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INSTALLATION MANUAL

Size, place and maintain Forespar Marelon vented loops (anti-siphon valves) so heads, bilge pumps and engine systems cannot back-siphon sea water into the boat.

DocumentForespar Marelon Vented Loops Installation
FormatPDF · 3 pages · 0.5 MB
LanguageEnglish
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Installation ManualPDF · 3 p.Placement diagram (D100)PDF · 1 p.
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Forespar Marelon Vented Loops — Installation

Why a Vented Loop

  • A vented loop lets air into the line and breaks the siphon, so water or other fluids (not fuels) cannot siphon back into tanks or the bilge after the pump stops.
  • Uses: head discharge to the holding tank, bilge pump outlet overboard, engine aqua-lift systems, gen-sets, air conditioners, ballast tanks and other equipment with a thru-hull intake.
  • The 1-1/8" barbed loop allows compliance with A.B.Y.C. standard H-22.8.7.2 (dated 7/98) requiring a vented loop on the bilge pump outlet hose.
Part # Hose size
903008 ½" View part →
903003 5/8" View part →
903001 ¾" Ask us to source it →
903007 1" View part →
903009 1-1/8" Ask us to source it →
903000 1-1/2" Ask us to source it →

Installation & Placement

  • Vented loops only work when fluid is "pushed" through them. Pulling draws air into the line instead of the heavier fluid: make sure the pump is on the correct side. The loops themselves are not directional.
  • Place the loop as high as possible above the waterline: the fall of water on both sides creates the vacuum that opens the vent cap. Minimum 10" above the waterline (M3).
  • Check the heeled case: when the boat heels, will the toilet and outboard vented loops end up below the outside waterline?
  • The D100 diagram shows loops mounted on the centre line of the boat, min. 250 mm above the waterline at the greatest angle of heel.
  • Head system loops shown in D100: (1) water inlet between the pump and the bowl; (2) toilet via diverter valve to holding tank; (3) toilet via diverter valve to seacock; (4) holding tank discharge pump to seacock.

Maintenance & Vent Cap

  • Check and clean the vent cap / duckbill periodically: debris can stop the duckbills closing and let odours escape through the cap.
  • To clean, soak the duckbill in white vinegar for a few hours to dissolve salt and calcium. Do not over-tighten the cap on reassembly.
  • Replace the vent cap / duckbill if it is clogged, deformed or not working properly.
  • Whistling (most common with electric flush heads) is a resonance at the duckbill: remove the cap and rotate the duckbill, repeating until you find a quiet angle.
Part # Description
903002 MF 841 replacement vent cap (one-way duckbill valve inside) Ask us to source it →
903004 MF-846 vent barb assembly, optional ¼" hose barb in place of the cap (eliminates the one-way duckbill valve) Ask us to source it →

Need a part that isn't listed? Ask us to source it and quote the part number from the PDF.

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