RO output pressure and hot water dispenser


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Old 04-13-07, 06:39 PM
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RO output pressure and hot water 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?
 
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Old 04-13-07, 06:50 PM
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1.) I have good water pressure in house (60psi regulated, ~100psi unregulated) so what can I expect for pressure out of the RO system?

The holding tank used in common ROs has a static pressure of 4-7 lbs @ sea level. That is the delivery pressure from the RO (slightly lower pressure at altitude).

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?

Yes you can add a pump to increade delivery pressure. You should discuss that with the manufacturer of your RO to see what they recommend.

I'm curious about the Artesian filter and would appreciate you posting back after you've had it a while. I've had a simple 4 stage RO for over ten years providing cooking and drinking water and feeding my icemaker with no "agressive" water problems at all. I do have the only icemaker in the neighborhood that works and my RO water tastes fine.
 
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Old 04-14-07, 08:12 PM
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RO output pressure and hot water dispenser

Be happy to. Doubt that I'll have any problems except......

The same posting at another plumbing forum (different website) yielded a different response, stating that the pressure would be nearly the same as my household pressure, and that's what I'd get as an output. So I'd like some more opinions...

Is the 4-7 psi the pressure in the tank when there's no more water in the tank? I believe there's a check valve in the system to shut it off at a certain pressure, anyone know offhand what the "standard" check valve shutoff is?
 

Last edited by mical_k; 04-14-07 at 08:16 PM. Reason: wrong information
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Old 04-14-07, 09:22 PM
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There can always be some confusion if the parameters are not defined so let me qualify and then you take you pick of who's offering you accurate info and who's not...

IF you're talking about a "standard" design RO then the static pressure in the empty bladder tank is 4-7 lbs/in2 @ sea level. That is the delivery pressure at the RO faucet of the RO'd water. If the house service pressure was applied to the RO output faucet it would splash all the way to the ceiling of the kitchen.

The only pressure on the processed water at the delivery side of the membrane is applied by the pressure in the bladder storage tank.

If you want to verify this fact then go to... http://www.pwgazette.com/
Email them or phone them up. They specialize in ROs and know their stuff.

With respect to "another plumbing forum", they are incorrect.
 
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Old 04-14-07, 11:08 PM
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Thanks for the link. I got answers to almost all my questions. That's the good news. The bad news is, you came up a bit short on the correctness scale. Sorry. From their website...

"A standard reverse osmosis unit is controlled by an automatic shutoff system that shuts down water production when pressure in the storage tank reaches about 2/3 of the inlet water pressure. In other words, if your city water pressure is 60 pounds (psi), your reverse osmosis unit will stop producing and storing water when pressure in the storage tank reaches about 40 psi. For most household purposes, this is plenty of pressure and the RO unit will run wonderfully."

They add that a permeate pump will increase the efficiency of the system (more water with less waste) and will pump up the pressure to nearly the inlet pressure (60psi in the example above.)

Thanks again for the link!
 
  #6  
Old 04-15-07, 12:04 AM
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I have not seen anywhere near that pressure at an RO output faucet. In fact, many newer refridgerators with icemakers have an "attention tag" that feeding the icemaker with an RO may not work due to the low output pressure of the RO.

See #12 here... http://www.frigidaire.com/support/FAQ-Refrigerator.asp#12

You can prove this with your own eyes and RO. When you get your RO hooked up, and it's made a tank of water, open the RO faucet and observe the water flow. Now, bleed all the air out of the storage tank and observe the flow out of the RO faucet. I realize that the bladder (with 4-7 lbs behind it) will be progressively compressed as the volume of water increases in the tank but the delivery pressure at the faucet never seems to approach the numbers you quote.

Remember, to pressurize the tank it has to be empty of water and you need a small pump, like a bicycle pump. A digital pressure gause makes setting the pressure easier.
 

Last edited by justalurker; 04-16-07 at 07:34 AM.
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Old 04-16-07, 01:22 PM
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R O Pressure

Mical,
You are correct regarding what pressure the R.O. will build up to. Depending on what the inlet pressure is, it will build up to approx. 15-20lbs. less than that and with units with a automatic shut off, it will stop making water. Remember though that as soon as you start using water from the storage tank the pressure will decrease proportionally towards the tank bladder pressure the more you use your water. So the 40 PSI will only last a very short time in proportion to what size your holding tank is. The main problem I've encountered over the years is the small tubing generally provide by manufacturers from the storage tank to the faucet. Pressure loss due to friction is your greatest deterrent to good flow at the R.O faucet. Try to locate your storage tank as close to your faucet or refrigerator as possible to try to keep this loss to a minimum. Use 3/8" tubing from the storage tank to the faucet will also help with pressure loss. A Larger storage tank will help keep your pressure higher longer. Remember; be careful of ratings of the units and storage tanks. If they are rated at 50 gallons per day, that rating is at 80 psi and 77 degree water, probably not your pressures or temp, which will dramatically affect actual make up of water. If the storage tank is rated at 4 gallons of storage, probably expect it to be approx. 60% of that with the bladder taking up some of the storage space. In that example your usable pressure would drop from 40 to 7 psi in just 2.4 gallons of use IF your tank was full to start with. The emptier your tank gets, the lower the pressure will be. Down to 7 PSI just before it's empty. Don't confuse pressure with flow, they are related but not directly. A wild example would be, I can get 200 gallons per minute through a 6" pipe at 2 PSI with out any loss due to length of pipe. But at 100 PSI I can't get 5 gallons a minute through a 1/4 " tube! Long story short, Buy an larger storage tank or DELIVERY pump from the tank for more even pressures, use 3/8 tubing from the storage tank to keep pressure loss to a minimum. Keep storage tank as close to appliances as possible. Buy units figuring, as a simple rule, you will receive 50 percent of what it's rated at, both for storage tank and rated make up.(EX. 50 gallon per day, figure at 25-30 per day meaning slightly just more than 1 gallon per hour, So if your rated four gallon tank is empty, it's going to take 2-3 hours to fill it( 2.3 gal.) and reach 40 psi again) Remember also the refrigerator itself has over 15ft INTERNALLY of 1/4 tube and storage adding to this problem. Some Manufacturers are just now starting to change to 3/8 tubing internally in their units to help combat this pressure/ flow problem with RO units. Hope this helps. Good Luck

