When a debris clogs the through hull fitting to the air conditioner, you need to call a diver to swim under the boat and clear the debris or pull the hose and push the debris clear with a stick. Both of these options are either time consuming or costly.
When an air lock forms in line and the water pump stops flowing water, the hoses need to be disconnected and bled. Bleeding the lines is necessary to remove any air bubbles from the system so the pump can continue flowing water to the air conditioner. This is a very wet and messy job.
All of these issues can be easily overcome with a simple hose and valve added to the system.
Simply connect a hose from the pressurized fresh water side to the intake hose between the strainer and the sea cock.
If you get a bag or other debris covering the through hull fitting, opening the valve will flush water out the through hull and can clear lightly obstructing debris quickly and easily. There is no need to get a diver to pull a small bag from the hole or the hassle of poking a stick through a gushing sea cock. Simply open the valve, wait a few moments, then close it again!
My through hull is rather low, so I don't typically suck up the bags floating along the surface, but I do get air in the line every time I go sailing. If you heel over far enough, the through hull will come out of the water; and if the sea cock is open, it will drain the whole system out. When you get back to port and plug the boat in, the A/C will not be pumping due to the trapped air.
This is where this system shines! Open the valve and let the water pump prime the system for you. I like to close the sea cock that way all the water is running through the pump. I recommend checking the discharge through hull to make sure that a steady stream of water is coming out. If you have a bunch of bubbles coming out, wait until the stream clears up.
Once the flow is continuous, open the sea cock and let the pump flush any air that could exist between the T connection and sea cock. Now the system is fully primed and ready to be turned on.
I like to listen to the pump when it first turns on to listen to air noise. If there is a bubble in the impeller, it will make a rather loud rattle sound. If it's a small bubble, it will sound like sand falling on concrete. If it's completely quiet, the system is perfectly primed and no air exists in the line.
If I hear the rattle, I will open the fresh water valve again to further flush the system until the rattle fades away. If I hear the falling sand sound, I will let it be. You can waste a lot of water to perfectly bleed the system when it will work just fine with a small bubble present in the pump.
As you can see, this system greatly relies on the fresh water pump to provide the necessary pressure to prime the system. I have a 2 gallon per minute pump flowing at 25PSI to feed a 500 gpm air conditioner pump. A larger water pump would make the priming job easier, but I like to conserve as much water as possible on a daily basis by using a small water pump.