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Old 07-04-2018, 11:43   #1
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Vented Loops

I have a Wilcox-Critteron (sp) Skipper manual toilet that is around 20 years old. It was rebuit once during that time and has operated fine until just recently it was overflowing with seawater coming from the intake line. That intake line comes from the thru-hull to a vented loop and then down to the pump on the toilet. When I first installed it about 20 years ago and when I rebuilt it some 6 years ago I had to prime the pump by letting the bowl fill with fresh water and then it would pump. The Skipper toilet had a foot pedal which must be depressed to allow water intake and when released it stops the flow of intake water. Last week the water continued to flow even after releasing that pedal and the bowl filled up and flowed over. I thought the vent value would prevent this from happening even if I had a defective pedal. I took the vent value apart and it was sealed shut so it was not really operating as a vent. Bought a new vent valve and made sure the duck bill was installed properly and now I could not get water to flow unless I put tape over the vent which defeats the purpose of the vent. I now pump the sea water in with the tape over the vent and then release the tape to keep the bowl from flooding. The only thing I can figure is that the pedal has become defective whereas for years that was the only thing keeping the water from cyphing (sp) into the toilet bowl. (The vent valve is installed high between the thur hull and the pump. It seems that in order to work that valve must be installed after the pump...ie between the pump and the intake to the rim of the toilet)
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Old 07-04-2018, 12:11   #2
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Re: Vented Loops

The duckbill is not going to help this. Think about what is happening and you can see the problem. The pump is trying to suck seawater up and over the loop. But the duckbill is designed to let air in if there is suction on the line. So it just lets in air.

I suspect your installation is wrong. The sea water pump should be plumbed in between the seacock and the anti-siphon loop. That way the pump is pushing water past the loop and not sucking on the loop. Most heads have fittings that allow the anti siphon loop to be connected between the pump and the bowl.

If the anti siphon loop is connected directly to the thru hull that is wrong.
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Old 07-04-2018, 13:01   #3
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Re: Vented Loops

The air valve ("duckbill") in the vented loop is designed to allow air into a line through which water is being pulled (i.e. the intake line)...which is why, on most toilets the loop belongs between the pump and the bowl. But your Skipper isn't designed like most toilets...there is no line between the pump and the bowl. And if the toilet is below the waterline, the foot pedal only opens a valve to LET flush water come into the bowl (the pedal has to be pumped to bring it into toilets installed above the waterline). I think you're correct that the valve (not the pedal) has become too worn over the years to shut off the flow of water...and unfortunately Thetford discontinued the entire W-C product line when they bought W-C. so it's unlikely a replacement valve is still available--at least not from Thetford. You might get lucky and find one on eBay or other aftermarket source, but meanwhile about all you can do is continue to cover the hole in the top of the loop to allow the pump to bring in water and uncover it to shut off the flow of water.

I just an afterthought that MIGHT work: The Groco Model K is very similar to the Skipper...and uses the same flush water pedal. I doubt if they're interchangeable, but maybe...and even if they aren't Groco might be able to modify theirs to work in a Skipper. You have nothing to lose by giving 'em a call: (410) 604-3800 Ask for Patrick.

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Old 07-04-2018, 17:40   #4
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Re: Vented Loops

Thanks Dan and Peggy. I suspect all these many years thinking that I had a working vent value, I was wrong. That old vent valve(no duck bill) seems to have some sort of silicon plugging up the hole which allowed the pump to take suction. It was plumbed by the marina when the boat was brand new 33 years ago.

Which brings up another question about vented loops. I have a vented loop on the seawater side of the Perkins 4-108. It is after the raw water pump and is plumbed high above the engine. Again that vent is to prevent cyphening (sp) of water which could fill up the exhaust muffler and back water into a shut down engine. However to have a cyphen it must go through the raw water pump and I would think the impeller would prevent a cyphen from developing. Sorry about the spelling.
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Old 08-04-2018, 13:42   #5
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Re: Vented Loops

Your engine cooling is plumbed correctly. The rubber vane pumps are not completely one-way. Water can get past the vanes even if the pump is not turning. So itís critical that the anti-siphon work correctly. On our boat the duckbill is replaced with a small nipple. Then a small hose goes from there to a spot on the hull high above the water line. That way I can see water dribbling out when the engine is running. Then i know the anti-siphon system is working and not plugged up which is common.
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Old 28-04-2018, 21:04   #6
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Re: Vented Loops

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Originally Posted by transmitterdan View Post
Your engine cooling is plumbed correctly. The rubber vane pumps are not completely one-way. Water can get past the vanes even if the pump is not turning. So itís critical that the anti-siphon work correctly. On our boat the duckbill is replaced with a small nipple. Then a small hose goes from there to a spot on the hull high above the water line. That way I can see water dribbling out when the engine is running. Then i know the anti-siphon system is working and not plugged up which is common.
Why is their water dribbling out when the engine is running? My loop is not vented but the point where it enters the exhaust riser is above both the waterline and the exhaust hose outlet in the transom so I donít see how a syphon could form. Boat is 30 years old with no iproblem so far. However, I thought I might add a vent as all manuals seem to recommend but not sure I will if it is going to dribble constantly while underway.
Do I need a vent in the exhaust loop and if so will it dribble constantly? Thanks
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Old 03-06-2018, 20:31   #7
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Re: Vented Loops

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ID:	171034 What type of anti-siphon valve have I got here on my Perkins 4-108 raw water outlet? Anyone know! Thanks.
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Old 04-06-2018, 02:52   #8
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Vented Loops

I don't know the brand for sure but this is a fairly common gravity type one way valve. Water pushing up to the top closes the valve. When the water flow stops then gravity lowers the valve and lets air into the pipe thus breaking the siphon. This one looks like a Groco design but not sure.

These valves need to be checked quite regularly because salt deposits can freeze up the mechanism and they get stuck in either the open (will leak water) or closed (stop workin as an anti-sihon) position.

Edit: I think it is not a Groco. Probably not a US design.
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Old 04-06-2018, 09:27   #9
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Re: Vented Loops

Why is a vent needed on the engine raw water line? Isnít it a closed system with the inlet seacock at one end and the exhaust at the other? The only potential problem is if the water backs up through the exhaust riser to the exhaust valves, but this canít happen if the water inlet point on the riser is below the top of the riser on the outlet side as it is on most designs??
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Old 05-06-2018, 00:17   #10
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Vented Loops

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Why is a vent needed on the engine raw water line? Isnít it a closed system with the inlet seacock at one end and the exhaust at the other? The only potential problem is if the water backs up through the exhaust riser to the exhaust valves, but this canít happen if the water inlet point on the riser is below the top of the riser on the outlet side as it is on most designs??
A vented loop is usually required to prevent water siphoning into the stopped engine. Many engines have been ruined because the raw water system siphoned salt water through the seacock. It will eventually fill up the muffler and then flow into a cylinder via an open valve. Read the installation instructions for any marine engine to see how and where to put the anti-siphon device.
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