Installing a Holding Tank Flush System

A holding tank flush system makes rinsing your holding tanks a whole lot easier and more sanitary than other methods.   Old style methods include back flushing clean water through the dump hose, inserting a wand down the toilet to flush the black water tank, and back-flushing devices that are built in to special dump hose connectors.  Each of these methods often exposes you to unpleasant and unsanitary back spray and there is no way to tell how well the tanks are being cleaned on the inside.  Short of completely refilling your holding tank with clean water, none of the methods mentioned will rinse the whole inside of your holding tanks.  Flush systems are designed to spray the entire inside of the tank and, when properly installed, have no back-splash or even leakage of clean water.

Many late model RVs have flush systems already installed as standard equipment.   If you have an older RV or any RV that does not, they can usually be installed fairly easily.  If your RV has enclosed holding tanks, the installation will be a little more difficult but is still usually possible. Sometimes you may have difficulty finding room for the fittings or for the tools needed to install them.  Once installed you need only connect the male end of a garden hose to the female flush fitting on the side of the RV to send a cleansing spray into your holding tanks.   Continue flushing until the water running from the dump hose is clear.  The black water tank is the most important place for a flush system but using one on the gray water tank will help minimize the possibility of odors from left over food deposits from the kitchen sink.

For many years I used a wand inserted through the toilet to flush my black water tank.  This method is fairly effective and the wands are not expensive.  The biggest drawback I found was if my aim was off I'd get nasty splash back up in the face!  Wands have a directed stream to hydraulically scrub the inside of the tank.  If you don't get the wand far enough down inside the toilet before turning it on it splashes back up at you with surprising force.  Always have a  shutoff where the hose connects to the wand so you can turn it off to bring it in and out of the RV and turn it on once you've inserted it deep down in the toilet.   Flushing systems are installed in the side of the tank with wide-angle spray heads that are intended to shower the entire inside of the tank to flush away residue and debris without any back splash on the user.  They operate inside the tank with the toilet valve closed so there is no back spray anywhere.

If you want to install a flush system on your RV, you need to do a little planning even before you purchase the installation kit.   Inspect the holding tanks to be sure there is access to make the installation.  Consider not only whether there is a spot on the tank to install the nozzle, but whether there is room for the plumbing and for the tools necessary to install it.  You need room to drill 1" inch holes in each tank (plus pilot holes for the mounting screws) and to run the hoses from the nozzles to a convenience location on the side of your RV where you will install the connector.  The connector is the same kind that is used for your city water hook up: a female hose connector in a recessed mounting.   It is a good idea to label the flush system so someone doesn't accidentally connect your fresh water hose to it.   Measure the distance from where you are going to install the nozzles to where you will install the connectors so you know how much hose you will need.   I usually buy a 25' length of 5/8 garden hose plus male and female connectors.  That way I can make hoses to connect to the gray and black water tanks.   I like to use a "Y" with shut offs between the outside connector and the holding tanks so I can flush each one separately from the same connection.   With that in mind, you may need two sets of male and female connectors so you can make up three hoses:  one from the outside connector to the "Y", one from the "Y" to the gray water tank, and one from "Y" to the black water tank.   The outside connector should be installed near the dump valves where it will be convenient for you to use.  If your dump valves are in a cabinet, install the hoses so the "Y" connector is accessible in the cabinet.  If your valves simply hang down under your RV, run the hoses so the "Y" connector is near the dump valves and convenient for you to reach when switching from one tank to the other when dumping.   If access is difficult you may want to take your RV to a qualified RV technician for installation.  They may have tricks and tools to make it work.  Enclosed tanks may required removal of access panels or body parts to facilitate installation and that is often best left to experienced professionals.  If there isn't anyplace to install a "Y" where you can access it you may have to install separate female connectors for each tank.

Your installation kit will come with easy, detailed instructions.  Follow the instructions carefully for proper installation.   You will need to drill a hole about 1" in diameter in about the middle of each tank.  Center it top to bottom and end to end, preferably in the long side.  The nozzle will be inserted in the hole and is usually attached with 3 self-tapping screws.   You may be advised to drill pilot holes for the screws.   Make sure to install any required gaskets and use appropriate sealant to ensure a leak-free installation.  The fittings on the nozzle are usually ordinary hose fittings.   You may have to add one on the back side of the inlet.  You will need to drill a hole in the side of your RV to install the water inlet.  Make sure there is enough room behind the wall to attach the hose.   Sometimes it may be more convenient to install the inlet in the rear instead of a side wall.  The flange can be attached with self-tapping screws or pop-rivets.  You should use some of the same butyl tape (some times called "dum dum tape) that is used to install windows and vents to seal the flange to the RV body. some folks like to use silicone for an additional seal around the fitting, but if the butyl tape and the fitting are properly installed it shouldn't be necessary.  If you can position the lines from the inlet to the tanks so that you can install a "Y" connector with shutoffs to direct the flow from one tank to the other you will need only one inlet.   If you can't find a place where you can reach the shutoffs, you can either just run water into both tanks at once after dumping them or install separate inlets so you can rinse each tank individually.  Without access to the shutoffs you will want to dump both tanks completely, then close the gray water valve while flushing the black water tank, then close the black water valve and flush the gray water tank.   Don't flush the gray water tank TOO long or you will have a lot of water in the black water tank.   Since it is often suggested you should put about 1" of water in the black water tank when adding chemicals, letting some water accumulate while flushing the gray water tank is acceptable and perhaps even desirable.  However, anytime both valves are open at the same time on a system with one dump port, there is a chance of cross contamination --  you could end up with residual sewage from the black water tank in the gray water tank.  Definitely not a good thing!

Once everything is hooked up you're ready for easy, sanitary rinsing of your holding tanks. Connect a garden hose from a faucet to the flush inlet.  Do NOT use your potable water hose!   Make sure the on/off settings on your "Y" connector are set properly to direct water to the tank you are going to dump and flush.  Turn on the faucet and open the dump valve.  Allow the tank to drain until clear water flows from the dump hose.  Then switch to the second tank and repeat.  When you're all done, turn off the faucet and disconnect the hose.  Don't run the flush system with the valves closed or you will risk pressurizing the tank and damaging the plumbing or forcing sewage back up into the unit.

What if there isn't room or access to install a holding tank flush system?   In the first place, you can always resort to flushing the black water tank using a wand connected to a garden hose and inserted through the toilet.   Another fairly inexpensive and easy solution is to purchase a back flush adapter.   This is a connector that installs between your dump valve and dump hose and allows you to connect a garden hose to back flush the holding tanks.  They are usually made of clear plastic so you can monitor the flow as your tanks are dumped and rinsed.  These can be used to back flush both the gray water and black water tanks.  They should come with an anti-siphon valve to install on the garden hose to avoid any chance of getting sewage into fresh water system.  They won't deliver the powerful rinsing that directly plumbed internal flushing systems will, but they generally provide enough flushing to clean tanks and prevent odors.  They usually have a control valve attached to the hose connection that lets you direct the spray alternately back into the tank or down into the hose for final rinsing.

Pleasant flushing!