How to Inspect, Clean and Replace Vented Loops (Anti-Siphon Valves)

BoatUS says they're usually tucked behind bulkheads or in a corner of the engine space. Look for a looped hose or an inverted U-shaped fitting with a hose going in, one coming out, a vent cap on top, and sometimes a small tube running to an overboard fitting. Typical spots:

• Engine: in the raw-water line, often between the heat exchanger and the exhaust elbow (where water is injected into the exhaust)
• Generator: same idea, on its raw-water line
• Head: on the intake line and/or the discharge line
• Bilge pump discharge hoses often have a plain riser loop; some have a vented one

Electric toilets that flush with raw water are a special case. Practical Sailor notes that air drawn in through a vented loop on the intake line can make the toilet's pump struggle, so some installations use a solenoid valve that closes the vent while flushing, or a solenoid in the intake line. If yours has one, it's part of the anti-siphon system: a solenoid stuck open or closed changes how the head behaves. Check the toilet maker's instructions before changing anything.

Make a quick list with locations and photos in BoatMend. While you're at each loop, look at the hoses and clamps too: Practical Sailor's maintenance series suggests checking and cleaning anti-siphon valves along with an annual look at every through-hull hose, because the valves clog with salt and debris and hard, stiff hoses split from vibration.

With the boat floating normally, estimate how far each loop's top sits above the waterline outside. On a sailboat, think about the boat heeled: a loop off to one side drops a lot more than one near the centerline. Compare with West Marine's 12-inch guidance and your engine or toilet maker's instructions. A loop that's too low, or off to the side on a sailboat, is worth a pro's opinion.

BoatUS lists salty crust, green corrosion stains and (on head loops) odor or worse. Crust around the cap is a sure sign the valve needs service. Weeping at a hose joint: tighten the clamp, and plan to pull the hose to check the fitting's nipple for corrosion or wear, which BoatUS says is common on metal fittings in head lines.

Engine off and cool. For an engine loop, close the raw-water seacock. For a head loop, close the head's intake and discharge seacocks. Put a rag under the loop to catch drips.

BoatUS's method: remove the small tube from the vent spout (if there is one), then unscrew the cap at the top of the loop. Some loops use bolts to hold the top down instead. GROCO says its oversized vent valve comes out for inspection without tools. Keep track of every small part.

Many loops use a small rubber duckbill. Forespar's, for example, uses a duckbill under the cap. Pull it out carefully so it doesn't tear. BoatUS says you'll often need to replace it because calcium and other deposits make it stiff, though you may be able to clean it. Soak it in warm water with white vinegar, work deposits off with your fingers or a soft brush, and blow through it with low-pressure air. Replace it if it's torn, misshapen, hard or doesn't close on its own.

• Diaphragm valve: a flexible membrane over a frame, usually held by small bolts. BoatUS says to replace it if it's deformed, stiff or torn; otherwise clean it and the area it seats on.
• Spring, ball, flap or float valves: clean the parts as the maker instructs, check that nothing is bent, and lubricate only with a product the maker recommends. BoatUS's advice: if in doubt, replace it.

If your loop vents through a small hose to the outside, check that hose and its fitting are clear. BoatUS warns that a vent line blocked by insects has the same effect as a valve stuck closed: the siphon isn't broken. Blow it through gently and check the outside fitting isn't painted over or plugged.

Some makers require that vent hose on engine loops. GROCO, for example, says its bronze vented loops must be vented overboard when used in an exhaust system, and both GROCO and Forespar sell caps that accept a small vent hose. If an engine loop has no vent line, ask a mechanic whether the loop's maker calls for one.

Refit the valve right side up (the way it came out), the cap and the vent tube. Reopen the seacocks. Run the engine (or pump the head) and watch the loop: a clean, sealing valve shouldn't spray. A steady drip or spray means the valve isn't closing; recheck it. Afterward, listen and look as flow stops: the valve should let air in.

BoatUS describes an alternative some owners use on the engine loop, never the head discharge: remove the valve and run a hose from the vent to an overboard fitting, so a small stream of water shows whenever the engine runs. The passage is always open, so there's no valve to stick, and you can see the water flowing. The diverted flow must be small enough that it doesn't hurt exhaust cooling. Have a mechanic set this up.

If the body is cracked, the nipples are corroded or the cap threads are stripped, replace the whole loop with one of the same hose size and type. Forespar's loops are molded composite (Marelon) and GROCO makes bronze ones; both offer caps that can take a vent hose. Mount the new loop solidly, at least as high as the old one or higher, with new stainless clamps. SAIL Magazine's writer, after a hydrolock, notes a loop placed high and close to the engine, near the boat's centerline, sees the least change in height when the boat heels or pitches.

West Marine suggests inspecting vented loop valves every year and replacing them every three to five years, because the rubber can harden and fail without any sign. BoatUS adds that flushing vinegar through the head regularly reduces deposits in head loops. Put both reminders in BoatMend.