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Old 01-10-2016, 08:22 AM   #1
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Vented Loop Use

I am closing in on the install of an anchor washdown rig. When we were in the yard last month, we had them install a new thru-hull (3/4") to dedicate to the task. As I put the plan to paper, I am wondering if I need to run it (raw water) thru a vented loop prior to the pump's intake side?

And while I am at it, I noticed the air conditioner system is not looped above the waterline either. Should I consider making that change as well?
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Old 01-10-2016, 09:57 AM   #2
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Vented loops are to prevent siphoning. They are only needed if the outlet is below the waterline. For a wash down system the outlet is at deck level, so no vented loop is needed. Similarly, a vented loop isn't used for a below waterline system that is closed where siphoning isn't a problem. For example, there is no vented loop on the raw water intake for the engine although the engine is below the waterline since getting water into the raw water side of the engines cooling system is not a problem. On the other hand there is often a vented loop between the outlet of the engine's raw water system and the injection point of the exhaust elbow (if the exhaust is below or only slightly above the waterline) since siphoning of water into the injection elbow can result in filling any cylinders with open exhaust valves with water.

Another example is overboard discharge lines for marine heads. If the head is below the waterline and there is a direct overboard discharge option, there will be a vented loop in the discharge line to prevent siphoning water into the head. The vented loop isn't needed if the head is above the waterline.

Another example is bilge pumps. Some boats have bilge pump outlets below the waterline for aesthetic reasons. That sort of installation definitely requires a vented loop and the vented loop needs to be checked FREQUENTLY. However, if the bilge pump outlet is above the waterline at all times there is no need for a vented loop.
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Old 01-10-2016, 10:01 AM   #3
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TD

Good reply. It perfectly describes the 5 vented loops on my vessel and the other locations that don't require vented loops, such as the 3 ACs.
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Old 01-10-2016, 10:15 AM   #4
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Purely for a siphon break and or vent. If this were done on a supply line it would most likely be a bear to prime.
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Old 01-11-2016, 07:17 AM   #5
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Thanks soooooo much for the thorough explanation. That helps a lot.
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Old 01-11-2016, 06:15 PM   #6
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They are only needed if the outlet is below the waterline.
Just to be specific, a vented loop is only necessary if the outlet could ever be below the fully heeled waterline of the boat.

So for instance, if the outlet is near the water line when the boat is upright, it is likely below the waterline when the boat is heeled over.
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Old 01-26-2016, 07:17 PM   #7
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And the pump if it is a rotary impeller the water cannot freely flow through the pump to the outlet, so no vented loop needed.
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Old 01-26-2016, 07:40 PM   #8
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Originally Posted by sdowney717 View Post
And the pump if it is a rotary impeller the water cannot freely flow through the pump to the outlet, so no vented loop needed.

Not sure I'd bet my boat on that. Unless you can always guarantee that the condition of the pump is near pristine. No vanes missing, and no pump end plate or housing wear.

Risk low, consequence medium.

That said, I'm one of the few that keep hull valves shut to isolate the system when not in use.

I don't consider a check valve to be adequate isolation either.
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Old 01-26-2016, 07:43 PM   #9
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Originally Posted by sdowney717 View Post
And the pump if it is a rotary impeller the water cannot freely flow through the pump to the outlet, so no vented loop needed.
Not sure that's is true....I would have to double check...there is still a remote possibility of siphoning back if the impeller is damaged or worn.

I think I actually had it happen to me with a rubber impeller macerator so when I replumbed, I used vented loops.


Thanks Spy...beat me when I was editing...good catch.
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Old 01-26-2016, 07:59 PM   #10
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Ok, if the impeller is destroyed, maybe some flow, just worn, less flow.
And then some electric impeller pumps have an electric shutoff valve, or one way valve. I have several of those on the boat for pumping fresh and raw water. So if the impellers failed, they could not reverse siphon.

Most likely a worn out impeller is not going to be sucking water so you will notice it and fix it. Most of us having power boats, they don't heel over normally like a sailboat and my head is above the waterline. I actually have zero vented loops on my boat, not even for the Lectrasan head. Any such bilge pumps or heads, the discharge line loops up high maybe 18 inches or so, then drops back down and exits a few inches above the water. The head discharge is slightly submerged. The raritan head also has a joker valve.
I have in pounding waves seen the head fill up somewhat with raw water, must be back flowing in past the Lectrasan and that joker valve.
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Old 01-26-2016, 08:14 PM   #11
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Well I have seen pretty chewed up impellers work good enough equipment doesn't overheat or you notice a flow reduction....then I have seen some that look great and the set makes them unusable.


And like I posted...I am almost sure I had a standard rubber impeller macerator siphon that was working fine with a good impeller. Pretty common on smaller vessels.
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Old 01-26-2016, 09:03 PM   #12
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Any such bilge pumps or heads, the discharge line loops up high maybe 18 inches or so, then drops back down and exits a few inches above the water
That is pretty much the case, and I should have eliminated the term "vented" in my last post on this thread. My Hatt had several discharges right at the water line, for bilge, AC, sumps, etc where the "source was below the waterline. All had hose loops well above the maximum heel of the boat, except for the waste macerator, which discharged at the bottom of the boat, well and always under the water line, I added a vented loop to that which greatly improved performance. (The original design of the boat had valved individual head discharges to overboard or the tank, but not for a tank macerator pump).
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Old 01-28-2016, 07:01 AM   #13
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I tried to imagine how a suction pump getting water from a thru hull could ever be a reverse siphon risk.
My own situation must be similar to all others, the thru hull water source is at a low point in the hull.
The pump is up much higher, mounted on plywood shelf.
The output hose is secured higher than the pump to the hull
The end of the hose is up on the deck, so way above the water.
The end of the hose being out on deck, cant ever fill up the bilge.
So where is the siphon risk? The entire water circuit is sealed away from the bilge.

