Dear Boat Doctor,

I decided to upgrade my head to a new electric model. I chose a new model with the pump integrated into the base and it fits in the same spot as my old head. I followed the instructions to place a vented loop in the seawater inlet line to prevent the head from siphoning water and flooding the boat.

Everything is installed and it looks great, but the head doesn’t pump in seawater when I hit the flush button. If I put water into the bowl with a bucket, it pumps it out just fine. What is going on?

Ed Smithfield

Chicago, Illinois

Dear Ed,

Seawater siphoning into and through a head is a serious issue. It can sink a boat. Water wants to flow downhill, driven by gravity. One of the first lessons we teach guests on our boats is the “wet bowl, dry bowl lever” drill, but electric heads don’t have a lever and oddly no automatic valve to prevent the siphon.

Placing a vented loop in the inlet line of the head is correct, it will stop the head from siphoning. It stops that first gulp of water from pulling the rest with it. But, it will also prevent the pump in the head from priming, it is easier for the pump to suck air through the vent in the loop than to pull water up and over it. If you run the pump you will hear air being sucked into the vented loop. A good test to verify if this is your problem is to block the vent with your finger and try to pump the head. It may take a little time to prime but it should work.

As a side note, it’s important that the top of the vented loop stay above the waterline, even when the boat is heeled over. The standard guideline is 8 inches above the static and heeled waterline.

Fixing this problem takes a little engineering. You need the vented loop to break the siphon so you don’t sink your boat, but you can’t have a vent when you are flushing. Jabsco (www.jabsco.com) makes a vented loop solenoid valve that closes electrically. This valve is installed in place of the check valve in the vented loop. In its normally open state it acts as the vent in the loop, allowing in air to break the siphon. It gets wired in parallel with the pump in the head and closes when the pump runs. The result is that magic we wished for above: a vent under normal conditions and one that closes automatically to allow the pump to prime.

The valve threads directly onto many vented loops, but in some cases you may need to replace the vented loop with a version that presents the correct threads to mount the solenoid valve. You can find the valve online for about $85.

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