Forespar Vented Loops
Forespar Vented Loops: Corrosion-Free Anti-Siphon Protection
When it comes to protecting your vessel's plumbing from the silent danger of back-siphoning, few names carry more authority than Forespar. A trusted manufacturer in the marine industry since 1966, Forespar engineered its vented loops line around one of its most celebrated innovations: Marelon®, a proprietary glass-filled marine-grade material that forms the backbone of the company's corrosion-free plumbing product range.
Vented loops — also called anti-siphon valves — serve a critical role aboard any boat. They are commonly installed in marine toilet (head) systems and engine cooling lines, where the risk of water being back-siphoned through a through-hull fitting into the bilge or cabin is a constant concern. Without a properly functioning vented loop, even a minor pressure imbalance can allow seawater to silently flood below decks. Forespar's solution addresses this threat with elegant simplicity.
What Sets Forespar Vented Loops Apart
The defining material advantage is Marelon® itself. Molded entirely from this glass-filled compound, Forespar vented loops are highly resistant to corrosion, UV degradation, and abrasion — problems that commonly plague traditional bronze or metal fittings over time. The result is a fitting that maintains its structural integrity in the harsh marine environment season after season.
At the heart of each Forespar vented loop is a duckbill one-way valve built into the cap assembly. This valve allows air to enter the hose line when the system is not in use, effectively breaking any siphon effect that could develop. The moment fluid begins flowing through the loop, the valve seals securely — and critically, it is engineered to seal properly even when particulate matter is present in the fluid, a common real-world condition in head and raw-water cooling systems.
Forespar's vented loops are available in multiple barb sizes — including 1/2", 3/4", 1", and 1-1/8" — to accommodate a wide range of hose diameters found on different vessels and applications. The loops themselves are not directionally sensitive, simplifying installation. Hose barbs are threaded onto the loop body and sealed with Teflon® tape, and the fittings use NPSM (National Standard Free-Fitting Straight Mechanical Pipe) threads — an important distinction from tapered NPT threads commonly found in non-marine plumbing.
For boaters who want to route any venting overboard, Forespar offers the MF-846 Vent Barb Assembly as an optional upgrade to the standard MF-841 cap/duckbill configuration. This accessory connects a 1/4" hose to the vent port, allowing moisture or condensation to be routed outside the hull rather than dripping below decks. Regular activation of the valve and periodic lubrication with winch grease or a compatible marine grease is recommended to keep the duckbill mechanism functioning correctly for years of reliable service.
Forespar backs its plumbing products with a 36-month limited warranty against factory defects in materials and workmanship, a reflection of the company's confidence in Marelon® construction and long-term durability.
Whether you're re-plumbing a head system, servicing an engine raw-water circuit, or simply replacing an aging metal loop that has begun to corrode, Forespar's Marelon vented loops represent a proven, long-lasting solution trusted by sailors and powerboaters around the world. Browse the full selection of vented loops or explore the complete Forespar product lineup to find everything you need for reliable marine plumbing.