Vacuum breaker on cooling system

[waterman46]

I may misunderstand, but seems like some have mentioned a siphon break between the water pump">raw water pump and the heat exchanger. I don't see how putting a siphon break in that location does any good. It should not be necessary there.

You do need a siphon break in the connection between the raw water outlet of the heat exchanger and where that water enters the exhaust elbow because without that, when engine is shut down, water can enter the exhaust pipe from the outside and fill up the water lift muffler and then flow into the exhaust manifold. Isn't that the only place where water could enter the cylinders?

[skipperpete]

Originally Posted by waterman46

I may misunderstand, but seems like some have mentioned a siphon break between the raw water pump and the heat exchanger. I don't see how putting a siphon break in that location does any good. It should not be necessary there.

You do need a siphon break in the connection between the raw water outlet of the heat exchanger and where that water enters the exhaust elbow because without that, when engine is shut down, water can enter the exhaust pipe from the outside and fill up the water lift muffler and then flow into the exhaust manifold. Isn't that the only place where water could enter the cylinders?

The siphon break loop can be at the raw water pump but it’s the least favorable option because it can allow a larger volume of water to drop down into the mixer elbow and aqualock at shutdown and the pressure in the vent loop is higher at the pump, so if the overboard piddler is chosen as the method of siphon break the flow out the vent will be strong and may steal too much cold water from the raw water system. There is occasionally no option though as on the little Solé marine engines where the mixer is integral and no hose connection exists.
Pete.

[kapnkarl]

My loop & exhaust elbow. Sorry for the rotation on the picture.

[skipperpete]

Well done and very tidy ,but fatally flawed. The clear plastic tube becomes very pliable with heat and can kink at the attach point.
The lower of the two zip ties that secure it is very close to blocking off the air flow needed to break the siphon.

[a64pilot]

Originally Posted by waterman46

I may misunderstand, but seems like some have mentioned a siphon break between the raw water pump and the heat exchanger. I don't see how putting a siphon break in that location does any good. It should not be necessary there.

You do need a siphon break in the connection between the raw water outlet of the heat exchanger and where that water enters the exhaust elbow because without that, when engine is shut down, water can enter the exhaust pipe from the outside and fill up the water lift muffler and then flow into the exhaust manifold. Isn't that the only place where water could enter the cylinders?

The purpose of a siphon break is of course to break a siphon. It really shouldn’t matter where the siphon break occurs, just so long as at the highest point air is allowed to be sucked in, air being much less dense than water will “break” the siphon, but of course the break must be well above the waterline or it won’t work.

Although it’s not done apparently, but I see no reason why a single hose coming from the exhaust elbow itself and going to a siphon break up high wouldn’t work just as well as the loop we use. Just so long as the elbow is the high point of the engine. It would I think be a neater installation, and it’s odd that the engine manufacturers don’t include provisions for a siphon break.
A siphon break is of course nothing but a simple one way valve, and that valve can clog up and or stick, so in my opinion the small hose to simply let water bleed off does two things, first it gives a visual verification that there is water flow, and it’s not likely to clog, and if it did, you would know cause it would stop flowing water. I’m my opinion it’s a superior installation to just a siphon break like I have.
However any good siphon break should have provisions to install a small hose on it, so that if the valve sticks, you won’t be pumping salt water on top of your engine. Just put that hose in the bilge, it should never have any water in it, unless of course the break’s valve has stuck open.
If you choose to not have a valve and just let the water flow out of a small hose, the raw water flow is way more than needed, so a tiny diversion of a small amount is insignificant to cooling.

[Sailmonkey]

Originally Posted by a64pilot

The purpose of a siphon break is of course to break a siphon. It really shouldn’t matter where the siphon break occurs, just so long as at the highest point air is allowed to be sucked in, air being much less dense than water will “break” the siphon, but of course the break must be well above the waterline or it won’t work.

Although it’s not done apparently, but I see no reason why a single hose coming from the exhaust elbow itself and going to a siphon break up high wouldn’t work just as well as the loop we use. Just so long as the elbow is the high point of the engine..

