Forespar MF 847 Vented Loop - 1-1/8"MF 847 Vented Loop 1 1 8" Installing one of our vented loops will ensure that fluids will not siphon from tank to tank. Vented loops are commonly used in the marine toilet system and engine cooling system. The simple valve system incorporated in the top of the loop allows air to enter the line when not in use, yet effectively seals as water or other fluid passes through. Molded in Marelon, neither vent nor loop will corrode. The duckbill one way valve