Replacing Holding Tank

mregan · October 17, 2014, 05:43:23 AM

I'm replacing my holding tank either this weekend or next. Any tips from those who have tackled this before. My main concern is leakage when I disconnect the piping.
I plan on pumping out, filling with water, pump out again, fill again then pump out using the macerator pump. Then start cutting.

Stu Jackson · October 17, 2014, 09:44:54 AM

I typed in "holding tank" with the quotes in the search engine and found this in five nanoseconds. That search engine is pretty good.

http://c34.org/bbs/index.php/topic,6593.0.html

Dave Spencer · October 17, 2014, 04:03:43 PM

mregan,
I pulled my tank this spring to paint under the tank and replace all of the hoses. The job isn't too bad. I made one improvement when reinstalling the tank - I placed a length of PVC pipe at the bottom of the pump out hose thereby (hopefully) extending the permeation-free life of the hose since "stuff" in the tank will sit in the PVC pipe that won't permeate. Being in the Great Lakes, I don't have a macerator fitted to my holding tank. Good luck with the job. Maybe have a change of clothes handy.

mregan · October 17, 2014, 04:59:35 PM

When the tank is pumped out is it pretty empty. I'm more worried about some "sludge" leaking out into the bilge then I have to clean all that up. I thought I remembered someone saying they used diapers under the tank openings to catch any drips.

Dave Spencer · October 18, 2014, 03:26:01 AM

That would work. You'll never get every drop of waste out of the tank via pumpout. The positive thing is all of the fittings are on the aft end of the tank so once the tank if free, lift the aft end so you don't get any drips from the fittings.

Indian Falls · October 19, 2014, 08:48:02 AM

I had a shop vac running to catch what ever came out that couldnt be pumped out. Lots of paper towel underneath. I strongly recommend painting that compartment before you put the new tank in. I used rustoleum oil alkyd paint, you could use anything like bilge paint etc...

mregan · October 20, 2014, 08:09:43 AM

Got the tank out yesterday. No leaks.
I cut he pumpout hose about 6" above the tank.
I was trying to figure out a way to cut the tubing to the macerator without having anything leak out. Ended up disconnecting the macerator hose to the thru hull and pulling out the tank and attached macerator all as one item. Once I had it out on the dock, I cut the tubing to free the macerator. Tipping the tank is sounded like there was still a bit of water in it. Glad nothing leaked out.

Now I realized I made a couple of mistakes when I ordered my new tank. With the vent elbow threaded into the top of the tank, it sticks up into the board covering the opening. On the original tank, in the center of the tank there was a recessed (about 1/2") threaded connection for the vent. I moved the location when I ordered the tank. I assumed the new vent location would be recessed but it isn't. There is a recessed location in the new tank, same spot as the old, but they capped it. Must be a standard feature. I may have to router out the bottom of the seat cover board to get it to sit flush.

I moved the hole, from the head to the tank, to the top outermost point, close to the hull, on the side of the tank.. Now that I put the new tank in, I realized when I'm heeled on the port side, everything in the tank may flow into the hose back towards the head. My thought is to put an elbow coming right out of the tank, run a 12" piece of pvc across the tank along the side up near the top of the tank, then another elbow or 45 looking down. The hose from the head would tie into the elbow or 45. Now heeled to port, whatever was in the tank would have to travel up the 12" long pvc before it could flow into the head hose.

Does this seem like an adequate idea? Not sure if having 2 elbows in the line is good. Worried about blockage with a large clump of toilet paper. Was thinking of using long radius elbows to help out.

