Blake’s Lavac Can Someone Please Explain How it Works

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KeelsonGraham

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Yes, I know, I’m being dim, but I can’t for the life of me figure out the principle of operation of our LAVAC.

Ours works really well but I just don’t understand how a simple Henderson pump can pump out effluent and also create a vacuum to let flushing water in.

Does the waste get drawn up into the pump before it’s then ejected back via the bowl? Is there a non return valve at the base of the bowl? It’s a complete mystery to me.
 
So simple! It works by creating a vacuum in the bowl - you can see the lid move down slightly as you pump and the seal on lid seats against the rim of the bowl. The vacuum brings in the new water. When you stop pumping the seal slackens off, the vacuum is released and you can lift the seat for the next use. You may well find a little vent hole in the top of the inlet loop which helps release the vacuum. All explained here marinechandlery.com/media/userfiles/lavac.pdf
 
The pump draws waste out of the loo bowl, from the large spigot in the base.

This reduces pressure in the bowl, so water is drawn in through the smaller spigot.

That's all there is to it.
 
I also think they are rubbish. Too clever by half. A solution in search of a problem.
It is neccessary to be able to pump in flushing water with the lid open, so you can see what's in there, and clean the bowl for the next person. Had the designer ever used a toilet, I wonder? Perhaps he had a bit of a phobia about looking at skidmarks.
They belong on the scrapheap of history.
 
Tranona said:
So simple! It works by creating a vacuum in the bowl - you can see the lid move down slightly as you pump and the seal on lid seats against the rim of the bowl. The vacuum brings in the new water. When you stop pumping the seal slackens off, the vacuum is released and you can lift the seat for the next use. You may well find a little vent hole in the top of the inlet loop which helps release the vacuum. All explained here marinechandlery.com/media/userfiles/lavac.pdf
Click to expand...

Thank you for responding, but this isn’t an explanation of what must be going on. Nor is the PDF.

A vacuum in the bowl, with ambient pressure outside won’t expel the waste. Rather it would cause waste from the holding tank to flow back into the bowl. As this doesn’t happen, there must be some other mechanism at work. Also, to prevent said reverse flow, should there not be some kind of non return valve at the bowl outlet? No sign of one in the diagrams.

As I understand it, the purpose of the vacuum is to draw fresh water in, after the wate has been expelled.

So, what process does the expelling?
 
KeelsonGraham said:
So, what process does the expelling?
Click to expand...

The pump does the expelling - the inlet of the pump is connected to the toilet bowl, such that it sucks from the bottom of the bowl (part 4 on the exploded drawing), and then pumps out to either to the holding tank, or out of the boat via a sea cock. The inlet and outlet valves in the pump act as non return valves.
 
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I don’t understand this response. The inlet from the seacock goes to one side of the pump. The outlet from the pump goes to the bowl. So, I can see how inlet water is drawn by the pump through the seacock through the pump and into the bowl.

The pump absolutely doesn’t expel waste via the inlet seacock.

So what forces the waste out of the bottom of the bowl???????
 
KeelsonGraham said:
Thank you for responding, but this isn’t an explanation of what must be going on. Nor is the PDF.

A vacuum in the bowl, with ambient pressure outside won’t expel the waste. Rather it would cause waste from the holding tank to flow back into the bowl. As this doesn’t happen, there must be some other mechanism at work. Also, to prevent said reverse flow, should there not be some kind of non return valve at the bowl outlet? No sign of one in the diagrams.

As I understand it, the purpose of the vacuum is to draw fresh water in, after the wate has been expelled.

So, what process does the expelling?
Click to expand...
The first few pumps expel the soil and a joker valve in the pump prevents return to the bowl. At the same time this creates a low pressure which draws in flushing water. Wait a few seconds and a few more pumps flushes the system through and draws some more water into the bowl which can be left or pumped out by lifting the lid and pumping. If the vacuum tkes too long to break, just enlarge the bleed hole a bit.
They are excellent bits of kit. Previuous boat had one for 18 years until sold, current boat has one we have owned for 22 years. Neither have given any trouble despite some some epic attempts to beat them.
 
KeelsonGraham said:
I don’t understand this response. The inlet from the seacock goes to one side of the pump. The outlet from the pump goes to the bowl. So, I can see how inlet water is drawn by the pump through the seacock through the pump and into the bowl.

The pump absolutely doesn’t expel waste via the inlet seacock.

But how does that same process expel waste.
Click to expand...

No, the inlet from the flush water inlet seacock goes direct to the toilet (part 53 on the drawing). This is the pipe with the air inlet valve to allow the vacuum to break.

The outlet from the pump goes to the outlet seacock (or holding tank).

I think you are confusing the plumbing of a Lavac with the plumbing required for a Jabsco or similar. In the later, the pump is a two way affair, which pumps both flush water in, and waste out.
 
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DanTribe said:
The first few pumps expel the soil and a joker valve in the pump prevents return to the bowl. At the same time this creates a low pressure which draws in flushing water. Wait a few seconds and a few more pumps flushes the system through and draws some more water into the bowl which can be left or pumped out by lifting the lid and pumping. If the vacuum tkes too long to break, just enlarge the bleed hole a bit.
They are excellent bits of kit. Previuous boat had one for 18 years until sold, current boat has one we have owned for 22 years. Neither have given any trouble despite some some epic attempts to beat them.
Click to expand...

So, the pump creates overpressure in the bowl first, and then a vacuum?
 
Kwik Decision said:
No, the inlet from the flush water inlet seacock goes direct to the toilet (part 53 on the drawing). This is the pipe with the air inlet valve to allow the vacuum to break.

The outlet from the pump goes to the outlet seacock (or holding tank).
Click to expand...

…..Oh! I must have got it completely wrong, looking at it through a cramped inspection hatch. I thought the pump was plumbed in series with the inlet side of things. Now, looking at the PDF it looks like it’s actually plumbed in series in the outlet side of things. So, all the waste goes through the pump’s chamber. Is that correct?
 
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KeelsonGraham said:
So, the pump creates overpressure in the bowl first, and then a vacuum?
Click to expand...
The pump takes from the bottom of the bowl and pushes soiled water out towards seacock outlet or holding tank.

Its what one side of all other toilets do but without their separate pump for flush water. Instead here the flush water is connected to inlet on the bowl so if bowl sealed by lid, then water leaving at bottom pulls fresh in from top
 
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