View Full Version : difference between low flow and regular bowl?

07-18-2007, 05:33 PM

Does anyone know authoritatively what the difference between a low flow and an older regular toilet bowl is? I know the tank is different, I just wonder if I can replace an old tank with a new 1.6 gpf.

If you know, I'd really appreciate the info! Thanks in advance.

-Rich :confused:

07-18-2007, 07:06 PM
If you could do this, then just adjust the float to only allow 1.6 Gallons into the old tank... Try it, it won't work.


07-18-2007, 07:38 PM
One reason many low-flow toilets got a bad reputation (and some still do) is that they didn't do the engineering to change their designs so they would work with the mandated low flow. There's more to it than just limiting the flow. Most of the new ones have a taller tank so there's still some water pressure when you flush and other small things.

07-18-2007, 09:30 PM
Thanks. That makes sense, that there would be engineering changes necessary to a-commode-ate (heh, heh!) less water. I guess I'll just ante up for the Toto Drake and pull the old toilet one ---- more ---- time ---- ugh! So sick of wax rings!

07-19-2007, 06:31 AM
Most companies designed their 1.6 tanks so you could not do it anyway. But if you could, the 1.6 gpf tank does not supply enough water to flush the old toilets properly.

07-20-2007, 09:28 AM
Yes, I think you're right. Still, I'd love to know, from an engineering standpoint... what changes did they make in the bowl to make 1.6 gallons accomplish the same thing as 7 gallons? ... anyone... ???

07-20-2007, 09:54 AM
i guess a good avenue for curious minds to explore would be to look deep into the bowls designed to bolt onto Euro style wall mount carriers. Like Duravit as one example.

They were designed as "flush-down" technology right from start. They sort of trickle water down the sides and build up just enough momentum to push solids through.

American style toilets were originally designed to use the "siphon" technique, which works well when water builds up a head, breaks through the pressure, and then relies on momentum in the pipe to suck forward everything behind it including a huge amount of extra water added "for good measure". I can remember seeing solids first float upwards for a number of seconds before then being pulled downwards disappearing fast along with a lot of extra water and making a lot of sucking slurping noises. Not a happy image or sound to remind you all of, in casual conversation, but since you asked, there's your answer.


07-20-2007, 02:10 PM
There are three prominent features on most recent low-flow designs, the combination pioneered by Toto. These were first used on one-piece Toto models, to improve performance with the low head pressure of the lower water level in the tank. Eventually they were designed into the two-piece Drake, and now most of the Toto line.

1. Oversized flush valve: Toto used a 3 inch flush valve. This provides a 125%increase in cross-sectional area over the common 2 inch, and dumps the water much faster.

2. Direct-fed jet: The water for the siphon jet is fed directly to the jet from the input at the tank through a channel or channels on the outside of the bowl, rather than branching off the front of the rim. This shorter, straighter, bigger channel provides higher pressure at the jet.

3. Flex-trap: Toto added a second hump in the trapway, just before it exits. After the first flush, a pool of water remains in the lower loop, shrinking the size of the trapway for a stronger pull by the siphon, but without impeding movement of solids through it.

Some or all of these features have been copied by other manufacturers for most of their recent designs.

07-20-2007, 09:28 PM
Toilet engineering.......begs the question, why do they still clog? Why not a
4" trap way? Anyone?


07-20-2007, 10:30 PM
Caroma makes a 4" trapway.
It has a very small bit of water in the bowl.

07-21-2007, 07:07 AM

There is a cutaway view at the Toto web site, complete with labels. It shows the older sled type flapper, but the idea is the same. You can see how the water branches to the rim and to the siphon jet at the input from the tank. You can also see the extra hump in the trapway, just before it turns down.

07-21-2007, 08:20 AM
Thanks, all! I took your advice to look at the Toto website and I found a great video of the whole system. I already trusted the information I got here that mounting the 1.6 gpf tank on the old bowl wouldn't work, but now I know why. Here is the url : http://www.totousa.com/innerbeauty.asp just click on the streaming video link.

Now... Terry... a 4" trapway? Suppose one of the children fell in!!! Where will it end??? ;).

Again, thanks everyone. I'm pickin' up my Drake 2 pc. today (replacing my 1968 Amer. Std.) and expect to be done by tomorrow.

07-22-2007, 10:52 AM

Well, I installed the Drake yesterday and it's way better than the old high flow. It is a little noisier than I expected, but I guess there is no way to have power and quiet at the same time. I'm also not crazy about the fact that, in order to get a full flush for solid waste, I have to hold the handle down until the tank empties. Does that seem a little hokey to anyone else?

Question: the installation directions refer to 3 points of contact for the tank onto the bowl (two in the front... one in the back). The diagram shows these 3 points as being raised. The actual toilet does not have these 3 points raised and, in fact, I torqued those bolts down about as hard as I dare, and I still don't have actual contact between tank and bowl. Has anyone else recently installed a Drake and know what I'm talking about? Am I missing something, or did they just portray the points as being raised in the picture to illustrate where they are?


07-22-2007, 01:50 PM
The first person you find that can lay a BM that requires a 4" trapway...whew...not sure I want to see it.

07-22-2007, 04:22 PM
When they first went to 1.6 gallons, the manufacturers realized right away that 1.6 gallons of water was not sufficient to initiate the siphon action which is what makes a toilet work. The fix was to make the trapway smaller. This flushed well, but of course was prone to clog due to the small size.

Your old toilet with a very large trapway...the 1.6 gal is barely enough to fill the pipe, but will not initiate the siphon to remove the waste.

