Holding tank vent fitting options

Seems like there are several holding tank threads going right now (must be the season), but this one is specifically about the through hull fitting at the end of the vent line. We have ¾" hose leaving the top of the holding tank on a continuous rise to the inside of the through hull fitting. Used to have a filter, but that was removed a couple years ago. Turns out that was only part of the problem.

Pumping out always resulted in a lot of gurgling at the forward (and most used) head, but the vent problem surfaced clearly when we started using the day head. Flushing it often resulted in bubbles coming back up from the holding tank. Flushing larger volumes down the forward head also causes "air" to burble back through the day head. Unpleasant.

Rather confident that the line is clear, I finally took the mushroom cap off the outside of the through hull fitting and found a lot of black matted growth on the screen. The pictures below are after cleaning (albeit rather half-assedly) the screen with a brush. Two weeks later the screen is again not passing air out of the holding tank and instead venting the flushed volume back through the day head.



What is the best option for replacing this holding tank vent? Like most, we have a lot of spiders and flying insects ready to inhabit any convenient hiding place. The richly organic environment of the holding tank vent is a popular one. I feel like an open traditional through hull fitting would make a great home to the most aggressive spider on the dock and present a regular maintenance item of blasting the spider, web, and whatever it has in there down into the holding tank every few days. Some Miss Muffet's might extend the interval, but it only lasts so long. Is there a better designed vent fitting?

I can't imagine any holding tank vent being attractive as a hiding place?? But no experience so anything is possible.
With 3/4 hose I would just go with a large mushroom thru hull and flush and refresh with some Miss Muffet after. That stuff does seem to do wonders with spiders and I doubt you will have any plugging with large hose.

An open mushroom through hall fitting like this one?

Yes, mushroom through hull is the way to go. Then you can easily use a hose to back flush the vent to clean out any bugs.

I'd agree with @Comodave and @Bacchus . On my last two boats I replaced screened vents with the open mushroom. Easy to do if you can access the back side of the vent. The screen may actually be providing a substrate for the bugs to build on?

I went with a mushroom through hull during my refit and also increased the hose size (your 3/4" should be fine). As mine is located in the stern with easy access from the swim platform, I flushed it after every pumpout. Never had an issue, and believe removing the screen increased air flow into the tank, which promoted the breakdown of contents, reducing smell.

Ted

On our current boat I added 2 1.5” vents with mushroom through hulls. One vent to each side of the boat so no matter which way the wind blows I get air flow through the holding tank. No smells.

Did those dual vents have their own taps into the holding tank? Mine is fiberglass and all ports are used.

No, I used Uniseal fittings to hook the vents to. Not sure if they would work with fiberglass or not. But you could always glass a fitting into the fiberglass. Fairly easy to do.

So it turns out my vent line is clogged after all. It seemed like the mesh in the vent cap because it was clogged too, but cleaning the vent cap did not enable air flow during flushing or pump out. Neither did aggressively spraying a jet of water into the open vent line.

The vent line is also ½" hose rather than ¾" like I thought, and nearly 10 feet long. It takes a circuitous route from the through hull diagonally across a bulkhead and across the top of the holding tank to this fitting in the access plate. Guessing that white tape repair was a split at some point. And the old Simrad rate compass is on its way out.



Question is, should I try to snake that line with a fish tape, or should I just grow up and remove the hose from the fitting and deal with the potentially messy result?

I would replace the whole thing and go with a much larger vent hose. I like 1.5” but even 1” would be much better than 1/2”. I didn’t even know that any builder used 1/2”. Usually 5/8” is what builders use and that is too small. Also use a quality hose not the corrugated hose. If you can go to 1” then use Raritan SaniFlex hose. It won’t permeate and it is very flexible. Defender sells it by the foot. Then use a mushroom through hull fitting so it is easy to back flush.

Shouldn't be a problem getting 1" hose through the mouse hole between the top of the tank and the structure, but the tap in the tank plate is ½" female thread. Does someone make an adapter to get from that level sender to pipe thread? I don't use that level unit any more either.

I would just cap the old tank fitting and use a Uniseal to install a whole new tank fitting. The tank is plastic, correct? Then get a Uniseal to fit 1” PVC fittings. The Uniseal is simple to install, just use the correct size hole saw to drill a hole in the tank. Then use a round file to cleanup the hole edges and insert the Uniseal. Then put a short piece of PVC pipe into the uniseal and put a PVC barb fitting on the PVC pipe. Then put the new hose on the fitting and run it to the hull side. Run the new hose as short as possible and a continuous uphill run.

It's a fiberglass tank. Guess that's not much different, but getting to the inside of the tank to tighten the nut side of the fitting sounds daunting.

The Uniseal does not have a nut inside. I think it would work with fiberglass but check to make sure. It is just inserted from the outside and stays in place well.

Guess I’m not familiar with that product. Is it just a friction fit in a seal?

I guess. When you stick the PVC pipe in it expands a bit and creates the seal. I have tried pulling one out and couldn’t. I file the sharp edge on the PVC so it doesn’t cut the rubber bung and then a bit of Super Lube to halp slide in the PVC.

HeadedToTexas said:

Guess I’m not familiar with that product. Is it just a friction fit in a seal?

Click to expand...

Uniseal This should help.

dhays said:

Uniseal This should help.

Click to expand...

