Backflow?

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Sep 23, 2005
13
Catalina 36mkII Blaine
We have a catalina Mark 11 with a manual pump head. While docked we have air bubbles in the tank - about an inch or so in diameter. The tank will bubble for 10 minutes or so and then it will stop. We thought that it was backflow from an overfilled tank but now we are beginning to wonder if a vent is plugged? Where would we find the vent? Perhaps we are not flushing with enough water? We are doing 3 or 4 dry pumps followed by a couple of wet pumps. Also, does anyone know how we can determine the size of the tank? Thanks.
 
Dec 2, 1997
8,959
- - LIttle Rock
When is the tank bubbling???

You say "while docked"...does that mean you're only using the tank at the dock? Does it happen after each flush? Only immediately after returning to the dock? Is the tank overboard discharge thru-hull open? Till I know more, I can't imagine what would cause a tank to "boil" for up to 10 minutes! Sudden thought: is it possible that the P.O. installed some kind of aerator in the tank...and that the pump starts when you get back to the dock and reconnect to shore power? T 'Cuz the source of the air that's creating the bubbles can't be above the surface, it has be low in the tank...well below the surface. A correctly installed aerator would explain it. The vent line is the smaller hose coming off the top of the tank or on a side at the top. The other end exits the hull somewhere...you'll have to trace the line to find out where. Or depending upon the size and age of your boat, it may end inside a rail stanchion. However, if the vent is blocked, flushing the toilet wouldn't cause it to "boil," it would pressurize the tank. Flushing the toilet wouldn't cause it either, unless you're flushing some product that's effervescent each time you come back to the dock. However, I don't think you're pumping the toilet enough times to move bowl contents all the way to the tank, which leaves waste sitting in the line...which will cause other problems. And you've got the wet and dry sequence backwards. I suggest you check out the link below.
 
Sep 23, 2005
13
Catalina 36mkII Blaine
bubbling/ overflowing head problems

No, it does not only happen when we are docked. It does not happen with every flush - we usually notice after a number of flushes. No, the tank is not overboard discharge thru hull open - our boat is a 36 foot 1998 Catalina. We wonder if a vent could be plugged. We also wonder how we can determine how full our holding tank is. Thanks.
 

Ross

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Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
If I were trying to determine the holding

tank capicity I would pump it dry. And then with the selector valve on "dry" I would pump one gallon of water at a time through the toilet until the holding tank was full. I would also count the gallons.
 
Dec 2, 1997
8,959
- - LIttle Rock
Install a tank level indicator

The Profile gauges http://ferriellosales.com/default.php are the best quality, very reasonably priced, and can never clog. Meanwhile, there's a MUCH easier way to determine the size of tank: If it's rectangular, multiple L x W x H in inches, divide that 231 (the cu. in. in a gal). Because you're measuring outside dimensions and you can't use every single milimeter without overflowing the tank, the usable capacity will be a be less. So if you get a number something like 26.32447, what you have is a 25 gal. tank. If the tank isn't rectangular, you'll need the math skills to arrive at the number of cu. in. You can never go wrong making sure your vent isn't blocked, but I don't think there's any way that blocked vent can cause a tank to "boil." That requires a source of air below the surface of the contents...the vent is above--at least it better be!--the contents.
 
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