Fresh water tank vacuum forming.

[Arnac]

 

We are still slowly discovering our narrowboat's workings and so the following may be obvious to some, but I post simply in order to help any others who may find themselves in a similar position. 

 

When filling our stainless steel water tank we would have a Vesuvius of air and water as the tank neared the end of its fill with no obvious overflow or vent pipe apparent which I thought was odd. 

 

After replacing the filler cap, a small vacuum would form as we used the water which would then became progressively worse, resulting in the tank banging as it flexed and the pump struggling. This would be solved by releasing the filler cap to allow air in to get rid of the vacuum.

 

Having trawled the forum for advice, I lifted the front cratch inspection cover and with a torch and some yoga moves, discovered that the fresh water tank did indeed have a flexible hull venting pipe but that it was not accessible from the inspection hatch. It was also coiled in two loops.

 

I threaded a flexible piece of thin, hard nylon tube through the hull vent outlet (from the exterior) removed it and then gave the vent pipe a few short blasts of high pressure water to ensure it was clear. Hey presto, problem solved. Upon filling the vent pipe now overflows as it should and no more vacuum.

 

Thanks to all previous contributers on the various threads. Couldn't have worked it out without your contributions.

Edited May 11, 2023 by Arnac

[Tony Brooks]

Thanks for this report and well done.

[blackrose]

Make sure not to fill it up to the very top just before a cold snap in winter. If the vent pipe is full of water and freezes the same thing will happen.

[Arnac]

Thanks for the tip.

[system 4-50]

I'm baffled as to why it was blocked.

[blackrose]

Well it was coiled so any debris getting in there is likely to stay put. Spiders tend to crawl into small spaces and set up home causing blockages. I'm not sure what the OP's pipe is made of but my mild steel integral water tank vent pipe which sticks up from the bow deck became blocked with rust. I ended up chopping it in half with an angle grinder drilling the rust out and putting it back together with a piece of hose pipe which just happened to be a perfect internal diameter.

Edited May 11, 2023 by blackrose

[alexjcrawford]

On 11/05/2023 at 14:04, Arnac said:

 

We are still slowly discovering our narrowboat's workings and so the following may be obvious to some, but I post simply in order to help any others who may find themselves in a similar position. 

 

When filling our stainless steel water tank we would have a Vesuvius of air and water as the tank neared the end of its fill with no obvious overflow or vent pipe apparent which I thought was odd. 

 

After replacing the filler cap, a small vacuum would form as we used the water which would then became progressively worse, resulting in the tank banging as it flexed and the pump struggling. This would be solved by releasing the filler cap to allow air in to get rid of the vacuum.

 

Having trawled the forum for advice, I lifted the front cratch inspection cover and with a torch and some yoga moves, discovered that the fresh water tank did indeed have a flexible hull venting pipe but that it was not accessible from the inspection hatch. It was also coiled in two loops.

 

I threaded a flexible piece of thin, hard nylon tube through the hull vent outlet (from the exterior) removed it and then gave the vent pipe a few short blasts of high pressure water to ensure it was clear. Hey presto, problem solved. Upon filling the vent pipe now overflows as it should and no more vacuum.

 

Thanks to all previous contributers on the various threads. Couldn't have worked it out without your contributions.

I've been having this exact problem. We have an added complication where our breather outlet is almost on the waterline for some bizarre reason.

Daft question, but is there a risk of anything going back into the tank while trying to unblock by blasting water or air back through?

[Jen-in-Wellies]

1 minute ago, alexjcrawford said:

Daft question, but is there a risk of anything going back into the tank while trying to unblock by blasting water or air back through?

Definitely. If there is something blocking the pipe, or vent, then forcing it from the outside will send it to the tank, if it moves.

[Tony Brooks]

10 minutes ago, alexjcrawford said:

I've been having this exact problem. We have an added complication where our breather outlet is almost on the waterline for some bizarre reason.

Daft question, but is there a risk of anything going back into the tank while trying to unblock by blasting water or air back through?

 

I would add that it is probably more hygienic if you ensure there is a high swan neck between the tank and hull fitting. If it does not have one, there is always the chance that canal water will get into the tank.

[alexjcrawford]

18 hours ago, Jen-in-Wellies said:

Definitely. If there is something blocking the pipe, or vent, then forcing it from the outside will send it to the tank, if it moves.

 

17 hours ago, Tony Brooks said:

 

I would add that it is probably more hygienic if you ensure there is a high swan neck between the tank and hull fitting. If it does not have one, there is always the chance that canal water will get into the tank.

Thanks for the replies and advice 🙂

[JoeC]

My fresh water filler is fitted to the gunwale and not the floor. A previous owner drilled a hole in the filler cap and glued a small piece of mesh over the top to allow for air flow. As mentioned above, the overflow should give air flow but this cap hole may also come in handy.