RV Water tank filling and use

We are new to RV'ing and got lots of good help here last summer when we were in electrical crisis mode. Now I hope you can help as we plan this year's trip!

I am sanitizing the water tank for the first time to prep for our trip; we've never actually filled it before. I hooked up the hose to the external outlet and used that to fill the tank. The tank itself is located under the bed, which I can lift to see the tank and also see/use the valve to empty the tank. On the top of the water tank is a circular cap. I had it closed but realized that the tank would not fill that way, because there was no air release. So I first loosened the cap, then removed it entirely while it filled.

My question is this: Should the cap be removed when filling and when using the water tank? It provides a place for the pressure to equalize, but it also seems less sanitary to have the tank just open. And would the water slosh around, and eventually out and into the interior of that space or the underside of the bed, if it was not covered at all? What is the best way to handle this?

Thank you!

Never heard of anyone needing to remove a cap to add water or use it.

I would think there is a vent somewhere and/or at least an overflow tube in case someone started to fill
the tank and then walked away or got distracted - then the water should come out of the overflow tube.

Can you see any tubes going to or coming from the tank other then the one used to fill the tank?

Like RedandSilver, I've never seen a system that required you to open a cap on the tank to vent it while filling it. If this is a gravity-fill system, there "should be" a small vent tube near the filler spout. (The hole you put the garden hose into to fill the tank.) The vent tube usually looks like a clear rubberized piece of plastic tubing. I suppose in the interest of cost-cutting, it's possible that the manufacture vented the tank differently, but It would surprise me if it would require someone to unscrew a cap. In any case, a vent tube would certainly be attached up high on the tank... possibly even on top.

Kev

kriemermann said:

We are new to RV'ing and got lots of good help here last summer when we were in electrical crisis mode. Now I hope you can help as we plan this year's trip!

I am sanitizing the water tank for the first time to prep for our trip; we've never actually filled it before. I hooked up the hose to the external outlet and used that to fill the tank. The tank itself is located under the bed, which I can lift to see the tank and also see/use the valve to empty the tank. On the top of the water tank is a circular cap. I had it closed but realized that the tank would not fill that way, because there was no air release. So I first loosened the cap, then removed it entirely while it filled.

My question is this: Should the cap be removed when filling and when using the water tank? It provides a place for the pressure to equalize, but it also seems less sanitary to have the tank just open. And would the water slosh around, and eventually out and into the interior of that space or the underside of the bed, if it was not covered at all? What is the best way to handle this?

Thank you!

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First the fresh water tank should never be put under pressure, not even overfilled so water gushes out the vent tube.  Pressure will expand the tank walls and over flex the corner joints and if enough water pressure is applied or over filing is done to often the tank will most likely start leaking.

Questions: 
--  Did you fill by connecting a water hose to a water faucet and then to the shore water connection and then turning a valve labeled something like "shore water/fresh water fill"?
--  Or did you use the gravity fill tube?
--  Either way are you saying the fresh water tank would not fill at all unless you removed the cap?  Most unusual if so.  Usually, as stated by others, there is some sort of vent for the air to escape while filling.

As the others have mentioned there's usually a vent tube. If not air will still escape through fill tube but it may gurgle some as the water goes in and the air comes out. Removing the 'cap' is not a good idea. Inevitably an over fill WILL happen at some point and drench your interior.

Thanks for your help! Yes, I attached a garden hose to an exterior spot labeled "fresh water" (or something like that; not checking at the moment) and I heard water trickling into the tank inside. When I went in a few minutes later to check, I could see the tank filling. There is an overfill spout, yes, which is the same tub that empties the tank directly and has a valve to empty it. When I looked at the tank, it was bulging, obviously from pressure, though the water was still going in. When I removed the cap on top (it wasn't enough to loosen it; removing it made the difference), air rushed out and the tank returned to normal shape. So at the moment, the cap is off so I can fill, sterilize, and drain. But I agree that it doesn't seem sanitary or practical. I didn't see anything in the manual about this, but I can contact the previous owners if need be. I just prefer to figure it out without bothering them!

Perhaps if you identified the year/make/model of RV we could be a bit more helpful. Water systems in RVs are all quite similar, but details for filling and using vary.

No RV water system in the last 35 or so years was pressurized, so you should not have to remove a cap from anything.

It's a Sunseeker, 2006 I believe. Here is a picture of the top of the water tank with the cap off. I don't know if it helps, but it might clarify what I am talking about. There is also a picture of the sticker on the tank itself, if it helps to ID the tank.

I looked in the manual again today and there is a mention of removing the cap if you need to take water from the hot water tank to the fresh water tank because the fresh water tank is empty. Removing the cap will cause air to rush into the fresh water tank, thereby pulling water in from the hot water tank? I read it quickly. But perhaps that's the only reason to remove the cap.

Removing the cap will cause air to rush into the fresh water tank, thereby pulling water in from the hot water tank?

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Nah!!! The two systems are completely separated. Not sure why anyone would want to do that anyway!!! The opposite might be true, you might want to put fresh water into a hot water tank for heating but is usually done using the water pump or from the city water inlet.

That 'cap' could be used to add bleach to the tank but it is definitely not the air vent unless someone has modified the system and replaced the usual fittings and connecting tubes with a blank plug.

In the second picture, immediately to the right of that plug the OP has out, is what I believe is the vent/overflow line, with the fill line right behind it.

kdbgoat said:

In the second picture, immediately to the right of that plug the OP has out, is what I believe is the vent/overflow line, with the fill line right behind it.

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Good catch. The OP needs to follow those 2 lines back to see where they go.  When it's that type of application, there is probably a door on the outside of the RV. Maybe 8" square. Open the door and the fill line where the hose would stick into would be a fairly large cap and the vent is (usually all you'll see is a screen) right next to it.

On our Southwind we have the same two lines kdbgoat picked up on.  While I haven't actually looked to see where they connect to our tank that is what is present at the gravity feed behind the door on the right side, just behind the entry door on ours. 

Somewhere ... maybe the Bounder forum , we were warned to always open the gravity feed cap when filling the water tank from the city water connection with the hose.  Could cause possible over pressure on the tank. 

My procedure is to hook up hose, select the valve to fill the tank and walk around and open the gravity fill cap.  I always get an audible whoosh of air when I take the cap off, more or less depending on how soon I get there.  The small vent line doesn't seem to keep up with the pressure build as you fill from the hose.  The small vent isn't blocked as it spits air and water later in the fill process before the tank is full.  While it is a vent, I thought of it more as allowing air into the tank as you used the water.  ???

So leaving plug in, do you just need to open gravity feed cap when filling?

Or could the plug be where an overflow line should be that never got plumbed in?  I'll have to take a look at mine and see if I can see an overflow line on tank.  I normally fill until the open gravity feed line overflows.

Howard