Why Is My RV Black Tank Flush Not Working – Solved!

9 reasons why your RV black water tank flush is not working are:

  1. Seized drain gate valve. This usually is caused by waste drying out at the edges of the valve seat.
  2. RV is not level. RV tanks have a slight slope toward the drain, but if the RV is not level, waste will not flow toward the exit pipe.
  3. Design of tank. Older tanks were boxy and flat, and solids merely sat where they fell.
  4. Excess paper or solids. Using too much paper or using paper that is not biodegradable causes sludge to build up, particularly around the valve where it might catch on any edges.
  5. Insufficient flush liquid. To save on emptying a tank, some of us tend to use too little water when flushing the toilet, so the solids are not carried away from where they fell.
  6. Accumulated waste in corners. Because many tanks are rectangular, the sludge builds up in corners over months or years of use.
  7. Dried and solidified waste due to improper season-ending flush. If you do not thoroughly flush your tank at the end of the season, waste will dry and harden in the tank.
  8. Gauge inaccurately showing full tank. Gauges are notorious for giving false readings. Frequently, the gauges show “Full” when the tank really is empty.
  9. Poor operation techniques. Irregular cleaning and flushing, leaving the gate open all the time, not draining grey water after draining the black to help with the flush, inadequate season-starting, season-ending maintenance, and incorrect day-to-day use all contribute to tank problems.

RV blackwater tanks are not designed in the same manner as household plumbing, from the toilet unit to the sewage lines.

Most RVs (not including park models) use a toilet that consumes minimal water and flushes straight into the holding tank, without the use of a p-trap built into the toilet. This means that waste falls directly into the black water tank, generally to the rear, distant from the drain outlet. By using minimal water, the toilet does not immediately wash the solids toward the drain and it often accumulates near the toilet discharge.

When you flush, liquids may drain away, leaving a mound of solids. These solids may build up over the camping season, or flow partially toward the outlet, eventually clogging that gate. The residue may build up an unpleasant odor Why Does My RV Smell Like Sewer signaling that waste is not properly removed.

Why Is RV Black Tank Clogged?

The solid wastes from your toilet do not flow as quickly as the liquids and will separate, accumulating until the RV tank clogs. Several factors contribute to the flow of solids and liquids in your tank toward the gate valve and discharge.

Most tanks are designed in a rectangular shape, with slightly rounded edges. As waste builds up, it may flow back into the far reaches of the tank, where it may congeal. This reduces the effectiveness of your flush.

Many campers leave the gate valve open whenever hooked up to a septic tank. This allows liquids to leave before enough liquid has accumulated to flush solids along with it. Ideally, you should keep the valve closed until the tank is nearly full of both solid and liquid waste, then flush the entire contents. Additionally, you should flush the grey water tank into the discharge line after flushing the black water, so that the additional liquid can clean out any remaining residue.

How Can You Unclog A Full Black Tank?

There are a few ways to release or unclog a full black tank, including a soapy flush (see below), using a plumbers snake (difficult and not always effective because of the design of the tank storage), and freeing the gate valve before flushing water through, but you should never use back pressure or a plumber’s compressed air tank to blowback sludge.

This can cause an eruption of waste wherever the pressure is least, either through the toilet or back through the pipe or point where you are pumping air into the system.

Is The RV Black Tank Gate Valve Stuck Or Broken?

A common problem with RV black and grey water tanks are that the gate valve sticks or breaks, and if it is broken, you may notice that the release handle moves freely but the gate does not open. The high acidity of wastes can rust the metal draw shaft on the gate, even though the majority of the gate is built of a rubber-like material. or the mechanism may bend and break.

Sticking of the valve also occurs when waste piles against the mechanism and the mechanism cannot move. Most often, the waste builds up at the point where the flapper gate draws back, and sludge gets compressed against this slot in the gate valve housing. If it dries, it may feel almost like it is of concrete consistency, but softeners and detergent, soaking into the area overnight, may help to relieve the buildup.

If your gate mechanism does free up but seizes occasionally again later, it may be time to replace the entire mechanism. Each time it sticks, the polyvinyl gate may warp, fray or break, causing leaks or failing completely. If it seizes permanently, you will be compelled to deal with a full tank of waste when you need to work on replacing the valve—an entirely undesirable outcome.

How Can You Unstick Your RV Black Tank Gate Valve?

If the gate valve is seized because of rust or debris, gently pilling the handle back and forth may release it, you can spray penetrating oil or rust remover (WD40) on the exterior metal shaft of the gate valve, or you can use a rubber mallet and tap gently around the circumference of the pipe where the valve is installed.

If there is no material in the tank, you can disassemble the drain line and either replace or repair the gate valve. While this is a fairly simple process, it also can be quite messy. You should ensure that there is no or minimal material in the tank and that safety precautions are taken when you are working on this area of the plumbing system.

How Can You Clean Out Your Black Tank?

Three ways to clean your black water tank are:

  1. Add a cup of liquid laundry detergent, or dissolve a pod of detergent in two cups of hot water, mixed with two cups of water and a cup of dissolved water softener granules or soda to the tank. The softener acts as an emulsifier, removing waste that adheres to the corners and walls of the tank. Fill the tank with water through the toilet, then drain as you normally would. Make sure that you also bleed the grey water tank into the drain as the solids tank empties, to act as a venturi, drawing the solids mix along. The greywater already will have soaps in it.
  2. Use a pressure washer or garden sprayer to reach accessible parts of the tank through the toilet. Once the tank is drained, place a pail under the drain vent and carefully spray water into the drain line with the gate still open.
  3. Regular cleaning involves making sure the gate valve is closed, cleaning the toilet, flushing water into the tank (with detergent or a small amount of bleach) to about the halfway level, letting it sit for several hours, and connecting your garden hose to the backflush connection. Drain the tank, then flush water through using the garden hose until the water runs clearer. Close the valve, fill the tank and let it sit for an hour or so. Then drain, noticing if it is fairly clear. Do not leave bleach in the system, as it may dry out the gate valve. Follow these procedures to drain your tanks.

Do Macerators Work?

Macerators work by grinding solids into residue, then discharging it through a smaller (2-inch) line, instead of the 3- or 4-inch waste pipe. This allows solids to remain in suspension as smaller particles in the waste liquid and run freely into the holding tank. Macerators can be installed on the discharge pipe, or, in some cases, between the toilet and tank.

This requires elevating the toilet platform so that the macerator hardware can fit under the toilet and grind the waste before it enters the tank, in the same manner as a garburator in a kitchen. However, you also can purchase macerator toilets as a complete unit, albeit at a much higher price than purchasing the components separately.

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