TJ
 
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Old 04-16-07, 01:59 PM
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TJHornet,

That explains why the delivery pressure at the RO faucet seems so much less than the numbers would indicate and the pressure does fall off rapidly. It also expplains why the loss of only a pound or teo in the bladder redecues the flow so much.

I stand corrected and thanks for the info.
 
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Old 04-16-07, 11:12 PM
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After thinking about it, the physics works like this. Assuming constant temperature, Pressure x Volume = a Constant. As volume of bladder goes down (tank is filling with water) pressure in bladder goes up, and the tank pressure must equal the bladder pressure. So if bladder is 1/8 its original size (1/8 x 4 gallon = 1/2 gallon) then after emptying a half gallon the bladder doubled its size and water pressure is half.

I'd gladly trade all the physics I learned in high school to be that age again. Not that that age was great, but what a ride between then and now.
 
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Old 04-16-07, 11:31 PM
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I've been fiddling around with the RO tonight (still without a pressure gauge) and the pressure does fall off rapidly as the tank empties. Getting the first quart out is much quicker than the second quart and the third is MUCH slower.

So, after all these posts, all over the internet, it seems an RO won't properly supply an instant hot gizmo. I wonder if there isn't an instant hot dispenser designed to work off an RO?

Agree about High School and the ride since then has been memorable.
 
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Old 04-28-07, 01:24 PM
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Justalurker, I noticed you mentioned about the fridge water not having enough pressure to run off an r/o system. What can be used to compensate? A high pressure storage tank? Or would you recommend a pump?
 
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Old 04-28-07, 06:03 PM
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Mabulok,

If you want to run an icemaker from an RO here's what I recommend...

Only use PE (polyethylene) tubing. RO water is too agresssive for copper tubing.

Try hooking the RO to the icemaker. There should be an unused "T" at the output where the RO goes to the faucet. If there's not a "T" there, add one. You can get nifty 1/4" John Guest (push-in) style "Ts" at Home Depot. While you're at the Depot you may want to add an in-line valve at the output of the RO to the icemaker. Makes it easy to service. They are right there where the "T"s are.

The "T" looks like this... http://www.pwgazette.com/images/parts/jgtee14.jpg
The in-line valve looks like this... http://www.pwgazette.com/images/parts/inlinevalve.jpg

You have to "copy and paste" as HTML is turned off on this forum.

My fridge works fine on the RO. If your's doesn't one answer is a permeate pump and you can scope them out here...

http://www.pwgazette.com/permeatepump.htm

and here...

http://wattspremier.com/watts/showdetl.cfm?&DID=15&Product_ID=161&CATID=1
 
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Old 04-28-07, 07:51 PM
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Not sure if this helps but,

As has been said an RO system has a very low delivery volume.
An icemaker is a device that does not require a very large volume and would work well directly connected to an RO system.

A point of use water heater on the other hand needs to have a constant delivery of water to the elements or they could burn out or go off on the limit stat.
Commercially, the way this is handled is to have an atmospheric storage tank that the RO system fills that then supplies water to devices with a demand pump that does not require a pneumatic tank.
 
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Old 04-28-07, 08:01 PM
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Greg,

You can do it that way and here's a nifty pump, but they are not cheap.

See delivery pumps here... http://www.pwgazette.com/6800pump.htm

What I've seen is that some newer fridges require a certain delivery pressure just to open the feed line. Vikings and Zeros are that way. A lot of icemakers work fine on an RO even though they say they require a minimum pressure that is not mentioned.

Actually I have been corrected recently and I now know that ROs deliver at approximately 66% of line pressure which is not that low and works more icemakers than people realize. I've had a high percentage of success just hooking the RO up and 9 out of 10 times it works fine.

Odds are if he tries it it will work fine. If not, then he can look at his options.
 
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Old 04-28-07, 11:13 PM
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Yes, that is the type of pump I was referring to.

They use those on most DIY RO water dispensing set ups in supermarkets and such.
I managed to scrounge a couple like that from servicing those store units.......Just need to make up something for myself to use them on.
 
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Old 04-29-07, 05:14 AM
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Well, I've already completed the job. I tee'd it off at the output line so I could continue to feed the sink dispenser. The ice maker works fine, and the fridge water works adequately if not a little slow. And it now shuts off slowly, leaving a slight drip or two before completely shutting off. I'm just going to monitor it for awhile before forking out 100 plus bucks for that booster pump. Maybe I'll "run across one in my daily ventures" and it won't cost nearly as much. Thanks for the input, it got me started on the right track!
 
 

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