Vented loop before mixing elbow.

[GuyT]

I have an H34 with a Yanmar 3GMF with a fresh water heat exchanger. I recently replaced all my engine hoses and found I could not stop a small leak in the vented loop between the hot water discharge and the mixing elbow so... I 86'd it!Now its a direct connection and no leaking.Any one with some comments of why I need it, or will I be OK without it.

 

[walmsleyc]

Yes !!!!!

There are a number of factors, but if the injection point in near or below the waterline, then there is a significant risk of a siphon of water and the possibility of flooding and damaging your engine. If your engine had one to begin with, then it has probably already been determined that it is necessary.Chris

 

[KorculaChris]

YESSS!!!

The vented loop is there to stop your exhaust manifold and any cylinder with open exhaust valve filling up with water. This can destroy the engine if you crank it with a cylinder with water in it as well as the corrosion issue.

 

[NYSail]

When a vented loop / siphon break

leaks, it either needs maintenance or needs to be replaced. Get a new one as soon as possible is my advise. Costs alot less than a new engine or rebuild due to water back-flowGreg

 

[RichH]

Remodel your next siphon break valve .....

.... take the valve OUT of siphon break, connect in a long run of tygon, etc. plastic tube and re-mount the valve MUCH HIGHER vertically than the normal siphon break valve position (on the elbow). With the valve portion MUCH HIGHER than elbow most times the water will not rise to meet the valve before the pressure closes the valve..... and the pressure and air flow only closes the valve. What 'spoils' such valves is the eventual accumulation of 'salts' on the valve (the water evaporates leaving a thin carbonate fouling which eventually makes the valve no longer operate properly .... if the water cant reach the valve (in its MUCH vertically HIGHER position) the fouling/failure will be much less often. ..... or carry spare siphon break valve inserts and change on a routine basis.

 

[Stu Jackson]

Try this link

from BoatUS

 

[GuyT]

OK, I've heard about backflow.

But, the exhaust hose has a large loop which prevents back flow from the transom, the Yanmar mixing elbow is a U to prevent backflow to the engine exhaust. Seems there is enough backflow prevention. However, I thought of one scenario that the vented loop is usefull for.If I clog the exhaust so no water can escape, the water pump has got to put it somewhere so, back into the engine it goes - this is the only scenario I can see why I need a vented loop. It only vents when there is an exhaust clog.Do you all agree?BTW, I have seen a ton of pics of vessels without the vented loop.

 

[Stu Jackson]

Not trying to convince you of anything

just that lots do come with them, and your scenario is a good one.Your boat, your engine, your choice.

 

[GuyT]

Great link Stu - Thanks alot.

I think that the additional siphon phenomenon is enough for me to put the vent back in!I am going to check the water line with respect to where the engine hot water exit is.That was a great link with great details of exactly what I need! Thanks again!

 

[RichH]

Not so Guy .......

When the engine cools down after stopping, the gas mixture still in the combustion chanber cools from approx 2200 degrees down eventually to ambient (~70 deg. F, etc.). Its the eventual decrease of gas temp. inside the combustion chamber that will eventually cause a vacuum and if there is no way to 'vent' the exhaust loop, the cooling engine will SUCK any remaining water in the exhaust loop back into the engine as the expanded 'hot volume' collapses back to ambient conditions. The siphon break is 'supposed' to open and prevent such vacuum from forming. If the vacuum formed is in excess of the water (column) in the 'riser' of the engine exhaust, the water will be sucked back .... unless such a siphon break opens.

 

[GuyT]

Good comments Rich H...

This is exactly why I like this forum.Not about the trash talk and digressions - I get enough of that from my kids in school.Good comments - Some of us learned from this thread. I certainly did!

 

[Fred Ficarra]

Guy, No matter what theories are presented here,

I have the same boat and engine as you. I got tired of that damn vented loop leaking water all over the top of my engine so I trashed it. Good riddance! Works great. Of course I read Yanmars instructions first. Vented loops are only needed if the top of your engine will be below the water line. On our boats, if that happened, a loop is the least of our needs.

 

[walmsleyc]

Vacuum?

RichH,If you suggesiton were correct (not saying that it is not theoretically correct, but practically), then it would seem odd that vented loops are only recomended when the injection point is at or below the water line.If the situation you describe were likely, would it not be likely regardless of the engine/exhaust location (above/below waterline)? In the worst case, then volume of water sucked back would be something less than total engine displacement. I'm sure that someone could tell us the maximum exposed cylinder volume for cylinders with their exhaust valves open. I can't imagine is would be more than 20-30% of total engine displacement. Just keeping the discussion going!Chris

 

[RichH]

Typically the 'valve' is a simple 'duck bill' ....

which is held closed only when there is some pressure of waterflow on the valve .... Its supposed to open when the pressure/flow is off; but, with carbonate formation it may stick closed. When the vented loop is depressurized the valve should open and thus break the vacuum by either the water attempting to back-up (gravity) in the loop or when the gas volume 'collapses' due to the cooling ... by the 'perfect' gas laws: (PV=nRT). Is that what you wanted?