Dave Spencer · October 20, 2014, 10:16:26 AM

mregan,
Your boat must have had a different set-up than mine if you moved the head to tank fitting to the outboard side of the tank. Have a look at my tank from my 1994 boat in one of the posts above and you'll see the head to tank fitting is exactly where you have it on your new tank. The hose from my head has a smooth bend coming from the under the hanging locker and nav station sole to the input fitting on my tank essentially running the same route you are planning to take. The second picture in my earlier post shows the fitting in the correct orientation ready to accept the head to tank hose. You won't need the PVC pipe. I've never had an issue with backflow into the head. Since the head discharge fitting is always lower than that tank inlet, there will always be some waste in the line which is kept out of the head by the joker valve. Other than the vent issue, I think you're in good shape. Let us know how you make out.

mregan · October 20, 2014, 10:35:05 AM

Dave thanks. Looking at the picture, my head intake is in the exact same spot. Since you don't have any problem, I'll keep it as is, put on the threaded elbow and run the hose intake into it. Glad to know the head doesn't back up. That was my main concern.

mregan · October 24, 2014, 05:32:06 AM

Am I correct in my thinking that I don't use any teflon tape or pipe dope on the plastic threaded fittings into the new tank? Just plastic on plastic.

Dave Spencer · October 24, 2014, 07:39:09 AM

I put teflon tape or pipe dope (I don't recall which) on the threads. Maybe not essential given plastic fittings whose threads will will deform to form a seal but this is an area where you really don't want any leaks.

Indian Falls · October 26, 2014, 10:05:23 AM

First choice should be dope or paste compatible with the plastic involved. Work it into both the male and female threads. Finger tight then no more than 2 full turns. Do not over tighten or the female may split.
You can use teflon but it won't set like paste will. In a moving situation as on the boat it can work back and forth and start leaking.

Fred Koehlmann · October 27, 2014, 07:15:09 PM

When we had our new tank made, the manufacturer recommended that we should only use tapered threaded fittings. They tightened to the point where you could not turn any more, and then they were tight. They haven't leaked in the last two years.

Rick Allen · February 04, 2015, 03:21:19 PM

Sorry to revive this old thread but I'm looking at replacing my 1988 holding tank soon and I had a few questions for Mr. Spencer (or others) about the use of the rigid PVC pipe at the bottom of the pump-out hose (See the photo at Reply #2). I think this is a great idea as waste will always fill that hose as your holding tank fills.

How has this improvement worked out?

Do we actually know if PVC (or ABS) is less permiable as the waste hose? Is there any other evidence of this?

Also, have you had any issues with what I perceive to be a weak point at the elbow as it enters back into the bottom of the tank? I wonder if the top of the rigid PVC pipe would benefit from some extra support to lessen any movement at the PVC/hose coupling. Any movement there IMHO might weaken the elbow below. Perhaps I'm over thinking it.

Assuming the new holding tank is custom, has anyone thought about moving the waste inlet (from the head) to the top center of the aft vertical wall of the tank? In it's current configuration it seems the intake hose would fill with waste while heeled to port. If the inlet was moved closer to the center, it may avoid refilling during heeling to port or starboard. This may not be an issue as we all should flush fresh water through the inlet at the end of the day anyway.

Great job, and thanks for the pictures.

Dave Spencer · February 04, 2015, 04:52:12 PM

Rick,
I have no regrets about the short length of PVC pipe that I installed. It has only been one year but no unpleasant surprises have emerged. Realistically, I don't expect the benefit to show up for several years when waste sitting in the lower part of the pumpout hose may start to permeate.

The top of the rigid PVC/ABS pipe would benefit from support as you suggest. Although the whole assembly was already quite rigid, this is not an area where you want a leak. I added a stiffener but I don't have a picture of it.

I think it's well known that PVC will not permeate the way hose will. Think about the PVC sink traps in your house and all of the toilet pipes are PVC (at least they are in my house). I've never had a whiff off any foul odours in my house.

It's a fairly simple change to do; especially if you are taking the tank out anyway. If you don't like the rigid pipe for any reason, it's simple to put it back the way it was for minimal cost of a new length of hose. I'm glad to discuss further on the forum or one on one. My email is spencerdd <AT> gmail <DOT> com