RVs and travel trailers have a unique set of problems associated with ventilating the toilet. They are the one case where you regularly have 60 mile an hour winds on the outside blowing for hours on end. The other exception being a shack in Antarctica. This of course is what happens as you go down the highway. The problem it creates is negative pressure inside the cabin which allows the positive pressure outside to force itself inside. It will do so by any means possible. If you have ever been driving down the road in your RV and someone uses the toilet and you get a strong blast of stinky air when they flushed it or you just have a constant slight smell of the holding tank present all the time, that is what is happening. As a retired firefighter, I call it “back-drafting.” You have opened a pipeline from the outside to the inside going through the vent pipe and the holding tank and through the toilet dragging all the associated smells with it. The outside pressure being greater than the inside pressure is the force behind it. With travel trailers, it is not such a problem because people rarely ride inside the trailer when you are driving down the road from one place to another. If the cabin of your travel trailer does develop a smell then it dissipates quickly when you open the doors and windows to occupy it.
But it can also be a problem for both travel trailers and RVs when you are camped out and it is windy outside. Open hatches, windows or doors can serve as the mechanism to create the differential in pressure inside and out and can also pull in air from the outside through the toilet. That can cause an annoying smell. So how do you deal with that.
If you are installing a stand alone C-Head using the jug system to manage the urine, then you really don’t need to do anything. You will not want to ventilate the toilet unless you have a very strong fan that the outside pressure cannot overcome. But this consumes electricity and creates noise unnecessarily. You shouldn’t have any bad smells coming from the toilet as long as you are not getting the pee mixed with the poo. This is true for both RVs and travel trailers, tiny houses, schoolies and other mobile units with holding tanks. If on the other hand, you are using the bottom exit kit (BEX kit) to direct the urine from your C-Head toilet into the holding tank using the existing toilet flange, then a couple of things will eliminate the problem of back drafting the smell from the holding tank.
First don’t install a ventilation system from the C-Head toilet to the outside. Second, The p-trap that comes with the BEX kit and fits inside the existing toilet flange will block the gas coming from the holding tank. To prevent the outside pressure from overpowering the p-trap, reduce the size of the opening in the existing holding tank vent pipe by covering the end of the pipe with a plastic cap or plastic membrane and drill a 1/4-inch diameter hole in it to restrict the air flow. Since your poop is no longer in the holding tank it will not be producing methane gas which creates pressure inside the holding tank. The urine will be off gassing ammonia depending on concentration of urine and flush water, but this will not produce enough gas pressure to overpower the p-trap. The smaller hole in the holding tank vent pipe cap will prevent the pressure outside of the holding tank from overpowering the p-trap that is installed with the bottom exit kit. Often both the grey water and the black water tanks share the same vent pipe. In such a case, the kitchen sink and shower use the same vent pipe, but the inflow of water into the grey water tank will not be at a rate sufficient to prevent a smaller hole from allowing displaced air to escape. If you think that it could be an issue, then install a Siphon 360 over the end of the vent pipe on the roof of your camper. These are designed to generate suction from the wind coming from any direction.
With respect to houseboats, the issues they face resemble those of an RV more than a boat. Usually they will have the holding tank below the toilet using gravity feed to empty the toilet. This is the exception with sailboats and many trawlers. The use a standard type flushing toilet that dumps the waste through a pipe flange in the floor into a holding tank below the toilet.
This is ideal for using a BEX kit. Usually they have a large holding tank so pumping out can be delayed significantly if you are only filling it with urine and a small amount of flush water. Since they cannot run off shore to dump their waste, they must use the pump out station and it is helpful to only have to do this occasionally. Worst case, the urine can be siphoned out of the tank using a garden hose which is not the case if there is solid waste mixed in it. Since houseboats are commonly opened up with big windows and sliding glass doors, there is almost never a problem with negative pressure unless it is cold outside. So usually no modifications are needed after installation and the existing holding tank ventilation will suffice. If there is an issue, use the same methods you would with an RV.
So, you can ventilate your C-Head relatively easily. Every situation has unique problems and solutions. Some experimentation will be necessary and some changes made as time goes on. But again, I suggest you wait until you see how things work without ventilation.
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