Gray Water Drain Slows & Stops Midway

[baldwinh]

As I drain our black tank all drains nicely and the fresh water flush works good to rinse everything out. Then when I pull the gray tank drain, there is a big flush of water out for a few seconds and then all stops. I can see at the clear collar up at the tank outlet shows no water flow (it is full of water). I can hear water trickling at the elbow into the camp site drain. After a few more seconds, the water starts and rushes down and drains completely.

 

I'm guessing that the drain gets air locked and it's taking some time for the water level in the hose to go down enough in order to allow air back up into the hose so that the rest of the water can come out. Isn't there an air vent on the roof of the coach that should inhibit this situation? Is this telling me that my air vent is blocked? This seems highly unlikely that a large diameter vent would be so totally blocked so as to not allow ANY air flow. Am I missing something else?

[chirakawa]

Even if it needed to burp, it would still continue to drain after a brief hesitation. Take a gallon plastic jug full of water and turn it upside down. The water would gush out, then pause to catch some air, then start again.

 

It sounds like maybe you have some object restricting the outlet on the gray tank. Years ago I read about one particular company where the employees who made the cutout for the drain actually left the cutout piece in the tank. It might go for months or years before it blocked off the drain, but eventually did. It caused the same problem you are having now and lot's of unhappy owners.

 

With respect to the overhead vent, it could be clogged with something either at the top (dirt dauber nest) or where it enters the tank (grease and such). However, the tank should still continue to drain, albeit more slowly.

[Kirk W]

It is possible that your plumbing vent line which normally goes up through the roof is plugged up. While I have not experienced this, I have heard of those becoming blocked by bugs nesting in them or such as that. You can tell by going up on the roof and removing the cap from that line to gain access to the pipe. If it is clear you should be able to trickle water down through it, into the gray water system. From your description, it does seem to not get air at first, then suddenly release to drain the tank completely. If that is true it might be that because of a plugged vent line the water draws a vacuum in the tank until the vacuum is able to pull the water out of your drain traps to let air in. If that is happening, I would also wonder if the plumbing vents that is located up under the sinks are plugged or somehow blocked. That could also play a part in this problem. If you are not sure what the plumbing vent is, you can see a photo of it here, on our website.

 

It definitely sounds like a venting problem to me. You may want to try removing one of the "P" traps under the sink before you dump the tank just to see if that changes anything. Removing it should provide ample air to prevent the vacuum problem, if that is the cause.

[Al F]

I also could be that with a plugged roof vent, the vacuum created is pulling the water out of one of the P traps and there by letting the air into the tank to allow it to drain.

[MrSeas]

Does this happen every time you drain the tank? At different parks?

[LFDR3116]

Could it be the plug from drilling the hole in the tank that is moving into the outgoing stream and blocking the valve? Never had this, but have read about it over and over on this and other forums.

[cpaulsen]

Friends of ours had that problem. Turned out there was a lot of soap buildup in the tank. They were using the cheapest dish soap you could buy which did not dissolve right. Took to an rv repair place where they cleaned it out and told to use Dawn from now on.

[docj]

Friends of ours had that problem. Turned out there was a lot of soap buildup in the tank. They were using the cheapest dish soap you could buy which did not dissolve right. Took to an rv repair place where they cleaned it out and told to use Dawn from now on.

 

This problem can be caused by soap buildup or laundry lint or calcified material (from hard water). A couple of years ago we had our gray tank cleaned by AllProWaterFlow which was able to remove lots of calcified materials to resolve a similar problem.

 

A few months ago the problem seemed to reoccur but this time one of the AllPro vendors suggested I get one of these to flush the tank. http://smile.amazon.com/Valterra-F02-4350-Reverse-Flush-Valve/dp/B008O9DV78?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s01 It's easy to use and provides more than enough force to dislodge the blockage so it can then drain out.

[baldwinh]

WOW! Thanks so much for all the input you all ... I'll borrow a ladder from one of the other campers here and take a look at the vent first (and run some water down). If that doesn't cure it, I'll pull one of the sink traps to allow air into the drain line to see if that changes anything. If it does, then I know it's an air trap somewhere. If things don't change, then I'll look at the tank and drain themselves to see about blockage from sludge or the cut plastic. It really acts like an air trap, so I'm hoping I find a nest of some sort when I get up top.

Thanks again everyone for your input and comments.

[I'mdonewiththis forum]

If you have a decent air inlet (vent) your tank will drain. If you have a trap 'going dry' that means you don't have a good vent AND your Studer vents are stuck closed. The Studer vent is what are also known as an Air Admittance Valve and you should have one right after every trap.

[bobsea43]

Interesting I have had the problem on this RV as well as our previous one. I have suspected the roof vent but it was never bad enough I have done anything about it. Maybe I will with this info/suggestions!

[Kirk W]

If you have a decent air inlet (vent) your tank will drain. If you have a trap 'going dry' that means you don't have a good vent AND your Studer vents are stuck closed. The Studer vent is what are also known as an Air Admittance Valve and you should have one right after every trap.

Interesting suggestion as this is not only different, but much more expensive than what I've always found under our RV sinks and also used. I have gotten mine from either Lowe's or from Amazon and for about $10.The one that you suggest from Studor is more like $30. Is there some reason that it is better, lasts longer, or................. ? I was not aware of this type so perhaps it is worth the extra cost.

$10 $25

[I'mdonewiththis forum]

Kirk, I wasn't trying to endorse one brand over another. That site that I linked to was just for illustration. When I was a licensed plumber we always called them Studer valves, so that's the first thing that came to mind. HOWEVER, I have found that when I could buy at a wholesale plumbing house that if I went to Lowes, Home Depot, etc and compared the quality of faucets with the same brand and part number there was a very noticeable difference in quality from one to the other two. I will let you draw your own conclusions.....