A single hose attached to the mixing elbow won’t break a siphon, all it’ll do is open the mixing elbow to the same atmosphere it’s already open to. The rest of the cooling system will happily dump water into the mixing elbow.

[a64pilot]

Originally Posted by Sailmonkey

A single hose attached to the mixing elbow won’t break a siphon, all it’ll do is open the mixing elbow to the same atmosphere it’s already open to. The rest of the cooling system will happily dump water into the mixing elbow.

The way a siphon break works is to open the siphon to atmosphere, and of course any siphon break is a single hole, it will most certainly work.
There is no reason to run a loop.

[Sailmonkey]

Originally Posted by a64pilot

The way a siphon break works is to open the siphon to atmosphere, and of course any siphon break is a single hole, it will most certainly work.

There is no reason to run a loop.

Think of it this way.
You’ve got a 5 gallon bucket sitting on a cinder block. From the bucket runs a hose, bottom of the bucket over the lip and to the ground.

At the end of the hose on the ground attach a “T” with another leg run above the lip of the bucket, include a valve on the end of the Leg above the top of the bucket.

Start a siphon. Water will flow out of the bucket onto the ground. Now open the valve. The water will continue to flow.

Why? You haven’t created a path that is higher than the top of the bucket with a lesser dense substance within it. All you’ve done is open another path to the already open hose.

Now if the siphon break was a T between the HX and the mixer, the siphon may be broken, may not. That depends on where in the system the fluid falling down weighs more than the fluid that is being pulled up.

[a64pilot]

It will work if you put that T at the highest point. I did say earlier that it had to be at the highest point, what will happen is the fluid levels on both sides of the high point will drop to water level.
To use analogies, put ten ft of clear vinyl hose under water, close off the end with your finger, hold it up and take your finger off.
Or of course start a siphon from one higher up bucket to a lower one, put a hole in the highest part of the loop, breaks the siphon of course, now put a T in the highest loop and once a siphon is established open the tube attached to the T, siphon breaks.

It doesn’t matter how air in introduced to the highest part of the siphon loop, and I assume my engine is like most, the exhaust elbow is the highest point in the raw water system, open a hole there and it will break the siphon, it doesn’t matter if the hole is on the end of a tube or not. You just need to be at the high spot is all, it doesn’t require a loop, the system itself is the loop. All that is required is the vent is above waterline.
But Surveyors etc are used to seeing a loop, no point in trying to educate them, just install a loop. Besides if the valve sticks and your engine floods, I guarantee you that not having a loop will be blamed, even though it’s of no consequence.

My engine installation doesn’t need one, didn’t have one for 30 years, but I added it for Insurence and to make a Surveyor happy.
My heads are above waterline and don’t need a vacuum break either, have never had one, but Surveyors still want to see one and will write up them not being installed, even though you can’t siphon uphill

[Sailmonkey]

Ok, yes, if your mixing elbow is the highest point it’ll work (unless your elbow is below the waterline when heeled).

On our installation the elbow can’t be higher than the waterline, I’ve got to have a loop to raise the siphon breakover point above the waterline at all angles of heel. For us, just breaking a siphon will still allow spillover.

[kapnkarl]

Thanks for your evaluation Pete but 2 points. The siphon break system has been in place for 9 years and is not in any danger of having the hose 1) heat up to a temp that will cause a problem at the point of attachment or 2) being constricted enough to cut off air flow.

The siphon break is taken apart and cleaned & tested annually, including the vent tubing. Thx for your thoughts of concern.

[skipperpete]

Originally Posted by kapnkarl

Thanks for your evaluation Pete but 2 points. The siphon break system has been in place for 9 years and is not in any danger of having the hose 1) heat up to a temp that will cause a problem at the point of attachment or 2) being constricted enough to cut off air flow.



The siphon break is taken apart and cleaned & tested annually, including the vent tubing. Thx for your thoughts of concern.

The condition of your engine room reflects your commitment to maintenance and no offence was intended, but not all cruisers on the forum have similarly high standards and understanding of their machinery.
Pete.