I can understand a bilge pump being a siphon risk since one end of the water circuit sits open in the bilge, but a pump like mine does not do that.
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Old 01-28-2016, 07:27 AM   #14
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Originally Posted by sdowney717 View Post
I tried to imagine how a suction pump getting water from a thru hull could ever be a reverse siphon risk.
My own situation must be similar to all others, the thru hull water source is at a low point in the hull.
The pump is up much higher, mounted on plywood shelf.
The output hose is secured higher than the pump to the hull
The end of the hose is up on the deck, so way above the water.
The end of the hose being out on deck, cant ever fill up the bilge.
So where is the siphon risk? The entire water circuit is sealed away from the bilge.

I can understand a bilge pump being a siphon risk since one end of the water circuit sits open in the bilge, but a pump like mine does not do that.
But...reverse it all.. what if the pump is 2 feet lower than the thru hull....and the tank it is pumping from is even lower?

Yep...back siphoning is possible, actually probable unless you stop the pump when it is cavitating from the tank being empty and there are enough bubbles in the hose to break the siphon or the pump is 100 percent backflow proof (cheaper macerators are not).

Like many discussions here in TF....there are many variations on a theme.
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Old 01-28-2016, 08:08 AM   #15
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All my pumps, except for bilge pumps, exist above the waterline, my situation may be different than others. Why design it so it can be inherently more dangerous, if you don't have too.
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Old 01-28-2016, 08:26 AM   #16
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I may end up splitting the difference and just mount the pump on a box that gets it at, or slightly above, the waterline in the bilge. Problem solved.

The big roadblock right now is finding a route from the pump location to a suitable output hose connection location. Several bulkheads are in the way and I doubt I will get it as far towards the bow as I would like. So I may end up punching it out of the side of the pilothouse. Still, even to route a 3/4" I.D. hose to there is going to take some doing.
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Old 01-28-2016, 08:44 AM   #17
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All my pumps, except for bilge pumps, exist above the waterline, my situation may be different than others. Why design it so it can be inherently more dangerous, if you don't have too.
Even with the pump above the waterline, it is still subject to siphoning if the holding tank is below the waterline and no vacuum break is installed.. assuming pump leakthough is possible. Raritan Engineering discusses it in their ELECTROCAN install.

It's deigned that way to make use of existing thru hulls, keep them to a minimum and place the pump in an easy to service location.

With a vacuum break it is no more dangerous than any other underwater install AND I learned my lesson about letting the pump go till it pumps air so there is a backup vacuum break to boot. Fortunately the pressure head for the siphon isn't great and it is only a 1" thru hull just barely underwater so dangerous isn't a word I would use...at least in relation to all the other boating hazards out there.
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Old 01-28-2016, 08:44 AM   #18
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I may end up splitting the difference and just mount the pump on a box that gets it at, or slightly above, the waterline in the bilge. Problem solved.

The big roadblock right now is finding a route from the pump location to a suitable output hose connection location. Several bulkheads are in the way and I doubt I will get it as far towards the bow as I would like. So I may end up punching it out of the side of the pilothouse. Still, even to route a 3/4" I.D. hose to there is going to take some doing.
would it be easier to use CPVC pipe and glue it together in shorter sections?
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Old 01-28-2016, 08:53 AM   #19
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Even with the pump above the waterline, it is still subject to siphoning if the holding tank is below if no vacuum breal is installed.. Raritan Engineering discusses it in their ELECTROCAN install.

It's deigned that way to make use of existing thru hulls, keep them to a minimum and place the pump in an easy to service location.

With a vacuum break it is no more dangerous than any other underwater install AND I learned my lesson about letting the pump go till it pumps air so there is a backup vacuum break to boot. Fortunately the pressure head for the siphon isn't great and it is only a 1" thru hull just barely underwater so dangerous isn't a word I would use...at least in relation to all the other boating hazards out there.
Yes minimal risks here. The OP is only talking of a salt water wash down.

I actually do not have a holding tank. I have Lectrasan and Raritan electric macerator head. It all is above the waterline and goes overboard with every flush. I have thought of adding a holding tank, and easiest solution for me is above the waterline placement. With my system, I have a clean smelling boat. Some people do have issues with holding tanks and odors. Having a vented loop venting the tank I wonder if that will make odors even with a hose running from it to outside of the boat.

I have 6 Rule bilge pumps. two 3700, two 2000, one 500, one 300. All are with their own hoses. I did that for redundancy and who knows when it might be needed, although it never has, not since I got the boat back in 1998.
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Old 01-28-2016, 08:57 AM   #20
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Might need to figure out a holding tank if you ever cruise south through North Carolina.


I always use the electroscan to pump into the holding tank.....no smells...well I never let it sit in there once out of the no discharge zones.
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