SV Pilgrim - 1979 Morgan 382 - Homeport: Beaufort, NC

Monday, August 4, 2014

To "Y" Valve or "T" Junction - That is the Question.

Note:  To preserve the narrative of Pilgrim's refit this post and diagrams will remain available to readers.  There are design flaws in the plumbing diagrams depicted on this post.  If you are seeking black water diagrams for use on another vessel, then please see our September 30, 2014 post - Holding Tank and Blackwater Plumbing Complete.  Thanks.

The most consistent critical feedback regarding my proposed head plumbing diagram is use of a "Y" valve just downstream of the holding tank.  In the diagram below I have labeled the "Y" valves 1, 2, & 3.
Valve #3 is the one in question.  It directs waste to either the deck pump out or the macerator. Multiple people have suggested using a "T" junction rather than a valve.  Below is a plumbing diagram with the "T" joint rather than "Y" valve #3.

My personal debate lead me to the following pros & cons of each set up

  • The total number of joints & hose clamps  (a.k.a possible failure/leak points) in the system is the same in either set up.  
  • The "T" joint costs significantly less than a "Y" valve.  But - we already own both a "T" joint and a "Y" valve so the cost for us is not a consideration.
  • The "T" joint will allow for a bit more waste storage since the volume of the hose is always open to the holding tank.
  • The "T" joint has no moving parts and thus is less prone to failure.
  • In the event of repairs or clogs downstream of the holding tank the "Y" valve would allow for greater control of waste and likely result in less direct contact with waste while making repairs.
  • In Pilgrim the 90 degree joints on the  "T" make the hose runs a bit neater than the 120 degree joints on the Forespar "Y" valves.
The lingering question I was unable to answer personally was... Is it detrimental to the macerator to have a strong vacuum pulled on it during a pump out.  I took this question to the owners of Burbridge & Wilson Marine Refit, Restoration, and Repairs part of the Jarrett Bay Marine Complex.  They recommend that "Y" valve be used as pulling a vacuum on macerator pump could damage the seals. Additional they noted that the "Y" valve also reduced the number of joints (possible sources of leaks) subject to negative vacuum pressure during a pump out.  They did cite a significant cost difference as a reason to consider using the "T" joint.

As a side note we have received comments from two people that recommend passing all the waste through the holding tank as a method of simplifying the system.  

Channeling all the waste through the holding tank greatly simplifies the plumbing. This design reduces the cost and number of possible points of failure.  I feel this is a good option for vessels operating in areas where pump out facilities are readily available.   When traveling in areas with few pump out facilities (i.e. Bahamas, Caribbean, Mexico, Central America, etc.) then a such a system is wholly reliant upon the macerator pump.  If the macerator pump fails or clogs and no pump out facility is at hand then the head is non functional until repairs are made.  I would feel more at ease traveling with this system if a manual pump (e.g. Henderson MKV) replaced the electric macerator.  Available space and access under the head counter on Pilgrim really does not allow the option of the manual pump. 

Thus we are inching towards installing the original design with three, yup three, "Y" valves.

Thanks to everyone that has contributed to this conversation both here on the website, on Google+, and over at the Morgan 38 Message Board.


  1. I am not a master of marine plumping, so not sure I understand all the issues. I am thinking like you are, I would not want the head operations tied to a mechanical bump of an operational electrical system. An idea would be to use the T joint and add an inline on/off valve just downstream of the T (between the Macerator and the T). this still deletes the costly Y valve and keeps the better hose run, but still lets you isolate the holding tank. In the end I think you must have a shut off for the holding tank. No matter how beautiful you design this system, one day you are going to need to do repairs and you will bless the day you can isolate the holding tank from the rest of the system.

  2. Mike - I agree fully with isolating the holing tank. My plans & drawing include a 1 1/2" valve screwed into the outlet of the tank. The #3 "Y" valve or "T" joint is down stream of the valve in my plans. Your idea to place the valve on the macerator side of the "T" joint is a good idea. Since we already own all the various parts - valves, "Y"s, and "T"s cost is not a factor in the assembly, but your idea may make for better hose runs.

