Holding Tank Vent

[saltsolution]

Hello all, the vent to our holding tank sprays the contents out when flushing. The tank is NOT full. Both heads do this. I am thinking there may be a check valve in the vent? Just seeing if anyone has any ideas before I start the unpleasant task of taking things apart.

2007 42DS.

[sitara]

This happened to me on a mates new boat, turned out the drain seacock had frozen shut, but the handle still turned so it appeared to open. The vent should be open to prevent gas build up in the tank and to allow air into the tank when it drains.

[allegria]

Hello,

I agree with Sitara, it's likely that your tank(s) is/are full and the hose at the bottom of the tank is blocked.
Not sure where the bottom of the tank is compared to the deck opening, but if they are vertically lined up, you could use a long stick and try to free it... a sail batten can do the trick (yes, you need to give it a good wash afterwards!).
The other option is to go from the bottom with something flexible... but you can imagine what would come down while you're in the water if you free it up and it was indeed full :-(

The same thing happened to me a few years ago and I did free it up from the top. I now give it a flush with hydrochloride acid once a year, let is sit for a few minutes and empty the tank... I haven't had a blockage since.

Hope this helps,
Cheers,
Allegria

[saltsolution]

Thanks for the response. If I am understanding you correctly you guys are saying the bottom hose to the through hull may be blocked? We are in a marina so, yes the valve is closed. Looking at the tank there is the pump out hose (which is fine as we were just pumped out) and a vent hose going over board (where the tank is spraying out). I had someone suggest that the vent is clogged, but if stuff is coming out I do not see how that could be...

[Don Reaves]

Perhaps the vent hose has a low area that has retained some effluent from a previous overfill. Then flushing the toilet would slightly pressurize the tank and push some of the contents of the vent hose out. If you have a filter in the vent system, it could be even worse.

Actually, this seems unlikely if you have pumped out the tank recently. The vacuum created during pump-out should have cleared the vent hose.

[Trevor]

Hello saltsolution,

This is indeed uusual given the holding tank has been emptied. Perhaps the tank is for some reason not emptied. Perhaps you could get a powerful light to backlight the tank and try to see the level of the contents. The level of the vent is at the top of tank so hard to see how it could happen if the tank is empty.

Good luck and let us know what you find.

Regards,

Trevor

[Mistroma]

I was going to suggest the same as Trevor. I fitted a small LED behind the tank to check the level and it works perfectly. A small torch at the back should show the level. It just appears as a dark shadow.

It's unlikely the vent hose is long enough to hold a significant volume. It simply goes from the top of the tank to the hull a little higher than the tank on my 2009 42DS.

The design means that the only way water can reach the vent is when the tank is full. I think it is certain that your tank is full if effluent is being pumped out of the vent.

However, you state that it happens with both heads and that is odd. The 2009 4DS has a single holding tank in the aft head but an optional 2nd one can be fitted in the forward head. They are independent systems with no connection and that would mean both tanks are full. Very odd situation.

Diagnosis should be really easy. Look at the aft tank first and remove the panel in front of the holding tank. You can see three hoses on top of the tank. One coming up from the bowl, another from the deck pump-out and the smallest is the vent.

I assumed you would be familiar with these as they are easily seen whenever you remove the panel to check the level.

I've never needed to rod out my tank but the deck opening is pretty close to the bottom hose position. However, the lower hose curves quite sharply to the valve and there's a 90 degree bend after the valve. I'm assuming the 2007 model will be similar.

One explanation might be that the pump out station isn't working properly.

Do you:

• Only pump out at the same station
• Never empty using hull valves
• Never confirm the tank is empty after pump out

[saltsolution]

well, we were pumped out and went about 2 days without water coming out of the vent. The tank was about 25% full when it started again. I cant find any way that the vent is blocked, I even took the pump cap off to make sure it can vent and still having the same issue. Not sure what is going on here. I guess when we leave the marina I will open the through hull dump valve and try and flush water through from the top...strange that both tanks are doing this.