Is there an effective way to clear a blocked vent pipe for a holding tank?

[Nick in Manitou]

Not long ago we had trouble pumping out our holding tank and investigated the dip tube which seems to still be in functional shape.

As we had just changed pump out services, I assumed the problem was with the new service.

A few weeks have passed and the pump out fellow usually has some difficulty with pumping out the forward tank, but has succeeded it getting it done after a bit of hassle...until yesterday.

Yesterday he said that three different times the system would start pumping material out of the tank and then stop. Only after he left did I think about the fact that if the vent was plugged it would be very possible for the pump out to start and be successful for a short time until the vacuum inside the tank would overcome the vacuum in the pump out hose.

So, my theory (at the moment) is that the vent hose is seriously plugged.

If that hose is plugged, is there an effective way to unplug it without removing it?

Accessing the hose is complicated by the fact that it exits the boat through the hull directly behind the washer/dryer installed by the previous owner...and he used quite a bit of glue on the woodwork enclosing the units.

I was thinking that if I could stick a flexible hose into the outside of the vent and get a good liquid seal around it somehow, I could raise the outer end of it up and pour in some liquid holding tank treatment...perhaps giving it a few days to soften whatever has collected in that hose over the years and break it down, liquefying it and draining it back into the holding tank.

I looked through a number of the threads on this forum about holding tank problems and didn't see one that addressed this question. (It may well exist and if it does, please let me know!)

Any suggestions?

Thanks once again for your help,
Nick

[SEVEN]

Nick, I’d try the simplest thing first. Put a water hose in the thru hull and seal it with a rag the best you can. Turn the shore water on at a low pressure. Back flush the system a few times. Try leaving the water on for 15 seconds then 20 and so on until it flushed into the tank or back out the thru hull. Let me know if it worked.

[captainwjm]

A couple of suggestions:

Although it does sound like a stopped up vent, just to be sure, see if you can pump overboard [just enough to know if the issue isn't the pump-out discharge pipe].

If it is the vent, take a shop-vac and try to suction out the vent from the outboard end.

If that doesn't do it, your suggestion of pouring a solution down the vent would be appropriate.

[SEVEN]

Nick, I’d try the simplest thing first. Put a water hose in the thru hull and seal it with a rag the best you can. Turn the shore water on at a low pressure. Back flush the system a few times. Try leaving the water on for 15 seconds then 20 and so on until it flushed into the tank or back out the thru hull. Let me know if it worked.

Nick you can also try a wire puller and insert it in from the thru hull into the boat. I’d try that after you loosen it with the water hose.

[Nick in Manitou]

I was concerned about where the pressure would go if I put a hose up to the vent on the outside of the boat, but I guess if I loosen the cap on the pump out fitting on the deck, that should allow any pressure that reaches the tank to push "stuff" up the dip tube a ways. (I won't leave the hose on long enough to push "stuff" all the way up to the deck!)

I will go out and give it a try...

I have not got a wire puller any more. We thought we were moving into an apartment and I ended up selling most of my tools before we bought the boat...some that I didn't sell are in a storage unit in Arizona...difficult to access from California. I may go out and pick one up if I can't borrow one from a neighboring boat. (Maybe I should't tell them what I need it for! )

Thanks,
Nick

[SEVEN]

I had water drains on my decks get clogged up with 40 years worth of dirt and leaves etc. I did exactly what I told you to do with the hose left it on for about 30 seconds, I pulled the hose off and outcomes the clog and gunk for all those years now it’s free-flowing.

[Pascal]

Do you have a charcoal filter on the vent line ? You need to be sure you don’t before trying anything

If you don’t you could try compressed air or water pressure from whatever end the most accessible. I doubt a shop vac isn’t to create enough suction to loosen anything

What is the tank made off? Some boats have been retrofitted with poly tanks which do not handle a vacuum very well. Personally I would never use a charcoal filter with a poly tank. The risks are too high to damage the tank.

[SKYCHENEY]

Remove the vent hose from the tank the next time you get pumped out. If he still can't pump it, its not the vent.

[dar636]

I had the same problem, on my vent line where it attached to the top of the holding tank. The tank connection used a 90' elbow, the elbow plugged up with salt or whatever. Check there first, it'll be the smallest I.D. of the entire vent system. I replaced the elbow, good to go, so to speak.

[jmooney]

I replaced my vent line with 1 inch hose ,as the fittings would accommodate a 1 inch hose , it had been previously been a 5/8 inch. Also be extremely careful putting pressure on the tank as the seams can leak, I think the max pressure is 4 psi , but I am not sure. As Sky mentioned, removing the vent hose from the top of the tank and then trying a pump out would tell you a lot. My onboard pump can empty the tank easily, most marina pumps cannot.