Toto was I think the first one to figure out that you could go back to a larger trapway if you could make the water go faster, hence the large flappers. After all, there has not really ever been a problem with 1.6 GPM flushometer toilets.

There are fine points, but that is it in a nut shell.

07-23-2007, 08:31 AM
If everything is working as designed, it shouldn't be necessary to hold the handle down until the tank empties. The typical human stool is less than 250 grams. The EPA recommends that toilets be designed to flush a minimum of 350 grams. The Drake has been tested to consistently flush 900 grams. Thats 2 pounds of poop.

The original 2 inch flapper has a cup on the underside that holds a bubble of air, causing it to float once opened, until all the water has left the tank. The Toto 3 inch flapper has a carefully calibrated hole in the side of the cup. It floats briefly, but then closes before all the water has left the tank, releasing exactly 1.6 gallons.

A sightly different flush handle operating technique is required. The handle must be pushed all the way down, pulling the flapper fully open, and released immediately. Watch with the lid off, to see if the flapper operates as described. If all is working properly, about half the water will drain from the tank. In the bowl the water rises momentarily, the siphon begins and the water starts to drop, then a wave of water from the large front rim jets sweeps everything into the trapway. Watch fast, it all happens pretty quick.

07-23-2007, 03:13 PM
Thanks for the education, Sam and Jimbo. I was assuming about having to hold the handle down. Clearly, I was wrong. I have now used the toilet extensively and I see what you're talking about. It is really very powerful and clean! I'M DONE WITH THE BATHROOM!!! :D :)

08-09-2007, 02:40 AM
There are the toilets with the measured pressure assist water flow.Sloan/Kohler. And the Caroma type with a supposed .8 and 1.6 dual flush. I installed 2 Caroma Caravelles in a customers home. 1. there specs give you a quarter of an inch tolerance from the back of the tank to the wall. Yet their toilets are cast poorly.Can't hold that tolerance I had to take 2 tanks back (cracks on inside corners ) The EPA has yet to test them.so testing was done in Canada at 20 psi or gpm,their results confirmed that it took multiple flushes to operate. They chalked it up to people liking to press the buttons . I don't see how they flushed because these won't flush at 50 psi on the .8 unless you depress the button longer (rendering it not high effiencency).By EPA standards if you can alter the amount of water flow per flush its not high efficiency
It also holds so little water in the bowl in between flushes it has to be brushed allot.Skid marks.I was thinking of making a holding tank for the sink water and using it to clean out the bowl:D
There is no written warranty given with the product, and on the one poorly mimeographed instruction sheet they do give, The phone number for spare parts in Canada is bogus. A guy named Joe answered and said he never heard of Caroma. These are $500 dollar toilets that are spares oriented .proprietary tank lid. Specialized $50 dollar seat, set on a self centering plastic cams that will not stay tight and only rated at 50 to 75 lbs.The internet is full of comments about . What ,no fat Aussies. And if you switched it with a seat with metal screws without really good washers. it would ruin the toilet . I wondered if it would void the non-existent warranty?
. And luckily both floors had been tiled so the flange was lower than the floor . I had to use a bare ring (the thinnest)otherwise it wouldn't seat. I would have had to shim it a good 1" and it rocked like a teeter totter because the screw holes are centerlined to the drain.At the plumbing store they had a blue rubber offset sleeve that replaced the wax ring on their offset models .The company referred me to an installation video. It was hilarious. A guy comes into a room. Picks up a toilet ..set it down on the flange(no wax ring) and leaves. no talk,nada. I found out this model was manufactured in maylasia by GW Limited.The tanks were made in Australia.because of the trademarked watersaving device. I only bring this up because the EPA is rating these toilets to get matching state and federal funds(water saver programs) yet there are problems with allot of them. Too many imperfect technologies foisted on the trusting consumer for the environments sake WE NEED TO CONSERVE Do not buy the Caroma Caravelle 305. thank you

The Caroma Sydney comfort height round 270 bowl

Mike Swearingen
08-15-2007, 02:54 AM
This thread illustrates exactly why this old mossback still has his two 1977 toilets. I will never change to these so-called high-efficiency, low-flow 1.6 gallon toilets. Most that I've seen require two flushes. Water savers? Ha! I've personally never seen one that works as well as my old ones.
They're gonna have to pry my cold dead hand off the handle of one of my old gushers some day. At least when I flush it once, it all goes away. LOL

Old Dog
08-15-2007, 03:19 AM
I am constantly amazed by all the good info I glean from this site.As a G.C. I like to try and stay ahead of the learning curve on products and procedures with the trades I work with on a daily basis.
I'm impressed with the passion that many here possess when it comes to their trade and livliehood.I don't think I've been in the company of so many men who literally know their s*** about toilet technology.This old G.C. tips his hat and raises his (toilet) seat in salute to you all...

08-15-2007, 08:49 AM
If you have a Caroma toilet, and you are having trouble with the flush,
Then it hasn't been adjusted.
They use a fluidmaster fill valve.
Pretty standard equipment as any plumber will tell you.
Just raise the water lever to 1/2" below the overflow, and you will see a dramatic improvement.
The Caroma works very well on one flush.
I sell plenty of them to the National Park system for their hotels.
You do, as an installer, have to adjust the water level though.

I would never, ever, consider going back my old 1977 toilets.
I was glad to see them go. And glad I could put the plunger where it belongs. By the way, where is that plunger? I haven't needed it for so long, I forgot where it is.

08-15-2007, 12:42 PM
I have two a/s toilets a one peice 3.5 and 1.6 two peice ,never had any problems with both .Although I am interested in the toto.