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Rule 3700 bilge pump air lock

Discussion in 'Technical Discussion' started by enproep, Jan 5, 2015.

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  1. enproep

    enproep New Member

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    Hello:
    I have a Carver 530 voyager which has a Rule 3700 bilge pump in the sump box. For some reason, the pump generates an air lock (typically after being underway for awhile or rocking at anchor) and will continuously run without pumping the box out. This has led to broken pumps and overheated wiring (it will run for hours). Anyone out there have this same problem and any suggestions on how to resolve it? Thanks, Bob
  2. dennismc

    dennismc Senior Member

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    Add a float sw
  3. enproep

    enproep New Member

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    Thank you for the reply. As part of the system, there is a float switch installed, which works fine. It's just that pump seems to get an air lock when the switch trips it and it just runs without pumping.
  4. jhall767

    jhall767 Senior Member

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    Can you post a picture of the pump and float? That type of pump shouldn't get an airlock. It is designed for its impeller to be immersed when running. There is no suction side therefore no "airlock" can occur unless there is not enough water. More likely the switch is failing in the on position.
  5. Pascal

    Pascal Senior Member

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    How big is th sump box? It s a pretty big pump to be used in a sump unless the sump is big enough.

    When you see the pump running and running how much water is in the sump.

    As others mentioned check th float switch. If this is a shower sump, switchez often get sticky with hair and soap scum

    Finally, make sure there is no check valve in the line. Many builders or yards use check valves limit back flow and cycling instead of routing the hose properly. Check valves may prevent bill pumps from starting
  6. enproep

    enproep New Member

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    Thanks JHall767:
    Unfortunately I live 100 miles from where we keep our boat, so no picture.
    Basically I think what is happening is, the float switch trips the pump to turn on, the pump turns on, then evacuates the sump box. All works perfectly when we're in our slip or at a very calm anchorage. Once underway, however, the remaining water in the sump box will trip the pump to turn on and the pump then sucks up water and air as the boat rocks. When the pump sucks up the air it just runs indefinitely not sucking up water anymore. The motor just runs and runs without sucking up water. When it is doing this, I will cut one of the bilge power wires to shut the motor off. Then when I hook the wire back up, it works fine. It's as if shutting the power down eliminates the air lock.
    The pump and switch are both new.
    Thanks again.
  7. enproep

    enproep New Member

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    Thanks Pascal:
    Please see reply to JHall767.
    No check valve either.
    Thanks.
  8. TeKeela

    TeKeela Member

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    How about this. While sitting at the dock, the water in the sump box remains level, switch is only on with incoming grey water. Nothing to pump unless the float is tripped from incoming grey. Head out, the same amount of water remaining sloshes indicting more water than is actually there, the float switch turns on the pump, the pump tries to send the water up a 4' head, water leaves box, switch turns off, no water makes it out, water goes back down to the box. Switch trips, do the circle again. The switch too low and sensitive when away from the dock..

    The pump will run and run if the switch is too low and there is not enough water to make it over the hump and reduce the quantity in the box. The pump does not know if it is getting water, it just follows orders from the switch.

    Or some slight variation thereof...
  9. enproep

    enproep New Member

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    Thanks Narwhal:
    Your description in paragraph one is spot on with the following underlined change "While sitting at the dock, the water in the sump box remains level, switch is only on with incoming grey water. Nothing to pump unless the float is tripped from incoming grey. Head out, the same amount of water remaining sloshes indicting more water than is actually there, the float switch turns on the pump, and the pump sucks up water and air until it stops sucking up water for some reason (the air it sucks up creates and airlock?) It then runs continuously without sucking up any water. When we get back to the dock we can take a shower and the sump box fills up and overflows even with the pump running (it's running but not sucking up water). I then disconnect it and then reconnect it and it works fine."
  10. TeKeela

    TeKeela Member

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    Well I assume it is a rule float switch you are using and if they haven't changed it I believe it is a simple mercury switch in there so I don't see how that can hang up as it is all gravity. Is the switch housing hanging up? binding against the side of the box? or as Pascal mentioned, hair and scum. You have not indicated the position of the switch and water in the box while the pump is running dry.

