Two water tanks

[jimi]

I've got two water tanks on the boat and normally I leave them both switched on and let the plumbing system decide which one to use. However on a recent holiday I decided to only have one switched on at a time as an alerter as to when to fill up. However when I did this the system refused to switch to the full one and would only use it when the empty one was filled up, run and then switched ... has anybody else experienced this? I can't think what might cause this so any hints would be greatly appreciated.

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[tony_brighton]

Are they both vented properly - maybe an air-lock?

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[MainlySteam]

Bit hard with so little information. But one possibility is that if you sucked the first tank dry it may be that when you switch the other one in the pump is not repriming. Most modern water supply pumps are multiple port positive displacement and should self prime, but if they have air in them and in the suction side piping, but the piping downstream towrds the outlets still has a head eg some water retained in it because of the piping arrangement, then they often will not prime (even though they are positive displacement.

We switch our tanks in the manner you tried and have no problem. However, the similar kind of pump used to pump our grey water sump out will often not prime itself if there is air in it and in the suction side piping (say after cleaning the suction side filter and the line has drained) against the head it sees pumping through the thru hull which is about 800 mm below the water line.

Long shot as I say without further info - you may not even have such pumps!

John

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[SlowlyButSurely]

Sounds like the empty one is not shutting off and the pump is sucking in air from the empty tank.

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[ccscott49]

Bloody hell! Not you and tanks again!

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[Spacewaist]

Happens occasionally on mine if I am at anchor. Simply opening the stopcock in a 3/4in pipe isnt enough to let the 350 or so litres of reserve water through to the bottom tank. Need to shake the boat around to get it started. Best to switch tanks at sea; once it has started it does keep going though.

I assume it has something to do with air having to get back up the pipe - cant be properly vented...... Never troubled me enough to fix it.



<hr width=100% size=1>A pontification from the Panjandrum of orotund bloviation AD2003

 

[MainlySteam]

praps there is a third water tank??

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[jimi]

Yes that was my first thought, I opened the deckfiller caps as well just in case but it still did'nt work. The pump is above the taps and as someone mentioned perhaps its a little bit of air getting through the shut off empty tank?

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[Ohdrat]

Re: third water tank

sssshhhhhhThat'll be the 2nd holding tank that Jimi has yet to discover/forums/images/icons/wink.gif

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[MainlySteam]

Jimi, if the pump is above the taps (unusual?) then either you have a pump suction lift problem, which to occur would mean that your second tank has to be lower than the first before it would be so (as the pump obviously lifts ok from the first) or you have an air leak into the suction side from either the empty tank through its isolation valve or the outlet pipework associated only with that tank (unlikely as it would likely leak water as well, which you would have seen). If the isolation valve is leaking it is obviously easy to check by just putting enough water in the empty tank to cover the outlet and seeing what happens when pumping with the valve closed.

John

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