Where to locate 2nd Holding Tank Vent

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Dougcole

Guru
Joined
Jan 21, 2008
Messages
2,189
Location
USA
Vessel Name
Morgan
Vessel Make
'05 Mainship 40T
Hi All,

A few years back I installed a new, larger (1.25" ID) holding tank vent on our Mainship 400. It is on the forward end of the tank and vents through the hull near the bow. I used the hole from the old fitting to install this new vent. It is a 1.25" fitting, but mainship originally necked it down to a 3/4" hose.

The original vent exits the hull midships on the stb side. It is inaccessible, so I plugged the tank end of hose and left it in place.

Though the new, bigger vent helped some with my odor problem (we get smell out of the vent when the toilet flushes) it did not fix it completely, so I am planning on using a uniseal and reattaching the original stb side vent along with my new fwd vent.

My question is where to place the fitting on the tank? I can access both the aft and fwd ends of the tank, but the hose is not long enough to reach the aft end, so I would have to add a double male fitting to lengthen the hose, which would bring that hose down to about 1/2" id. Is it worth it to have a smaller hose on the back of the tank or should I just install a second vent on the front?

Thanks,

Doug
 
Ideally your 2 vents would enter the tank at the front and rear or port and starboard to promote flow through air flow. But I think that if that isn’t possible then I would go with the larger size vent where you can fit it. I would want the more air flow. I added 2 1.5” vents in addition to the factory vent. I was able to install them on the port and starboard sides of the tank. I haven’t noticed any smells at all.
 
Two vents are opposite sides are a definite improvement. I ended up just adding a bait tank aerator and let it run most of of the time when the head/tank are being used. In other words, if there is poop in the tank, the aerator is running. When the tank is emptied, I turn it off.

This completely got rid of any and all tank smells, is quiet, and very little electrical draw. It wasn't my idea, but an idea I stole from another TF'er.

Thread is here
 
Two vents are opposite sides are a definite improvement. I ended up just adding a bait tank aerator and let it run most of of the time when the head/tank are being used. In other words, if there is poop in the tank, the aerator is running. When the tank is emptied, I turn it off.

This completely got rid of any and all tank smells, is quiet, and very little electrical draw. It wasn't my idea, but an idea I stole from another TF'er.

Thread is here
Yes, that is an excellent solution. I did that on several boats that I couldn’t get 2 vents to the sides of the boat. Absolutely no smells if you run it continuously. Very negligible current draw.
 
Hi Doug,
I had exactly the same issue - only one 1" vent and not possible to run a second to the other side of the boat.
My solution was to run a second 1" vent (from the opposite end of the tank) exiting just in front of the original vent. I installed a small clam shell over the original line, my logic being that in any breeze the clam shell would set up a very small venturi effect and suck air out and thus circulate the air - voilà - no smell, even after the tank sits for weeks half full and the toilet is flushed
 
Hey Doug, I looked into a second vent up forward in the chain locker. Good access to the tank, under the master bed, and climbed into the access hatch under the head of the bed to get to the backside of the chain locker fiberglass. I even bought the fittings. But I chickened out because I was worried about smells blowing back from the front of the boat while running! I ended up with the bubbler setup. Was super easy, costs next to nothing, and has eliminated problems. I installed a Metal Marine 110v bubbler in 2021, runs from April to October, still works great. Fish tank tubing, a barbed fitting into a short 1/2 PVC, and a Uniseal on the front end of the holding tank, accessible from sump hatch. I think the breaker for that outlet is labeled Freezer? I relabeled it Tank Aerator and just leave it on all the time. Problem has been completely eliminated. One note, if we're disconnected for a couple of days, we can definitely notice when someone flushes, but that's a rare occasion. A 12v pump would resolve that, but installing an inverter would be an even better idea! Bubble yer poop! It works! I'll send you the second Uniseal that I have if you want?
 
Hey Doug, I looked into a second vent up forward in the chain locker. Good access to the tank, under the master bed, and climbed into the access hatch under the head of the bed to get to the backside of the chain locker fiberglass. I even bought the fittings. But I chickened out because I was worried about smells blowing back from the front of the boat while running! I ended up with the bubbler setup. Was super easy, costs next to nothing, and has eliminated problems. I installed a Metal Marine 110v bubbler in 2021, runs from April to October, still works great. Fish tank tubing, a barbed fitting into a short 1/2 PVC, and a Uniseal on the front end of the holding tank, accessible from sump hatch. I think the breaker for that outlet is labeled Freezer? I relabeled it Tank Aerator and just leave it on all the time. Problem has been completely eliminated. One note, if we're disconnected for a couple of days, we can definitely notice when someone flushes, but that's a rare occasion. A 12v pump would resolve that, but installing an inverter would be an even better idea! Bubble yer poop! It works! I'll send you the second Uniseal that I have if you want?
Metal marine makes a 12v aerator. It is what I use. I tried it into the shower sump pump circuit. It was handy. Works great after a number of years.
 
If he does actually have a bladder tank it may or may not be able to install a bubbler/aerator system. I don’t know if you can add fittings onto a bladder tank successfully or not. But an aerator system will get rid of the stink.
 
This flexible tank has got me thinking re aeration. A hard sided black tank has an air gap above the sewage which needs refreshing with clean air to stop the smell bacteria forming. Not so in a bladder - there is no air gap therefore no matter how many air vents installed, no oxygenated air can ever be introduced, THEREFORE the only way is to force air/oxygen in via a bubbler.

Am I reading this correctly?
 
I built my own tank of about 35Gal close to 20 yrs ago out of Nytex fabric and then glassed it over with epoxy. Placed the tank on the Centre Line in the E.R.
For venting I installed two vents and hoses. One vent on each side of the boat and the vent hoses are 1" ID if I remember correctly. THe exits for the hose are also catty corner from each other. I won't say there has never been an odour but compared to the previous tank it is vastly better.

I have written the build up previously so won't repeat it.
 
This flexible tank has got me thinking re aeration. A hard sided black tank has an air gap above the sewage which needs refreshing with clean air to stop the smell bacteria forming. Not so in a bladder - there is no air gap therefore no matter how many air vents installed, no oxygenated air can ever be introduced, THEREFORE the only way is to force air/oxygen in via a bubbler.

Am I reading this correctly?
Yep, you are. Bladder tanks are ok for fuel and water because fuel and water don't outgas.... but not for sewage holding because they hug the contents, leaving no room in them for air/gasses to do anything but escape out the vent when displaced by incoming flushes. I'm not even sure that an aerator can be installed in a bladder because it would require installing a fitting in it for the air hose from the pump and owner-added fittings tend to be prone to blow out at the first sign of a vent blockage.

Do you have a bladder waste tank? If so, would you consider being a pioneer in this endeavor?

--Peggie
 
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