Forespar Marelon 1-1/2″ Vented Loop/Siphon Valve

Forespar Marelon vented loops are designed to help prevent dangerous back siphoning in marine plumbing systems. When the pump is turned off, the vented loop allows air to enter the line so fluid doesnt siphon backhelping reduce the risk of sinking or flooding.

Key Features

• Helps reduce the risk of back siphoning when pumps are turned off
• Vented loop valve system allows air to enter the line when not in use
• Glass-reinforced composite Marelon construction helps resist corrosion
• Designed for use above the waterline on a wide range of onboard plumbing and pumping setups

Product Compatibility

• Head discharge systems to holding tank
• Bilge pump outlet plumbing (where installed above the waterline)
• Engine and gen-set related discharge/outlet plumbing
• A/C condensate or discharge plumbing
• Ballast tank transfer/discharge plumbing
• Other marine equipment plumbing lines where anti-siphon protection is needed

Product Details

• Forespar Marelon 1-1/2″ Vented Loop/Siphon Valve
• Marelon vented loop valve assembly, part number: 903000 (MF series per Forespar catalog index) ([forespar.com](https://www.forespar.com/media/Marelon-Catalog-23-digital.pdf?utm_source=openai))
• Barb size / hose size: 1-1/2″ ([seamar.com](https://www.seamar.com/item/FPR903000/VENTED-LOOP-1-1-2-MARELON/?utm_source=openai))
• UPC: 028026148711 ([seamar.com](https://www.seamar.com/item/FPR903000/VENTED-LOOP-1-1-2-MARELON/?utm_source=openai))
• Includes: vented loop assembly (siphon valve/vent at the top of the loop) ([seamar.com](https://www.seamar.com/item/FPR903000/VENTED-LOOP-1-1-2-MARELON/?utm_source=openai))
• Common function: a valve at the top of the loop allows air to enter when the system isnt in use, while sealing when fluid is present ([seamar.com](https://www.seamar.com/item/FPR903000/VENTED-LOOP-1-1-2-MARELON/?utm_source=openai))

Parts Desk Insights

Best Use Case: Install on the discharge side of a pump/plumbing line above the waterline to help prevent siphoning back after shutdown.

Common Issues: If youre replacing a vented loop, check the hose/barb connections for tight seating and confirm the loop orientation and mounting location match the existing anti-siphon setup. If the system still siphons, re-check that the vented loop is positioned above the waterline and that airflow to the vent portion isnt blocked.

Related Products & Accessories

• Forespar Marelon Vented Loops (Other Sizes) – Browse the vented loop category for matching sizes and configurations (e.g., different vented loop diameters or vent styles).