Forespar Marelon 1-1/2" Vented Loop/Siphon Valve
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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 doesn’t siphon back—helping 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 isn’t 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 you’re 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 isn’t 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).
Shipping information
In-stock and OEM parts: ship same or next business day.
Locally warehoused items: ship within 1-3 business days.
Special-order items: timeline confirmed after order.
Returns & warranty
Returns & Satisfaction Guarantee
We stand firmly behind the quality of every product we sell. Your confidence matters to us — and we back it with a straightforward 30-Day Satisfaction Guarantee.
If you are not fully satisfied, you may request a return within 30 days of delivery. All returns must receive pre-approval prior to being sent back. Unauthorized returns will not be accepted.
To qualify for a return:
Item must be in new, unused condition
Product must be in its original packaging
All parts, components, hardware, manuals, and accessories must be included
Packaging must be undamaged and suitable for resale
Important:
Electronics are strictly inspected. No returns on open-box electronics. Exceptions may apply only with prior written approval. Any electronic unit that has been installed, powered, activated, or altered may not be eligible for return.
We are serious about product integrity — and equally serious about taking care of our customers. If there is ever a product issue, contact us immediately so we can make it right.
For return authorization, please contact our support team before shipping anything back.