Yup, that is it.

Ah yes. And that’s a common approach in the boat world?

HeadedToTexas said:

Ah yes. And that’s a common approach in the boat world?

Click to expand...

I've not used them, but have seen a number of folks here on TF that have used them to add access to a holding tank.

I added 2 1.5” vents with Uniseals to our current boats holding tank. I left the factory vent in place since it had the original filter in it. I am sure the filter wasn’t good anymore but it is completely inaccessible to get at. I have used Uniseals on 3 or 4 boats now. Never had any problems.

Beautiful. Thanks guys.

Being a vent line, it is important to keep the fitting above the liquid surface in the tank. That means as little pipe protruding below the Uniseal as possible. Is there a minimum spec on how much pipe must protrude below the fitting?

All of the fittings in my plastic holding tank are done with Uniseals (all in the top of the tank). It doesn't take much protrusion to get a good seal, 1/2" should be more than enough and you can probably get away with less. I hardly have any on the bubbler fitting due to how I designed it and it seals fine, takes a good bit of wiggling and pulling to get it back out. And that's only a 3/4" Uniseal, the bigger sizes have even more friction and are even harder to remove once installed.

I typically have .5” to 1” sticking out on the inside. They are really hard to pull the pipe out of.

Y'all managed to sneak this thread in when I wasn't looking. So I'm gonna stack all my comments into a single reply:

The Uniseal has become an accepted device throughout the marine industry...Dometic uses it in their "diptube kits" and a couple of tank mfrs are recommending it to replace the standard 5/8' (NOT 1/2" or 3/4" as most of you have guessed it is) vent line that boat builders use. The Uniseal is essentially just a rubber grommet...it doesn't expand, but it does stretch to fit the hose, assuming you've bought the right size and made the hole in the tank the right size and perfectly round. It is not recommend for use in or at the bottom of a tank.

Not that it matters, but "bulkhead fitting is the proper name for a "mushroom" fitting." One of you posted a photo of a plastic one...PVC is acceptable, stainless steel is better and bronze is best, but NEVER use nylon in an external fitting because nylon is not UV resistant...sunlight will turn it into powder.

The vent line is also ½" hose rather than ¾" like I thought, and nearly 10 feet long. It takes a circuitous route from the through hull diagonally across a bulkhead and across the top of the holding tank to this fitting in the access plate.
Ideally, a vent line should be straight with no bends, no longer than 5-6' and as close to horizontal as possible..if it must rise, the rise should be no sharper than 45 degrees, because ambient air doesn't fall and that's the most the gasses in the tank can rise. So you're gonna need a new vent fitting in the tank and new vent thru-hull that shortens and straightens the vent line--not necessarily all the way to 5-6' but as much as possible.

Ted said his tank vents out the stern, makes impossible for the vent to do more than exhaust the gasses--which have to stink because there's no air exchange--and be the source of air that a pumpout needs. Aeration is the only possible cure.,,,in fact a much better solution for any tank tank than adding a vent line. It doesn't have to be expensive...several people on the forum have DIYed holding tank aerators

And..that's my $.02 worth contribution to this thread.

--Peggie

My holding tank capacity is reportedly 30 gallons. The vent line has always been in the top of the tank as I understand most are. As the contents of reach 30 gallons, the vent line becomes an overflow line, true? The deeper the Uniseal tube protrudes through the seal into the tank, the lower the usable capacity of the tank gets and the higher my chances get to someday accidentally pump some poop into the bay. Are there not ABYC safeguards against that? I know the Y valve must be locked out, but the vent line seems far more likely to allow a spill than a less than familiar guest finding and opening that Y valve. Anyway, hence my obsession with the Uniseal fitting.

I was pretty happy with my 2021 vent line improvements, but plugged only a few years. Originally the vent line had an extra 4 feet of ⅝" hose and an orange filter assembly in line before exiting the hull. The original vent line took a far more twisty and steeply rising route from the holding tank fitting to the through hull. I took all that out and straightened the rise of the vent line across the bulkhead.


The photo below shows the aft edge of the holding tank nearly flush with the bulkhead separating the forward head and stateroom from the engine room and pilothouse. That does not afford any site in which to mount the Uniseal and new vent line fitting without the vent line taking a sweeping 90 degree turn forward and entering the top of the holding tank near the access plate shown in a photo posted further up in this thread.


The tank is fiberglass and while I could perhaps put a Uniseal in the aft face near the top of the tank, that would cut the already low 30 gallon capacity down that much lower. With the tank being on the centerline, I'm not going to get close to 5-6 feet in any case.

The vent line has always been in the top of the tank as I understand most are. As the contents of reach 30 gallons, the vent line becomes an overflow line, true?
That should never happen, but often does on sailboats due to heeling.

USCG regs require a means of determining when holding tank contents level reaches 3/4 full. Visual inspection is acceptable if the tank is in a location and is made of a material that makes visual inspection readily available...if not, a tank level monitor is required. There are tank monitors using internal sensors that only read 3/4 full,..legal but next to useless IMO. SCAD tank monitors Scad Tank Monitors are top rated, use external sensors that can be used on any material except wood (some people DIY tanks that are wood coated with fiberglass) and metal, are reasonably priced and available from a number of retailers including Amazon (and I THINK even Walmart), are easy to install and have a very low amperage draw.

--Peggie