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Old 08-16-2015, 06:48 AM   #1
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Fresh water tank venting

I'm looking at various fresh water tanks to install in my van and I've reached the conclusion that there are two basic configs that potentially would work for me.

My initial plan was to purchase a 6 gallon tank to mount somewhere inside the van (like under a hatch) with a large vented screw on cap. I like this idea since this eliminates needing a city water inlet/filler and the large cap would mean easy access for cleaning if needed.

My concern with this approach is the vented cap when accelerating and braking.........I don't need water seeping out.

My other thought presently is to order a typical fresh water tank with a couple of fittings for filling and water outlet, and perhaps put a city water inlet/filler in a door jamb or side door footwell, etc. I cannot install a fill port on the side of the van for neighborhood CC&R stealth reasons.

I have noticed that these inlets/fillers have check valves......does this filler/check valve also typically serve as the tank vent or is there typically another vent? I assuming that there is another small vent line routed under the vehicle that also lets you know when the tank is full by overflowing...... what are the typical RV fresh water tank configs?

2008 E350 RB passenger 4WD SMB penthouse
2013 KTM 350 EXC
2008 KTM 250 XCF-W
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Old 08-16-2015, 10:39 AM   #2
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Re: Fresh water tank venting

I'm 99% sure the check valve by the filler, on a typical RV, is your tank vent. Most I've seen have the additional overflow pipe/tube/vent from the top of the tank through the floor as well. SMB doesn't have the usual 'filler' type though, that you can put a free hose in, they usually use the screw on/hose only filler type and use a vent on the tank consisting of hose routed out of the top and through the floor. If you're aiming for stealth I would do the overflow through the floor, but either type of filler you go with putting it in a door jamb is a great idea.


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Old 08-16-2015, 11:02 AM   #3
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Re: Fresh water tank venting

I've installed fresh water tanks a few different ways on various boats. The typical way is with a deck fill, which is analagous to the outside side wall fill (so not what you want). But I have also had the fill inside, either with a screw on cap on the tank, or by using a deck fill but just putting it somewhere inside (counter top, wall with an angled fill, etc.). This works fine as long as you have your own hose (otherwise dragging dirty dock hoses through your interior is kind of gross) and/or a place where a drop or two of water won't hurt while filling. I've then used a hose with a "spray gun trigger" sort of end, so I can stop the water flow right when I want to.

If the cap is right on the tank, then you can't quite fill the tank as full, especially if you are on a slight slant, or rocking. If there is a neck on the tank, and/or a deck fill somewhere else inside (with a hose to the tank) then you can fill it all the way up (and even slightly into the neck/hose).

I have never had the vent incorporated into the fill. Instead, I have had a typical 3/8" or 1/2" vent fitting and run a hose from it. This can be run inside, and that can even be advantageous in terms of keeping airborne contaminants from being sucked into the tank constantly (when you draw water off). If it's a good bit higher then the fill, you shouldn't get water coming out.

I'm not sure how this last bit would apply to a pressure pump setup (as I had it on a foot pump setup) - there may be no difference but I'm just not sure - but in one setup I had the interior vent hose running up in a locker to around 4' above the tank. In that case the overflow (when filling - this was a deck fill so I just filled and filled until water overflowed vs. watching it) came out the sink faucet and then drained (overboard, as sink drains do on boats). That was handy.

Of course if you have no place BUT the interior vent hose or the fill for the "overflow" to go when filling, then you would want to watch it carefully (as I'm sure you would if filling indoors anyway) as it will burble up through the fill cap when it is full (presuming vent is much higher).

I like to put some sort of filter on the end of the vent. I think there would be "real" filters you could buy, but I have just have gauze fastened over the end of the vent hose. If you look at a typical vent hose that runs to the fill outside on an RV (terminates in little hole next to filler), you'll usually find it black with mold (gross!). It think this is mostly due to airborne contaminants being constantly sucked in (which of course likely get into your water tank as well). If it's exposed to light and is clear hose that could be a secondary reason.

Here are a couple of deck fills that you might consider using inside, if you have a place for one. These both take 1-1/2 hose; one note is that RV water tanks often take 1-1/4 or maybe 1-3/8" hose, so something to be aware of if you have that. These seal with an O-ring so will not leak even if water is right up to them. You use a little "deck key" to open and close them. They also have some with keyless caps, etc. but this is the classic time-tested style. ... -deck-fill

These are "gravity fills" so not "city water/pressure hose" type fills.

Then the vent fitting would typically be a 3/8" or 1/2" female threaded boss on the tank, into which you would put a male threaded adapter to either barb (for hose) or potentially PEX pipe fitting.
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Old 08-17-2015, 12:40 PM   #4
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Re: Fresh water tank venting

Accelerating and braking will cause a bit of pressure on one side of the tank or the other. This means the vent needs to be quite a bit higher up than just above the top of the tank.

I experimentally confirmed this by being lazy/running out of time while mounting my tank the night before a trip. So I didn't finish running that tube up into the body. At the moment I have a coiled bit it 1/4" ID tube that's just above the top of the tank under the body, leading some slight incontinence.

I really need to fix this since people on trails have a hard time believing that I'm leaking drinking water and not something I ought to be concerned about cleaning up.

I'll post up a thread with pictures when I get this all finished, hopefully sometime this week.

BTW, running water is really nice to have. I just did one trip with it and the payoff is almost as good as a fridge. Mounting the tank is less work than getting all the right fittings and tubing, all the bits and pieces probably come to $350-400.

'99 EB ex ENG KSWB news van, low rent 4x4 conversion (mostly fixed by now), home built interior.
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Old 08-17-2015, 04:16 PM   #5
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Re: Fresh water tank venting

I agree with MadScience, getting all the bits and pieces together is the hard part. My tank came with four bungs. Three were 1/2" and one was 1-1/8". The tank lays horizontal under my dinette with all the fittings coming off the end. The bottom ones 1/2" ones are used for the suction and drain lines. One of the top ones, 1-1/8", is the fill port the other 1/2" was used for the vent. The vent was routed horizontally to a cabinet and directly down through the floor. The vent is also used as the overflow so I can tell when the tank is full. Yes I may lose some water while driving but that cannot be helped with my setup. You can see my install here, viewtopic.php?f=13&t=3583&p=131412&hilit=water+fil l#p131412

When I replaced the filler connection with a new one I removed the spring check valve that was in it. That way I can manually fill the tank from a gallon jug if needed using an adapter, viewtopic.php?f=13&t=3583&p=157041&hilit=water+fil ler#p157041. This has come in handy several times.
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