Holding Tank Vent Size

Ericbsc · September 30, 2009, 11:14:47 AM

What size vent do most run for holding tanks? I would like to use 2" but it will be pretty tight. Either way it is going to look Look a snake with a lot of turns.

bigjohnkub · September 30, 2009, 11:24:40 AM

Eric, some of the gentlemen on the board have talked about running down in stead of up. Maybe they will get in .

Big John

NJT 5573 · September 30, 2009, 11:40:32 AM

Hi Eric,

I'm not a plummer so... I had my water system done professionally. It works well, its a success, 5+ years and no complaints. I think my vent pipes are 1.5, but I don't know for sure.

There has been some recent discussion here about odors. I don't know if there ever was an answer that worked for everyone.

The builder that did my vents put in 3 pipes. I thought to myself, "thats a lot of holes in my roof with all that ugly black pipe sticking out"!

Anyway, the point was that I have not ever had an odor issue in my bus and that I think perhaps the reason lies with all of the black pipe and holes in the roof...

bevans6 · September 30, 2009, 12:31:28 PM

The general ideal is to allow sewer gas to vent up out of the tank, and to allow for an exhange of gas volume when a big load of water comes into the holding tank suddenly. so there are two different, but related, requirements. the usual size of RV vent is a 1.5" PVC, vented on the roof. If you subscribe to the sewer gas lighter than air theory, then you think the vent line should run to the roof in an always rising route, with bends of 45 degrees or less.

My bus is vented down, into the rear axle compartment. It smells quite a bit inside the tank compartment, and I have issues with odor when driving at times. I do plan to vent it up to the roof in a constantly rising run, as soon as I figure out how to do that, since I do subscribe to the methane sewer gas is lighter than air and is bad for you theory, and I see no reason to camp sleeping directly over a black tank vent if I don't have to...

I don't see any reason not to use the main vent to route the fixture vents to as well, so I would say that you might be able to kill two birds with one stone there. I also don't see any reason not to tie the vent for a gray tank with the vent for a black tank, but I could well be wrong on both of these ideas. I only have one tank.

Brian

rusty · September 30, 2009, 12:47:55 PM

Eric, I used 1 1/2 vent pipes out throuh the roof. I vented both the gray and black tank at opposite ends of the tank and then connected the same ends of the different tanks together and ran two pipe throught the roof. We donot dump large amount of water into our tanks at one time so 1 1/2 pipe will be large enough. The reason for connections at opposite ends is to get a small air flow to help remove unwanted ordors. My system has worked for 6 years and I have never experenced any unwanted ordor.

Good Luck Wayne

Melbo · September 30, 2009, 12:54:55 PM

I was going to use 1 1/2 inch vent but had heard that some people had problems with that so I went to a 2 inch vent and it is connected above the black tank and has a T from the gray tank. I ran it straight up through the cabin of the bus but it has a horizontal run to where it goes up. The only time I had a problem was when I turned on the exhaust vent with it sucking air out of the toilet room and then flush the toilet. Did that one time and one time only. We have never had problems when driving or parked.

HTH

YMMV

Melbo

Sean · September 30, 2009, 02:24:03 PM

The minimum allowable vent for the black system is 1.5", for the gray it is 1.25". However, if any fixture requires a larger vent, then you must use that size all the way to the roof. If you use a wet vent system, which is common and convenient for conversions, you must increase by one trade size, which would make it 1.5".

Vents must be continuous all the way to the roof, and must exit the roof with few exceptions (none of which applies to a bus). Many here will argue this; we've had this argument before and it is now well documented in the archives. Notwithstanding various opinions on the matter, the code provides no leeway here.

-Sean
http://OurOdyssey.BlogSpot.com

NJT5047 · September 30, 2009, 07:22:06 PM

Quote from: Melbo on September 30, 2009, 12:54:55 PM
I was going to use 1 1/2 inch vent but had heard that some people had problems with that so I went to a 2 inch vent and it is connected above the black tank and has a T from the gray tank. I ran it straight up through the cabin of the bus but it has a horizontal run to where it goes up. The only time I had a problem was when I turned on the exhaust vent with it sucking air out of the toilet room and then flush the toilet. Did that one time and one time only. We have never had problems when driving or parked.
Melbo

Melbo describes how mine is plumbed also... 1.5" T'eed into a 2" vertical...he also describes what happens when the pottie is flushed, and the windows are closed, and the Fantastic Fan is running...don't do this.

Otherwise, been totally trouble free (and stink free) for many years now.
RV supply houses sell curved roof seals and neat little caps for 2" vent stacks.

JR

Chuck Newman · September 30, 2009, 07:45:34 PM

I found looking into the tank when flushing will give you an indication of how full the tank is and whether or not it has enough water -- the latter being very important.

That practice continued until I got a Fantastic Vent fan. You will get a blast regardless of how effective your vent system may be. Now my "tank inspections" are done with the fan off.

Chuck

Dreamscape · October 01, 2009, 02:46:53 AM

I have 1 1/2" PVC vented through the roof, seems to work for us!

With a vent through the bay floor with a hose venting from the top of the tank, down and out. We have a combined black/gray water tank.

Paul

Len Silva · October 01, 2009, 07:17:04 AM

There is also the option of a fan enhanced vent. I've not tried it but seems like a good idea, it would keep the tank at a negative pressure when flushing.

http://www.lslproducts.com/