Monday, May 17, 2010

Under Pressure

Have you ever lived on a well? We have, in fact for most of our lives. When you live on a well, your water is supplied by a pump, like on a boat. The pump does not start each time you open a faucet, tho. Instead, there is a pressure tank - a tank with a rubber diaphragm in it, with air above the diaphragm and the water below it. Thus, when the pump runs, it inflates the water side of the diaphragm with water, against the air pressure on the other side. When the system pressure reaches the setpoint, the pump shuts off, and water is then supplied by the pressure tank until the low pressure setpoint is reached.

Surprisingly, many boats do not have this arrangement. Instead, when a faucet is opened, the pump runs, and when the faucet is closed, the pump shuts off.

Can you install a pressure tank on a boat water system? Of course. But the "marine" units run $150-$300. There is an alternative. Looking exactly like the Groco unit, but with a different color of paint, there are small expansion tanks sold for use on domestic hot water heaters where backflow preventers are required by code. These are perfect for use as a pressure tank on a boat. The smallest size you can find will work fine as a boat pressure tank, and they are available for less than $40.

To install a pressure tank, tee it into the cold water side of your system, locating the pressure tank as close to your water pump as you can. The pressure in the air side of the tank can, and should be adjusted to match your water system. When the tank is empty, the pressure should be a little less than the cut-on pressure of your water pump. That way, your pump won't come on until the tank is nearly exhausted, but there will still be water flow when it does come on.


  1. I have a tank on the boat but the pump was pulsing the water..When I tried to pump up the pressure in the tank..I got water..Seems like I might have a hole in the bladder??

  2. Anon -

    Yup, that's it. Time for a new one.



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