[AC_Hacker]

I don't want to discourage you from making a rainwater flush system, I think it's a great idea.

I went through the same frustration for 20 years with a poorly working Low-Flush toilet also (it was not even dual flush). I even did a little research on the problem and found out that the early Low-Flush toilets had problems such as you described because the P-Trap that is built into the early toilets was not glazed (unglazed P-Traps were not a problem for standard flush toilets). So the toilet industry's fix was to glaze the inside of the P-Trap and that seemed to fix the problem.

When I did my bathroom remodel, I looked at every toilet I could find and I learned a lot. I noticed that most of the toilet manufacturers have some kind of flush rating system for each model they make, and that the rating system is not necessarily connected to price. You can actually find a cheaper toilet that is rated higher than some of the more expensive toilets.

I also learned that the diameter of the exit hole in the tank plays a big part. If the diameter is large, the water moves out of the tank more quickly and has more kinetic energy to do its job... look for the toilets with the bigger exit hole in the tanks.

I also looked into toilets that have a spring-loaded bladder inside the tank (the spring is compressed by water pressure) and pretty much shoot the poop down the hole, and use astonishingly little water to do it. The flush event is sudden and explosive and final. I thought that it would be really great, but apparently women are frightened by the whole thing. These kinds of toilets can still be found, but you might have to special order. I even found one that was run off of an air compressor and used extremely little water... these were for extremely sensitive environmental areas. I decided against this, too because I didn't want my toilet to quit working just because my air compressor was on the fritz.

I also found out a lot about code for waste pipes, slope and venting (this might actually be your problem, and not the toilet), etc. Turns out that 4" pipe is standard for waste, but 3" is ok. I was so sick of bad flush experiences, that when I re-did all of the waste lines in my house (my house was built in 1892, so it was a no-brainer) I found out what the minimum size waste line was for every particular use (tub, bath sink, kitchen sink, dishwasher, toilet, etc. and went one size bigger throughout. I also found out what slope requirements for the horizontal run was and went a degree or two greater. My waste vent stack was already 4" which is maximum, so I left it alone. I also learned that when waste lines go from horizontal to vertical (down) that a sharp turn is called for, but when lines go from vertical to horizontal, turns with a larger radius, called "90 degree sweep" is used.

Lastly, when I chose my toilet, because my bathroom was so space efficient (AKA: tiny) I used an in-wall flush unit by Geberit with built in dual flush, and I used a Toto wall hung toilet, which had a very good reputation for flush success. As an added benifit, the Toto has some kind of micro silver particles combined with the glaze that discourages microbe growth... it actually works. Sometime, after a month or two there's a hint of color on the bowl wall, but a hand sprayer will clean it right up.

Long story short, since all this work, I have not had a single marginal flush event (when I remember to use the proper flush button).

It really has been worth all the work and expense.

-AC_Hacker