woodsmitch
Sedro Woolley,Washington,USA
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Joined: 12/04/2003
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We had the problem of really slow draining in our kitchen sink for several months. Finally poured a large pan of nearly boiling water down the drain and it opened up and haven't had any problems for several months. I had tried taking the trap off(it was clean)and the route the piping takes makes it difficult to run a drain cleaner down it. Good luck!
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Dave H M
IL
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Joined: 08/11/2006
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Mile High wrote:Dr Quick wrote:Most RV's have a "air check valve" that acts as a vent for plumbing at the fixture. It allows air to enter drain pipe so water can flow, but it will not let tank odor back in to RV. If that is not working right that could be your problem. I would also think that you might have plugged line to tank. I would have to respectfully disagree that the check valve has anything to do with draining the sink. The check valves are installed on all of the tank vents to assist in draining of the tank itself by increasing the air flow displacement into the tank while draining out of the large diameter drain valves and sewer hose. Without it, the roof vent does not provide enough air flow into the tank to allow for rapid draining of the tank and it starts to slow until the airflow catches up. When not draining the tanks, those valves are held closed by spring pressure (hopefully) and don't contribute anything to the normal internal plumbing operation. If they had anything to do with standard drainage, they would be installed on your home plumbing.
It sounds more like a plugged internal drain line - or more often than not - the unnoticed galley tank valve.
My point is that I cannot see the logic or plumbing principle involved in putting one of those valves in all the drain lines to assist in dumping the holding tank.
If you watched the plumber rough in your stick home before the drywaller you would notice that each drain has a connection to the roof stack - they tell me as stated before that allows quick and unobstucted drainage. now since you don't have a connected stack system in the RV one of those spring loaded vent valves are installed.
That is my take.
Likewise if the only purposre of those valves in an RV was to assist in draining a holding tank, then the drains in a house would not have to be connected to the stack. 
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Mile High
Lone Tree, CO
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Joined: 02/05/2008
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Dave H M wrote: My point is that I cannot see the logic or plumbing principle involved in putting one of those valves in all the drain lines to assist in dumping the holding tank.
If you watched the plumber rough in your stick home before the drywaller you would notice that each drain has a connection to the roof stack - they tell me as stated before that allows quick and unobstucted drainage. now since you don't have a connected stack system in the RV one of those spring loaded vent valves are installed.
That is my take.
Likewise if the only purposre of those valves in an RV was to assist in draining a holding tank, then the drains in a house would not have to be connected to the stack. I believe I understand what you are saying. I do have three individual roof stacks from each tank, and I've even ran into the stacks in the wall adding things, but I cannot say if they are direct from the tank or if they connect to the interior drainage system. You could have a point I accept that.
I have been under the impression that because the roof vents are only 1-1/2" and your tank drains are 3", that the spring loaded vents open to provide a full 3" vent intake to displace the tank water as it drains. I still think that is true also.
Perhaps the earlier post is right and the spring loaded vents serve both purposes. That would especially make sense on the black tank, as it would not normally need any drain assistance yet mine has the spring loaded vent in addition to the stack vent.
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BruceStarkey
Ontario, Canada
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Joined: 05/02/2004
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Mile High, the 1&1/2" vent stacks into the tanks are more than enough to vent the tank when draining as the velocity & volumn of air accommodated by that smaller pipe is more than enough to prevent vacuum occuring from the out going 3" column of water/sludge. Those little vents under the sink counter's are more properly referred to as "vacuum breakers" as that is the function they're intended to perform. They are there to break the vacuum created by a column of water exiting the sink and taking the last of the available water along with it instead of allowing it to remain in the 'p' trap. Another part of that function is to allow air into the drainpipe to prevent that 'p' trap water from being pulled out by a vacuum created within the system by another fixture being emptied downstream.
In a sense; you are correct, they will augment your tanks main venting if that should ever become plugged with mud-dauber nest or if that stack should ever drop down into the tank sealing off it's bottom end when it contacts the floor of the tank (common occurrance) and preventing it from allowing air into the tank. Hearing those guys whistle under the cabinets when draining a tank is often the first sign a main vent stack is plugged with bug debris.
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Mile High
Lone Tree, CO
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Joined: 02/05/2008
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That makes sense Bruce. I guess I'll have to take a new look at how it works. I had been convinced they were only there for the tank draining.
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bogeyF250
Newark, DE. USA
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Joined: 11/20/2003
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Drgonslyr2 wrote:The sink must have some type of vent for the plumbing to work correctly, whether an actual vent line to the the roof or what is normally called a re-vent. Which is a vent pipe coming off the drain line in a vertical postion and it has an air gap mounted on the top of it. The air gap will close if water gets up that high so that there is not a leak. One of those two must be present near the sink.
The under the sink vent is call an AIR ADMITTANCE VALVE. It allows the sink drain to get air if there isn't sufficient venting or if the drain pipe is too long horizontally before the vent to the roof. You can purchase one of these AAV's at Lowes, Home Depot or a local plumbing supply.
That said, I would look to the drain valve as other's have stated. I too had just purchased a new RV and with the slideout extended I could not see the label that said GALLEY on it. I got the sink filled up and then started to search for what was wrong, and then found the drain valve just in front of the tire mounted on the frame rail. The other drain valves were two feet to the front and clearly visible and labeled but no one told me about the third Galley drain valve! Really had me scratchin my head for awhile! LOL
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