By Dustin Simpson
There are so many questions about the black tank flush system, so here is a post to help out. You or your spouse may have noticed this vacuum breaker under a sink or behind a cabinet drawer. Here is some information and some pictures to help you understand what it does.
The black tank flushing system
The black tank flushing system is a straightforward system that uses fresh water from a hose connection to flush your black tank to help clean it out.
It consists of a hose connection on your trailer, a vacuum breaker check valve, and the flush nozzle. It is typically piped using 1/2” white PEX tubing.
The vacuum breaker check valve
The vacuum breaker check valve is typically located up under the bathroom sink. If not there, then it will be in the wall of the bathroom somewhere. It may be behind the shower wall.
If you have two black tanks, there will be a separate valve for each tank.
The vacuum breaker check valve acts to prevent black tank water from flowing back into the domestic water system that you are using for flushing water, and thereby contaminating it.
It is common for the valve to be installed backward, which prevents water from flowing through the flush line. It is also common for it to leak out of the top, so keep a close eye on it when flushing the black tank.
The vacuum breaker check valve consists of a float that rises when water pressure is applied. This seals off the top of the valve that is open to the outside air through the top cap of the valve. When water pressure is removed, the float drops and air is sucked in to break the vacuum and prevent a siphon. There is also a spring-loaded check valve in the outlet of the valve that will shut to prevent backflow.
Some owners have advised that the plastic valves tend to freeze and crack in the winter. You can change out the valve to a brass vacuum breaker if you are in a cold climate.
Hope this information helps you better understand more about vacuum breakers and why they are used.
Please follow us on YouTube, see our published articles on RVtravel.com and other social media pages.
Thank you,
Dustin
RELATED
- Does Dawn dish soap keep RV black tanks clean?
- How to fix RV’s leaky black tank dump valve
- Gadgets to keep RV black tanks stink-free and clean
- What is the best black tank treatment?
RVDT2765
I’ve never had a problem with my plastic vacuum breaker freezing in the winter but I blow out my system with an air compressor.
I’ve found that the float inside the vacuum breaker may get stuck after sitting in storage. I learned to add water slowly to the flush line for the first time I use it after being in storage. The water will lubricate the float and release it. Once you hear water flowing through the breaker, you can go to normal water pressure.
I also have a battery operated water leak alarm under the sink below the breaker.
Smart!
Since I use A/F to winterize, I use a hand pump to force the A/F into the flush port. There is an anti-siphon check valve in the fill port, so the line from the fill to the vacuum breaker will have standing water.
You could also press the anti-siphon check valve, but that doesn’t guarantee the line will drain completely.
I haven’t found my Vacuum valve. I wonder if previous owner deleted it
Check inside near where the flush port is located on the outside. Usually located inside a cupboard.
Take it out and check for bugs. Cover with mesh and re install.
Ask Dave: I recently had a serious leak at the vacuum breaker/check valve in my black tank flush system. I replaced it with a brass unit, making absolutely sure it was installed with the correct direction of incoming water flow. But I cannot get any water to flow into the black tank. To be doubly sure the new valve was installed correctly, I opened the fitting at the exit of the new valve and plenty of water squirts out of there! I believe the tank “nozzle”, or nozzles, must be plugged. That perhaps being the reason for the failed breaker valve. My question, how can I un-plug/clean the nozzle(s). Thank you!