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4588 Water Heater issue, pulsing hot water, bypass shuts hotwater off-gctid375940

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    4588 Water Heater issue, pulsing hot water, bypass shuts hotwater off-gctid375940

    I should get a picture, but lets see if my descriptive skills will work. Boat is a 1993 4588

    Symptoms: Turn on the hot water in the galley, in 5 seconds we get hot water. Then at 7 seconds the pump kicks on and in 5 seconds we get cold water. When the pump kicks off 5 seconds later we get hot water. It will do this for quite awhile


    Cold water into a T at Heater #1, one leg of the T feeds into the cold side of the heater.

    On Heater #1, the hot outlet feeds into the cold outlet of Heater #2. The hot outlet of heater #2 feeds into a T and then to the rest of the boat. There is a pipe with a valve between the inlet T on Heater 1 and the outlet T on Heater #2. The pencil writing above the valve says "Hot Water Bypass".

    My thinking was when the valve is open, cold water can be pumped into the hot water lines. When the valve is closed, cold water is forced to Heater #1 and then to Heater #2 and then out to the boat.

    Interesting observations:

    When the bypass valve is on, the alternate hot/cold cycle starts. If I close the valve, the flow of water slows then stops.

    Heater #2 draws power, Heater #1 does not.

    The T that feeds into the #1 tank is brass and has an arrow pointing into the tank (what is that?) The one from the hot water side of #2 is a plain T.


    There is a blockage in tank #1 with an Air pocket in the tank. Water flows past the bypass, into the hot side where it is heated. When I draw water, the pressure from the air pocket puts hot water back out the hot line and into the boat. Pump kicks on and now forces water back into the tank pressurizing the air pocket. Pump reaches full pressure and the air pressure in Heater #1 pushes back.

    The plumbing is the gray rigid tubing with the press-fit connectors on them. I'm loath to fool with them in fear of breaking them and needing to rip "massive" amounts of stuff out. (I'm due for a hard core Murphy's visit, too much stuff has gone well).


    Is there a better simpler thesis?

    What is the best way to test mine? I'm thinking of removing the pipe between the two heaters and turning the water back on to see which way it flows.

    Is it possible for the water inlets/outlets to block?

    Yep, my 4588 Bayliner IS my happy place :whistle:

    This is common and a simple fix. Your water is cycling between hot and cold as the pump cycles and pressure varies.

    This is NOT fun in the shower.

    It is caused by resistance to flow differential in the hot and cold lines

    A simple check is to hook up your shore water supply and see if it stops doing this.

    If this is the case you need to remove your cold water pressure tank if you have one

    and replace your existing water pump with a variable speed model like the shuflo smart sensor.

    This will give you a constant pressure and eliminate the cycling of the pump.

    Save your old pump for a washdown pump.
    "Adios Dinero"
    1997 3988 with new 330 Cummins
    Photo Credit: Whiskywizard


      The simplest possibility is that there is one other water outlet that has hot and cold sources that can allow them to 'mix'.

      What I mean by that is that hot and cold water shutoffs can be open but ther water outlet slightly downstream is closed so there is no water flow.

      This will allow a place where the water pressure can backflow and move hot to cold.

      Most likely places where this can occurr on a 45 include each shower, transom shower, washer/dryer (if not fitted with soleniods).

      Please check that each seperate hot and cold fixture is completely off snd see if this helps.

      Hope this helps
      Northport NY


        OK, I'll check everything that uses water again.

        When the bypass valve is on, the alternate hot/cold cycle starts. If I close the valve, the flow of water slows then stops.
        Does your mixing theory cover this?
        Yep, my 4588 Bayliner IS my happy place :whistle:


          Check to see that you have not reversed the hot and cold connections at the tank. Sounds like the cold water is coming in and getting pushed directly out the hot outlet. Cold water is suppose to come in at the bottom and hot will move up to the top account cold water is denser. If reversed the flow in the tank is not right.

          What you describe is not normal. We have similar setup in 38 and hot water follows evenly - and hot.
          Ron O'Blenis
          B 38 175 Hinos 1989
          Completed Great Loop


            I had a similar problem a few years ago. Turned out I left the cold water valve on after winterizing the previous fall. This allowed the mixing referred to. I ended up replacing the water tank before discovering the real problem but didn't feel bad. The old was one was on its last legs and the new one was much better. Maybe too simple but worth checking.



              Cold water valve where?

              I'm also coming of the winter setup and it's not worked right since last fall.

              I checked all of the water outlets and I've closed the valves to the washer (on the theory that's where the mixing is happening.

              On Tank #1 is a long(er) T fitting with an arrow on it, the arrow points into the tank. What is that? Is it a one way valve?
              Yep, my 4588 Bayliner IS my happy place :whistle:


                Maybe I'm missing something in the picture here (not the first time that happened!), but why would you have two hot water heaters connected in series? Seems that one or the other would be doing most of the work, with more line restriction as well. I would think that if the idea is to have a larger supply of hot water than can be had with a single tank, you would connect them in parallel. Perhaps someone can help me out here.

                On the T with the arrow, I believe that is a one-way check valve in the T, and it is possible that it is stuck, as you stated that if you close the bypass (why would you open it?) the hot water flow stops. Clearly, water is NOT flowing through your two-tank set-up. I believe that it was put in there to insure that the bypass valve could be used as a mixer to lower the outlet temperature of the two heaters without getting back flow through the heaters. At this point, it appears that you are not getting forward flow, and the cycling is just as you have described, as water goes "in the out" and then back "out the out" as pressure cycles.


                  My theory on "why in series" was that during the "flow", the first tank would heat from hull temp up some and the second heater would heat from that temp to hot. But as a EE and a computer guy, my understanding of water thermodynamics is limited. It's the way the boat came and I'm loath to fool with it.

                  Thanks for the insight on the "one way T", that makes sense and makes my theory work.

                  To get to the T connector I need to remove the fuel selector manifold and that's not making me thrilled with my weekend. (It's a pretty cool manifold, for each engine and the generator I can select what tank and where the unused fuel returns to. But it has 10 hoses and 6 three way valves, so it's complicated and it works. Not wanting to tempt fate )

                  Yep, my 4588 Bayliner IS my happy place :whistle:



                    I left the cold water valve open at the swim platform shower. I didn't think to check it for some reason, all timers?



                      I had this problem and found the fix on my boat was re-pressuring the small water accumilator tank down flow from the pump. It should have 30PSI. It is a schrader valve filler like a tire. Cured my exciting showers!
                      Nick Patterson
                      1989 Bayliner 4588 Stockton, CA


                        I got pulsing when my original water pump broke and I replaced it with one that had a bit more flow. The pulsing with the more powerful pump was so bad that I replaced it with a variable speed pump the next weekend which completely solved the problem.

                        The shower was the worst location and it would switch from hot to cold as the pump turned on, quickly pressurized the system, turned off, the pressure reduced, etc. For some reason that caused the temperature to switch from hot to cold as the pump switched on and off. I could never understand why it did that.

                        This was back in 2007. The variable speed pump that I installed was a Jabsco Sensor-Max 14. It has worked great and completely solved the pulsing problem even on my boat which doesn't have an accumulator. Highly recommended.


                        Jim V.

                        1988 4518 in Seattle


                          Pressure valve on tank 1 to keep the expansion out of the cold water line was frozen by large chunks of dried minerals. No flow in or out. Tank 1 ended up full of air, perfect pressure bubble to take water in and compress then let hot water out.

                          Thanks to all that hazarded a guess.
                          Yep, my 4588 Bayliner IS my happy place :whistle: