Air in water lines

[AquaholicsToo]

We have a 2004 Hunter 41AC - we stored it for hurricane season on the hard in northern FL (cruisers for the past 2 years, previous storage was a winterization in southern Michigan in 2023). We've noticed since returning to the boat that once the water tanks get below about 50% we start to get air in the lines and the water pump stays on long (perhaps to push it out?).

Troubleshooting we've done so far...

Replaced the water pump
Checked all fittings and connections
Shut off the port tank to run just off starboard
Shut off the starboard tank to run just off port
confirmed the vents are clear of debris (we had a nest in the forward shower sump exit)

The only thing we have not yet replaced that we can think of is the "T" between the two tanks and the pump. We have inspected the existing and there is no sign that that is the problem, and as we are hopping from port to port finding a 15mm T in stock somewhere has proven a show stopper.

We considered a pinhole / crack in the pick up tube but as we isolated each tank and it was still happening thought that would be a bit of a coincidence that they are both cracked.

Any other suggestions on what can be checked? Certainly our first time storing in heat versus cold we've experienced a different set of "gotchas" but this one really has us stumped.

 

[heritage]

You might try filling both tanks full and check for any drips of water somewhere between the T-valve and the intake of the pump.

 

[Don S/V ILLusion]

As no one stores a boat with water in the tanks, it’s likely air is entrained in the filter(s), expansion tank or a high spot in the plumbing, any of which will happen when the tanks were emptied and then only partially refilled.

 

[Project_Mayhem]

Does your tank have a siphon tube or is the hose connected to the bottom?

If your tee is plastic, check to make sure it wasn't over tightened. I managed to create a leak when one barb collapsed under too much force from the hose clamp.

If all else fails, perhaps making a place for the air to escape to might work. A float valve would allow air to escape but not water

 

[Windpilot]

I am familiar with that problem.
The pump sits higher than the tank, when the water gets lower in the tank there is more siphon pressure at the pump.
My connection from the filter to the pump connection is the critical point. There is no leakage so hard to detect. On my 410 I would run the water until it ran as it should. I would sit down and watch that water filter for a while and noticed the bubbles coming up and water disappearing from the filter but everything seemed tight and no water leakage at all.
I ended up redoing that conection a few times and added the white silicon plumbing tape and the problem was resolved.
Again, it only happened when the lower tanks were getting low. My 410 has a third aft tank that sits higher, it never did that.
It took a while for me to figure this out and boggled my mind for a long time.
Hope this helps.

 

[Windpilot]

Keep in mind that the water in the line before the pump has no pressure on it really. Hence no leakage seen. But water wanting to siphon back into the tank does have suction pressure to go back into the tank. Turning on the water, that air in the lines is causing the problem.

 

[Johnb]

I noticed bubbles coming into my swimming pool from the discharge side of the pump. One day by chance I was looking into the outlet from the pool into the pump suction side and noticed a vortex from the water surface nearly 10 inches down into where the pipe exits. When anything is put in there to interrupt the vortex the bubbles stop.

The take away from this is the water level in the tank does not need to be down to the outlet level for air to get "sucked" into the pump - it just needs to be low enough for a vortex to form.

FWIW we routinely use our tanks to the point where the pump no longer works right and then switch tanks and it has always recovered on its own, i.e. just blows the air out.

 

[Windpilot]

[QUOTE="Johnb,

The take away from this is the water level in the tank does not need to be down to the outlet level for air to get "sucked" into the pump - it just needs to be low enough for a vortex

I do know that from swimming pools also.
But, the pump is sucking the water out from the top, its not a bottom drain. I dont think you'd get a vortex pulling water up from the bottom. Idk, maybe.
Good out of the box thinking, I'll keep running it through my head, good stuff for when im trying to go to sleep, lol.

 

[American_Mainsail]

I have had a lot of issues with the pickup side of the system on my 44AC. The push to connect PEX system just isn't great in vacuum. The fittings work well on the pressure side, but just from the fundamentals of how the push-to-connect fittings work they are very susceptible to leaking in vacuum.

To troubleshoot the same issue you're having, I start at the pump and work my way through each fitting between the pump and the tank. With the intake hose off where it enters the pump I have my girlfriend block off the hose before each fitting while I blow on the intake hose. If it doesn't hold pressure we adjust the fitting, replace it, cut off the end of the PEX, or replace the PEX run, until it does. A little silicone grease on the seal can help coax the fitting into behaving. Then we move onto the next fitting down the line. There are like ten fittings and this takes forever... okay there are only like 3-4 but it feels like ten. Serious boat yoga is involved in this task.

On my list of things to do at some point I am planning on replacing all of the push-to-connect fittings on the vacuum side with compression fittings like this:
https://pro.westmarine.com/flair-it-1-2inchbarb-x-1-2-barb-coupling-21269956.html

Or maybe barb fittings. I'm still up in the air.

Either way, I feel your pain.

PS: Check that your vent lines are working. I am doubting that this is the issue since both tanks are exhibiting the same issue, which is leading me to believe that it's the plumbing between the tank and the pump, but it's good practice to make sure that the vents are working.