RO system output pressure and HW dispenser

I want to install a hot water dispenser with the input coming from an RO system. (RO system has an artesian filter on the output, so I don't expect to have an issue with agressive water deteriorating parts prematurely.) The hot water dispenser (Insinkerator HC-Wave) states 30-125 psi input, and when I wrote them they said that under 20 psi, their faucet may drip. RO people won't commit to an output pressure, they want to sell me their HW dispenser.
Two questions: 1.) I have good water pressure in house (60psi regulated, ~100psi unregulated) so what can I expect for pressure out of the RO system? And 2.) Can I install a pump to increase the pressure in the holding tank if the system doesn't have enough pressure on its own?

If you have a bladder tank after the Artesian filter, the tank should be filled nearly to system pressure.
http://www.h2odistributors.com/bladder_tanks.asp?gclid=CKz4ytaBwYsCFQ8uUAodOBmsDg

Got a response to the same question at another forum yielding about the opposite response. A person there posted that the pressure on the air side of the bladder was about 4-7 psi, and that would be the output. Doesn't sound right to me, but I've only owned one of these that was serviced by company - never "built" one myself.

Is the 4-7 psi the pressure when the tank is nearly empty? Isn't there a check valve to keep the tank at a certain minimum pressure?

A bladder tank has a captive volume of air. You pressurize the tank with no water in the tank. That psi is back pressure on a pump or in this case an RO membrane. Most RO storage tanks have 7 psi air pressure. That pressure is what pushes the water to the faucet when you draw water.

So if you need RO water at a higher pressure, you need a delivery pump. They will not give you the pressure required by your hot water dispenser. They usually do not give you more than .5 gpm flow either.

I suggest giving up on this idea of feeding it RO water and if not then you must go to a larger storage tank and pump that can build the minimum pressure and gpm needed. Or you'll have to buy a HW dispenser that can use water from an RO, from a water treatment dealer.

You are correct when you say the pressure of the bladder pushes the water out, but it isn't at 7 psi when the tank is full. Going to my physics

PV/T = constant

Assuming negligible temp variation

PV = constant

Relating this to the bladder, as the tank fills with water the volume of the bladder decreases, and the pressure inside the bladder increases. So if the initial volume of the bladder is 2 cu ft at 7psi, then its volume at 60 psi is ~ 1/4 cu ft. The water pressure in the tank matches the air pressure in the bladder, of course, since the two balance each other. Conversely, as the tank is emptied, the pressure in the bladder (and water pressure in the tank) decreases. At some minimum pressure, the check valve kicks in and stops water flow.

If the pressure of the bladder didn't change as the tank filled, then the volume of the bladder would likewise have to remain unchanged and the tank wouldn't fill. Very zen.

This is why the tank must be empty when adjusting the air pressure in the bladder. And if you didn't have a bladder, the tank would fill to the same pressure, but you'd have no way to get it out - once the pressure of the tank was relieved, the internal pressure of the tank is the same as the pressure outside the tank.

You can prove this to yourself by checking the air pressure of the bladder when the tank is full. Is there a way for you to do that?

Thanks for helping me figure that out.

ro

The pressure in the RO system is controlled by the sensing unit that stops its operation when the set pressure is reached. It is not adjustable. Changing the tank pressure, when it is drained, only increases or decreases the volume of water before the tank's supply is exhausted.

Yes I didn't explain it as well as I could have and I see now I made it sound as if there is only 7 psi yet the bottom line is your hot water dispenser isn't going to work properly without a larger tank and a pump that can handle (aggressive) RO water.

The ASO (automatic shut off) valve in 99% of all ROs shuts off the product water to the tank/raw water to the membrane when the tank pressure equals the preset point psi of the ASO. And yes, if you measured the tank's air pressure then, it will be equal to the set point pressure of the ASO valve which IMO will not be the main line pressure. Since there is so little water in the tank, and usually the ASO is shut when you start drawing water, the pressure in the tank falls very quickly and then the ASO opens and the membrane starts producing water with a very small flow rate to the tank.