Anti-siphon valve

dlochner · Mar 21, 2022

Anti-siphon valves need to be on the pressure side of any pump and above the end of the fuel pick up line. If the valve is on the suction side the valve will open when the pump comes on and will suck air not fuel. The shut off valve should be close to the fuel tank.

dlochner · Mar 21, 2022

fred1diver said:
Not sure I fully understand, so after the pump?

Yes, after the pump. Anti-siphon valve work by opening when there is low pressure in the hose and and closing when there is pressure. Opening breaks the siphon by allowing air into the hose. It should be higher than the source of the liquid, i.e. the fuel in the fuel tank.

When the pump tries to draw fuel from the tank, it creates a vacuum on the suction side. If the valve is between the pump and the tank, the valve will open due to the low pressure and air will enter and fuel will stay in the tank. On the pressure, out put side, of the pump the hose will be under pressure which will close the valve and allow fuel to reach the carburetor.

The intent is to prevent fuel from siphoning out of the tank and into your bilge.

BTW, I'm assuming this is for a gas motor, not a diesel.

dlochner · Mar 21, 2022

The hose should go from the tank to shut off switch, to pump up to the AS valve and then down to the carburetor.

Routing the hose may be a challenge and finding an AV valve suitable for gas may also be challenging. The part you are looking for is a called a vented loop.

You might contact Moyer Marine, they are Atomic 4 specialists and they may have more specific solutions.

moyermarine

moyermarine.com

jeepbluetj · Mar 21, 2022

2nd Moyer. As an owner of an A4 you'll become well acquainted with Moyer marine.

My A4 (Catalina 30) did not have an anti-siphon valve on the fuel line or tank. IIRC, what was often used was one with a ball and spring that looked like this:

(pic stolen from amazon)

However, they failed often (plenty of threads over on the Moyer site. Thread where Dan M. says to get rid of it)

On an A4 a likely place to leak fuel is the mechanical fuel pump, and it often ends up leaking into the oil. Strongly suggest an electrical fuel pump instead.

I didn't mind having the A4, but followed the same rules as for a gas powered powerboat:
1. make sure the shut off is always off when leaving the boat.
2. Run the blower before start and during engine operation
3. Sniff the blower's output before start to smell for gas. Sailboats are pretty easy too, if it smells like gas when you open up the companionway, find out why.
4. Ventilate well during/after refueling.

dlochner · Mar 21, 2022

jeepbluetj said:
2nd Moyer. As an owner of an A4 you'll become well acquainted with Moyer marine.

My A4 (Catalina 30) did not have an anti-siphon valve on the fuel line or tank. IIRC, what was often used was one with a ball and spring that looked like this:

View attachment 203632 (pic stolen from amazon)

However, they failed often (plenty of threads over on the Moyer site. Thread where Dan M. says to get rid of it)

On an A4 a likely place to leak fuel is the mechanical fuel pump, and it often ends up leaking into the oil. Strongly suggest an electrical fuel pump instead.

I didn't mind having the A4, but followed the same rules as for a gas powered powerboat:
1. make sure the shut off is always off when leaving the boat.
2. Run the blower before start and during engine operation
3. Sniff the blower's output before start to smell for gas. Sailboats are pretty easy too, if it smells like gas when you open up the companionway, find out why.
4. Ventilate well during/after refueling.

The check valve is a little different than an anti-siphon valve (vented loop) although in this application they serve the same function, to prevent fuel from siphoning out of the fuel tank and into the bilge.

The check valve has a spring loaded gate that requires pressure on one side to open or a vacuum on the other side. These are similar to back flow preventers used in plumbing.

jeepbluetj · Mar 21, 2022

dlochner said:
The check valve is a little different than an anti-siphon valve (vented loop) although in this application they serve the same function, to prevent fuel from siphoning out of the fuel tank and into the bilge.

The check valve has a spring loaded gate that requires pressure on one side to open or a vacuum on the other side. These are similar to back flow preventers used in plumbing.

We agree. I wanted to mention the check valve because that's what many A4 installations used. (And many A4 owners removed them too)

thinwater · Mar 21, 2022

fred1diver said:
Hello everyone, quick question.
I need to do a few items on my new to me Pearson 30 to comply with the abyc norms following the survey.
They want me to install an anti-siphon valve, where on the fuel line would I need to install it? Before or after the shut off valve?

Ask them, to be sure.

There are also anti-siphon valves that go in/on the fuel tank, on the suction side. These are common on gasoline systems. But they function on a different principle (not air bleed).

Think about it. If the fire or chafe damamges the line between the tank and pump, what good will a valve after the pump do? none. Closer to the tank is better. This is a gasoline system, so it probably mounts on the tank. This is NOT related to vented loops. Totally different. I believe it is right in the code that there can be no line between the tank and the valve.

Rich Stidger · Mar 24, 2022

@thinwater is spot on. Previous posts have confused vented loops with anti-siphon valves.