Holding tank vent fitting options

[HeadedToTexas]

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?

 

[Bacchus]

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.

 

[HeadedToTexas]

An open mushroom through hall fitting like this one?

 

[Comodave]

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.

 

[dhays]

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?

 

[O C Diver]

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

 

[Comodave]

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.

 

[HeadedToTexas]

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

 

[Comodave]

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.