    1. Jeff, I refit my system with all waste going into the tank all the time - as you had considered doing. However, my evacuation is powered by a Henderson Mk V manual pump - not dependent on DC current - and I don't have a macerator (system's designed for off-shore only). I like the simplicity of the plumbing.

      Just curious - I did not include a vented loop, though considered it. I will aways keep that seacock closed unless I am pumping out the tank. I feared the vented loop would leak, or spurt "material?" Am I wrong? I didn't want to try and see. I did connect the pump backwards (by mistake) at first and managed to fill my holding tank with sea water! At that point, I had provisionally installed a vented loop, and it did indeed squirt water, but I was going the opposite direction. After seeing that, I removed the loop.

    2. Rick - On our previous boat - C'est la Vie - we also used a Henderson MkV to flush the head and evacuate the holding tank. A few times over the 9 years we traveled aboard the Forespar 1 1/2" vented loop just downstream of the pump leaked waste. This was always remedied by removing, cleaning, and lubing the rubber "duckbill" valve in the top of the loop. I used soap and water to clean followed by 303 as a lube. Be sure to re-install the duckbill in the correct orientation - bill pointed into the loop - or you will definitely get a squirt of material.

  3. if you are pumping everything thru the holding tank you may not require the vented loop. also your holding tank is above waterline and you would only open the valve when doing pump outs. less hose is better in this case and the vented loop is one more point of failure.

  4. Rick & Gonesail - I am fairly confident that even with the holding tank vented and using a manual pump it is possible to create a siphon back to the holding tank. Closing the thru-hull would stop the flow. Keeping this thru-hull is definitely a best practice.

    What I am unsure of is... If a siphon was created between the water outside and the holding tank would closing the thru-hull break the siphon or would the water continue to flow into the holding tank when the thru-hull was re-opened?

    If the siphon was not broken by closing and re-opening the thru-hull, then how would one go about breaking the siphon?

    I do believe that the vent in the tank would prevent the siphon from moving past the tank and back flooding the head.

  5. Jeff, reading your response made me realize I had the wrong kind of vent in my line. The one I installed temporarily didn't have a valve in it, that I know of. However, at this point I think I'm okay without it, seeing that the seacock is only open when evacuating the tank.

    A couple things about the siphon: a siphon is created by positive/negative pressure imbalance transporting fluid through a hose. It takes high pressure on the supply side, and low pressure on the exhaust side. Pressure can be created by a pump, or by fluid being forced from the source side (as in, water driven into the through-hull by forward motion of the boat, or by having the through-hull submerged deeper than when on a even keel, like when you are on a tack and the through-hull side is down). Once the syphon has begun, it is self-sustaining because a vacuum is continually created within the line as fluid falls out of it and the exhaust stays lower than the supply.

    Once the flow is stopped by closing a seacock, pressure balance returns to the conduit. Simply reopening the seacock should not reinitialize the syphon, unless there is (again) high pressure acting on the supply side (or vacuum being created by a pump or fluid falling out of the line).

    The Henderson pump is also a safeguard against a continuous syphon. The valves inside the pump which enable it to move waste out of the tank, would also prevent water from flowing back in reverse.

    I don't know if your macerator has back-flow valves, so that is probably a question you can answer by looking at the pump diagram. The tank vent would actually enable a syphon to fill the tank by allowing air to escape, its volume is being displaced by incoming water. If your diagram represents your installation, I'm not sure the vent would prevent back flow into the toilet: once the water level reached the top of your tank, water would simply fall into the toilet exhaust hose, moving backwards against the design flow.

  6. Oh, and I agree with the Y-valve. I'm pretty sure you would want to shut off the flow out of the tank if you wanted to repair or replace a component downstream of it.