Vented loop question

[ruvane]

Sorry of this is stupid or has been asked before, but I'm trying to figure out how the heads pump will pull water if there's a vented loop. Will the air not be drawn in more easily than the water which is harder to pump and also below the level of the pump?

TIA.

 

[Evadne]

Even if the vent is a simple hole, it is much, much smaller than the hole for the water to be drawn in from, so although they experience the same pressure the flow will be far greater through the big hole, though pumping will as you say be impaired. In practice I believe there is a spring valve (I've not had the urge to take mine apart and look) that lets air in at lower pressures but closes up at the higher pressure generated by a pump.

 

[alldownwind]

You don't normally have the loop in the section of pipe you are sucking from. You have it in the section you are pumping to, i.e. the section between the pump and the bowl (assuming you're talking about the inlet pipe).

 

[ruvane]

Alldownwind,

From what I have on my boat and from what I've seen in "typical" diagrams, there's a vented loop on the intake.

Is this not the way to do it?

 

[Evadne]

I would agree, as it's to stop siphoning, which shouldn't be able to happen through the outlet side of the pump anyway.

 

[Bilgediver]

This is how it should be on toilets and on engine cooling systems.....The vented loop comes AFTER the pump. Oh yes....water can syphon through a pump, that is exactly why we fit vented loops It can only syphon the one way, but that is one way too many.

John

 

[Salty John]

The vented loop comes after the pump, but to answer your original question: There is usually a valve in the vented loop to let air in but prevent it being pumped out - sometimes as simple as a plastic flap or a spring supported valve, sometimes more elaborate on electrical pumps. You need to clean the valve from time to time.

 

[ruvane]

Thanks guys, now I'm more confused than before!

These guys http://www.aquafax.co.uk/pdfs/03_cat.pdf recommend vented loops before and after the heads, although no pump is shown so it's still not obvious.

Surely the vent is to stop the heads syphoning water into the boat, either from inlet or outlet?

In theory, is there meant to be some water left in the bowl for "good" operation?

 

[Salty John]

There is a vented loop in the clean water line, the small pipe which brings water only from the pump to the bowl. Then there is a vented loop in the large discharge pipe taking the waste from the heads to the discharge seacock. There is no vented loop in the line from the inlet seacock to the pump.
The vented loop on the discharge side can be problematical because it can allow odour into the boat. They sometimes have a hose barb on them so you can run a small diameter pipe from the vented loop vent to the outside - being sure it is still above the water line of course.
You can pump the bowl dry. Before using it you can allow some water in.
If you pm me an e-mail address I will be pleased to send you a scanned picture of a typical installation.

 

[davidbains]

Techno are you talking about a Lavac or an inferior other make?
The Lavac should have a small hole on the inlet line, above water level. You can temporarily close
it with a finger to ensure more water remains in the bowl, apart from that it should be open.

 

[Blue5]

What a good question, I was told by a marine engineer that I should should put a vented loop on the inlet before the pump.

I dutifully did as instructed and the only way to fill the bowl is to put the finger over the vented loop while pumping water.

Even the experts make mistakes

 

[nealeb]

My heads have the pump integrated, so no chance of putting a vent between pump and bowl. Both inlet and outlet hoses (i.e. between each half of pump and respective sea cocks) have vents fitted and the inlet one seems to have no effect on pumping efficiency. At least, a lot less effect than the guy who fitted the joker valve on the pump outlet the wrong way round! If the heads are below waterline you are relying on the integrity of the pump seal to stop the boat flooding unless there is a syphon break somewhere, if someone leaves the sea cock open.

 

[Roach1948]

The amount of water left in the bowl depends on the size of vented loop hole/valve in the discharge pipe. The larger the size the more water is left in the bowl.

 

[Strathglass]

With the Lavac heads there is suppied two small plastic vents.

Those are fitted to the intake pipe at the top of the loop between the water intake and the heads.

Which one is used depends on whether the heads is above or below the waterline.

The vents have different sizes of holes.

The answer to your question really depends on which heads have been fitted.

Iain

 

[syfuga]

I don't blame anyone being confused!

In my view, the vented loop for the input should be between seacock and heads pump. This is how I have installed it on two boats and it works fine. The amount of air getting in is adjustable, but only needs to be enough to break the syphon.

These were and I suspect still are mandatary for charter certification. The need for it was demonstrated admirably when I changed the electric macerator pump on my Jabsco for a normal manual one. I woke up in the night for the usual and stepped into a shower tray full of water.... /forums/images/graemlins/blush.gif

 

[mono]

On my Lavac inlet tube, I have installed a tee pipe fitting and run a length of tube from that - terminating in a seacock (modified to operate like a tap). This is in a handy position - so no contortions to hold finger over hole in tube whilst pumping.

When pumping, the tap is closed - closing the vent. As soon as pumping is finished, the tap is opened to vent the pipe and the syphon is broken.

Goodness knows how the previous owners coped - I assume they had to turn off the inlet seacock every time they wanted to break the syphon.

 

[VicS]

A correctly installed Lavac (with the correct size of air bleed nipple fitted in the inlet pipe according to the position relative to the water line) requires no messing about with fingers on holes, tee pieces, small tubes extra taps or valves. You follow the makers instructions ie pump the specified number of strokes, pause for a while and then pump a few more. Simple . (You can leave the inlet and outlet seacocks open or shut between uses according to your personal whims.)
If you have to block the hole off then it is too big or the plastic insert with the pin hole is missing (or the pump is about a million years overdue for an overhaul)

Google will find a set of installation and operating instructions

 

[mono]

You're right. I had a Lavac on my first boat that worked ok without syphoning. I'm sure it had a vent on the inlet because I seem to recall it wheezing when I used the pump. I also recall that it was slow at allowing air into the system to break the vacuum seal at the heads after pumping.

My present boat had nothing fitted on the inlet - which became apparent the first time I pumped the heads! Hence my solution, which had the merit of both working perfectly - allowing air to enter the pipe much more quickly than a small vent, thereby breaking the vacuum seal at the heads quicker and allowing a faster pump out. It also had the merit of using materials to hand - i.e. it cost me nothing.

Happy ablutions.

 

[VicS]

[ QUOTE ]
I also recall that it was slow at allowing air into the system

[/ QUOTE ] Probably because the pin hole in the vent needed cleaning. They do from time to time.

In all probability the vent plug was simply missing from your present installation. If there was a 5mm hole in the pipe then that was definitely the case. It would have been a quick and easy soluton to have fitted a new vent plug. (A black one for toilets below the waterline, a white one if on or above). Then you would have had an installation that worked as it should with no long delays waiting for the vacuum to be released.
The full Lavac instruction manual is at http://www.blakes-lavac-taylors.co.uk/pdf/lavac.pdf

 

[mono]

Thanks for the info and link. The current system does work well but anything that simplifies using the heads when visitors are onboard is a good idea.