    While the pump is running dry, is the switch up and water holding it up? Or just up, semi-stuck and not enough water for the pump.
  11. enproep

    enproep New Member

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    Thanks Narwhal:
    Sorry if I'm not explaining the situation better, but the pump continues to run while the box is completely full of water. The switch is floated up that is why the pump is running, but the pump is not sucking any water. It just runs and runs. It's very strange and as mentioned, when I disconnect (cut) a power wire then reconnect it, it will drain the box and everything works great until I go out again.
    Thanks for your comments and sorry it's a bit hard to describe.
    Bob
  12. Capt J

    Capt J Senior Member

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    I have seen the Rule 3700 GPH pumps airlock easily and a lot. I've had several boats with 3700's do it. Your float switch is too low and underway when the sump box is tilted the switch doesn't shutoff prior to the pump sucking air. You can cut a block out of starboard and screw it down then screw the float switch to that, a piece of 3/4" starboard would probably solve the problem. An anti-syphon valve just after the pump will probably help as well. As what gets pumped out, doesn't return to the sump and slosh around.

    Or, you could change the float switch to an ultra switch jr. The cylinder type float switch (which is less sensative to rocking and rolling) and hose clamp it directly to the pump and adjust height as needed.

    Last ditch resort, I drilled a small hole on the discharge nipple of the pump, very close to the pump so the hose doesn't cover it. (make sure to do it angled downward and to the side a little otherwise it will spray all over in there.) This allowed the pump to not airlock.
  13. Capt Ralph

    Capt Ralph Senior Member

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    3700 may be a bit large for a small sump.

    Also, if you're using a smart or electronic switch, with a delay, you will have this problem.
    Never put any fancy electronic switch in a small sump.

    If you have ever tore a pump base apart, you will find a small nick in the upper ring to let air out and help prevent air lock. Sometimes it works. Sometimes it gets clogged by soap or junk. Sometimes it just will not prime and pump.
    After years of experimenting with lots of shower sump problems in different boats, There is one easy fix; Put a 1/2 to 3/4 inch starboard block under the float switch. The pump base will always have water in it now.
  14. Capt J

    Capt J Senior Member

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    Rcrapps. The sump might be fairly large. I think Carver did one large sump for the whole boat on some of their boats and it was fairly large. I'm also thinking they chose a 3700GPH pump (which is way overkill considering the boat's freshwater pump probably pumps a maximum of 400gph), because the sump has a lot of head to the discharge.
  15. Pascal

    Pascal Senior Member

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    You shouldn't have to cut a wire to shut down the pump... Or even disconnect the wire. You should have a circuit breaker or a fuse somewhere to protect the float switch and pump.

    In a normal instal, each pump has two CB. One for the manual feed at the helm(s) and one for the float switches. Ideally the CBs for the float switches should be on an auxiliary panel near the battery switches, but sometimes it could be an in line fuse near th battery

    An alternative to screwing blocks into the bottom of the sump, or hull :) , is to mount the switch on a small L bracket and use hose clamp to attach it higher on the pump body....
  16. Mikeyessir

    Mikeyessir New Member

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    Hello we had a similar problem in our air con sump box. Check your one way valve isn't sticking, trace pipe work to skin fitting. Should be a one way valve inline.

    Ours also air locked frequently but we also found a sticky one way valve to be an issue.

    Good luck.
  17. Capt Ralph

    Capt Ralph Senior Member

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    AND, I had a case where hail had restricted the hose. Pump never could over come the restriction to get primed real well. Just like Mikeyersir was describing above, any restriction may be giving you more problems.
    Monthly doses of genuine Liquid Plumber took care of that.
  18. Pascal

    Pascal Senior Member

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    How does hail get in the hose ? :)
  19. Capt J

    Capt J Senior Member

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    I thought the same thing. I think he meant to say Hair.....
  20. Capt Ralph

    Capt Ralph Senior Member

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    Oh hail, yes I ment hair.