Enough psi, but not enough volume(toilet with Sloan 3.5 gallon flushometer)

[john]

Hello,
 I have been finding conflicting answers to the following question and would like help on finding an answer.
Question:
What size expansion tank is needed to flush a toilet with a 3.5 gpf Sloan flushometer ?
Ok, here is what I am trying to do. I bought a toilet with a Sloan flushometer on it, which requires more volume of water than my 7/8" city water main can supply. There is more than enough pressure coming from the city line at 61 psi, but not the water volume. I have been told by several plumbers that the installation of a well pressure tank should provide enough volume to flush the toilet. The 61 psi city water line pressure is enough to fill the pressure tank without a pump , so again, all that is needed is a pressure tank put INLINE with the toilet. There is no need to add extra volume to the whole house, just the toilet, one appliance. And since I am the only person in the house, I do not have to be concerned about more than one plumbing fixture operating at a time.
I have been told that I will need a tank that will hold 3 times the 3.5 gallons needed to flush the toilet, which is 10.3 gallons. This is, as I understand, the amount of water needed in the pressure tank in addition to that of the tanks diaphragm volume. Can you assist me in finding the answer?  What size pressure tank will I need that will provide the volume of water to flush a toilet with a 3.5 gpf Solan flushometer?

Thanks!
John

[Cary Austin]

Hi John
If the system pressure stays at 61 WHILE the toilet is flushing a pressure tank will not help.  It would take a 40+ gallon size pressure tank to hold 10 gallons of water.  But the pressure will have to drop from 60 to 40 for this tank to express any water into the system.  If the pressure stays at 60 the tank will just stay full and you won't get any extra volume to the toilet. 

If when you flush the toilet the pressure is already dropping from 60 to 40 then a tank would be adding its draw down as the pressure decreases, so more volume. 

But again if the pressure does not drop when flushing the toilet you have a restriction to the toilet, not a volume problem.

[john]

Cary,
Thanks for your help. Before I invest in a pressure tank, I have one last comment. I haven't plumbed the toilet yet, so if I do, then I assume that the water supply pipe to the toilet will have to be 1 1/8" to match the inlet at the Control Stop on the Sloan flushometer. This will insure no restrictions from the pressure tank to the toilet. The pressure tank has a 1 1/4 fitting, so there shouldn't be any restrictions at this area....right?

John

[Cary Austin]

Gallons per flush and Gallons per minute are not the same thing.  I have looked at the specs from Sloan and for the life of me I cannot find a required Gallons Per Minute for that toilet.  A 1/2" line can supply about 5-7 GPM.  So if the flushometer takes 30 seconds to flush it should be fine.  But if the flushometer flushes 3.4 gallons in say 5 seconds, that is a flow rate of 68 gallons per minute. 

I tried calling Sloan and cannot get an answer.  You really need to find out the required GPM for this unit. 

I used a CSV on a large pump at a roadside park in Washington that had about 200 of those flushers.  I do not remember there being an increased line size to each toilet.  This makes me think a 5-7 GPM flow will do, and you won't need larger lines or a pressure tank.  Let me know if you can find the GPM requirement on those units?