  Put in a 3" drain line and you'll likely suck the water out of the P-traps as the smaller roof vent can't keep up with the outflow.

 

[00Buck]

Thanks Lou!  Although I didn't have the p-trap issue with the 3" gray water drain on Vista, your explanation hit the "Ah Ha!" button with me.  Makes sense.  I'll still be aggravated while waiting for the gray tank to drain, but now I'll know why. 

PS  I know the difference between sole and soul.  Homonyms kick my but when I'm using a keyboard.  OK . . . I know it's butt.

 

[Gary RV_Wizard]

The national plumbing code calls for 1.5" waste drain lines on sinks, showers, etc.  RV manufacturers & the RVIA (who sets the RV construction standards) seem to think that means  the gray tank drain also has to be 1.5". I suppose it is debatable whether the gray tank is like a sink or in fact is part of the sewer system, which is a standardized at 3" to the main drain and 4" to a septic or sewer line.

In any case, the gray tank drain is 1.5" because the RVIA's RV standards (ANSI 119.2) say it is. 

As has been noted, if it were larger the gray vent would also have to be enlarged to avoid sucking the P-traps dry. Oddly, the black tank vent is also 1.5", but RV black water systems with gravity feed or macerator type toilets don't have P-traps.

 

[Clay L]

My Sightseer has a three inch gray water drain pipe and it works fine.

The output pipe can be tilted up and down which can be handy - as when connected using a macerator directly to it.

 

[Gary RV_Wizard]

Usually there is a 1.5" gray tank drain emptying into a 3" main waste line. Are you saying you have a 3" line all the way from the gray tank itself to the 3" valve and external connector?

 

[tvman44]

The gray tanks should not have any solids in them and therefore a larger line is not needed.  Just like in the S&B.

 

[00Buck]

Usually there is a 1.5" gray tank drain emptying into a 3" main waste line. Are you saying you have a 3" line all the way from the gray tank itself to the 3" valve and external connector?

On my 05 Vista it was 3" all the way from the tank to the cap.  I was in and out of the dump stations in a flash!

 

[BruceinFL]

Bottom line...it's cheaper to use 1.5 inch lines.

 

[Ken & Sheila]

Our 2009 Camelot also has a 3" gray tank dump.

ken'

 

[Clay L]

Usually there is a 1.5" gray tank drain emptying into a 3" main waste line. Are you saying you have a 3" line all the way from the gray tank itself to the 3" valve and external connector?

Yep, the line from the gray tank is a 3 inch line like the one from thge blank tank. They both go  into the Y  output with the bayonet connector. The Y  swivels up and down.

 

[Mopar1973Man]

Yea... Even my Jayco with dual grey tanks are both 1.5" pipe on the grey's and 3.0" on the black. But I can see the bonus of having 3" on the both but no biggy only having 1.5" on the grey water pipes. Usually about 5-10 minute job from start to end for me.

 

[Jammer]

The main reason for the smaller pipe is that the valves and fittings are more compact, which, at least on some RVs, allows room for a somewhat larger greywater tank.  A few rigs have 2" greywater drain lines.

I've never found the drain times to be objectionably long.  The time spent waiting for the grey tank to drain is a small fraction of the total time at the dump station.  If it's taking more than a minute or two you may have a clog somewhere.

 

[Quillback 424]

Agree with Clay L's comment. My Sightseer also has a 3" gray line from the tank to the external connection, just like the black. As my salesman liked to say, "it's pretty difficult to clean a 3" line with a
1 1/2" line."