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Installing a RV 360 Siphon roof vent.


One of the biggest complaints that Recreational Vehicle owners have is a stinky bathroom. There are several reasons this can happen, but one of the "quick fixes" is to ensure proper venting of the holding tank. So I will be installing a new vent in my Thor Four Winds 28Z motorhome.

The claim the manufacturer makes for this Recreational Vehicle roof vent is it eliminates odors by creating a negative pressure which causes tank odors to exit via the roof-top vent rather than the toilet bowl. I suppose I need to take a skepical view until I see it working, but for now, the $30 price tag was not a lot to spend if it really works. And since the manufacurer also claims that this vent is OEM for many up-scale RVs, I figured it was time to try it. At the very least, if it works, it will take less encouragement to get family members use the RV's facility rather than the campground. While camping used to mean "roughing-it", a smelly toilet is not most people's idea of camping.



There are no moving parts, the vent pressure works by a venturi effect when outside air passes thorugh the vent cap. This venturii effect creates a negative pressure which pulls out odors from the holding tank.

Of course if this is an upgrade to your existing RV's vent, then you will have to remove the old one. Chances are that the lap sealant on your roof has a somewhat semi-hard "bubble gum" consistancy, which can easily be pulled off by using a PLASTIC blade putty knife. Take care here so that you do not damage the roof.

Once the lap sealant is predomanently removed from the roof, you will have to remove 5 or 6 screws, then the roof vent should easily come off.

You may use Denatured Alcohol if you wish to further clean the roof, however, be sure you liberally wash the alcohol off with soapy water or the lap sealent may not properly bond to the roof after installing the new vent.

I have a TPO roof on my RV, and when looking for Lap Sealant, I found that almost all of the Dicor Lap Sealants carried a warning that it was compatible only with some TPO roofs. However, I contacted my RV dealer's service department and they assured me that they use Dicor Lap Sealant on my brand RV's roof without issue. Still, try the lap sealant in a small spot first if you are not sure.


Removing the old cap.


After removing the original cap, you may find that the vent tube is too long. It can only be between 1/2" and 5/8" above the roof line. The style cap shown here typically requires a 2" high vent tube. You can use a hacksaw or any other favorite tool to cut the cap to the proper height (I used a Porter Cable Multi-Tool).



After further cleanup of the roof to get rid of any butyl tape left over from the original cap, attaching the new cap is simple. The 360 Siphon consists of two pieces, a center collar (the manufacturer calls it a "water collar"), and the cap itself. The water collar goes into the existing vent tube, then the cap goes over the collar. A short length of Butyl tape is supplied for creating a bond between the cap base and roof.




After installation of the cap and securing with screws (I used 1" #8 stainless steel sheet metal screws), then use a liberal amount to cover both the cap to roof joint and the screw heads.

The lap sealant cures to a skin coat in 5 minutes, and is waterproof in 4 hours. After 48 hours, the sealant attains 80% of it's bond, and 100% after 30 days.


Project video.


2017 Update

A new version of the 360 Siphon is now available - Gen 2. I will be doing a video on it soon.








Last reviewed and/or updated June 15, 2017