Community Forums

trebot · 04-13-2018, 06:29 AM

'07 Itaska 29R
1st time fresh water fill. Filled til water came out overflow vent tube. Water kept coming out until tank drained to 1/3. Checked manual and made sure valves in correct position. Tried again with same result.
Vent seems to act as siphon.

winniman · 04-13-2018, 07:40 AM

This works on my friends rv. When the tank begins to overflow, disconnect the hose, and reset valves to normal usage. Turn on pump, and run outside tap until water underneath stops coming out. Basically by pumping the water out of the tank, it must draw air in the vent tube, thus stopping the siphon action. The other option is just watch your gauges, and stop when the guage reads full. The extra few gallons you might get in the tank is not worth the hassle. His reads full about five minutes before it starts pouring out underneath.

Duner · 04-13-2018, 10:44 PM

If you dry camp a lot and need a full tank of water, it can siphon out while on the road too. I put a shower shut off valve in line of the overflow line. So I normally keep it open all the time but only shut it off when I want to get max water in the tank.

The USA valves are designed to have a small amount of bypass even when they are closed for safety reasons to prevent scalding in the shower. But that is a good advantage in this application because it lets some amount of air to escape when filling thru the city water connection so you don't burst the tank if you accidentally leave the valve closed while filling. I've been doing this for the last 19 yrs on two different motorhomes.

Here's the valve plus you'll have to get two barbed fittings to insert in the plastic breather line:

Tony Lee · 04-14-2018, 02:57 AM

Remember science lessons on water syphons? Seems like young school kids can grasp the fundamentals but highly-paid, very experienced RV designers haven't a clue and keep pumping out defective RVs for their new owners to puzzle over and post on forums to find out what on earth is going on.

Fitting a valve in the overflow line is effective but there is one big disadvantage because if you fill with the valve shut AND there is no gravity fill vent either, the tank will be overpressurised resulting in serious structural damage.

Better solution is to extend the overflow up into a wheel well or similar so the end is above the tank but that might be hard to arrange.
Drilling a 1/8" hole in the overflow pipe just as it exits the tank will also help to break the syphon.

Several methods depending on the individual layout

Blastvalve · 04-14-2018, 07:17 AM

We’ve had this problem as well. Had the best luck as others noted having someone watch the gauge and screaming out when reading full, seems as though if you can stop it before the overflow kicks in you don’t see 80% of your tank disappear.

I thought I remembered someone saying they had luck doing the above, but in addition leaving a couple faucets on, but with the pump off. The open faucet gave a “vent” to allow airflow and help soften the siphon effect. I can’t seem to find where I read that so I put it in there cautiously.

The above posted valve seems to be the best fix provided you don’t forget it one day and overfill the tank.