RO/DI system....How many GPD?

I have been looking at various units which vary in GPD ratings. I am assuming this is an output rating of "gallons per day"? That would be at a constant "on" it would produce 50 gallons of RO/DI water per day? If that is correct and I am not planning on drinking or cooking from this supply point than for my basic top-off needs of 1/2 gallon per day and maybe an occasional percentage of tank water change over, why would I even need that large of rating? I am curious as to why some of the same sized units with same number of stages show different output ratings? What limits or restricts this?

 

you dont need 50 gpd. but when it comes time to do a water change its nice to turn on the RODI unit for an hour and get all the water you need, otherwise you may be waiting 4 or 5 hours for 10 gallons of water. Thats really the main reason for the high GPD rating.
And from what i understand about flow restriction is that its based on what your RO membrane can handle. e.g. my membrane is rated at 70gpd while someone else's can be rated at 100gpd.
And just to note the higher the GPD rating the greater the water pressure is required to properly operate the RODI unit. e.g. 70 gpd needs 50psi while 100 may need 70psi.
Hope this kinda helps..
EDIT: also Bulk Reef Supply has some good demo videos about RODI units..
http://www.bulkreefsupply.com/store/brs-tv/ro-di-videos

 

I would say that is great info, more then I would have thought to provide lol. I would say that 50 GPD would be a happy medium. That way you are waiting for water all day long as stated above.
Mozilla/4.0 (BREW 3.1.5; U; en-us; Sanyo; NetFront/3.5.1/AMB) Boost SCP6760

 

The advantage to a 75 or 90 GPD system is it makes the water much faster so less time standing around with a faucet tied up. The cost and the physical size are the same so theer is no advantage to a 25 or 50 GPD system.

Look at the RODI-90-REFURB here for a great deal on a true reef quality system.
Untitled Document
SpectraPure Water Purification Products

You won't find a better deal or a better system anywhere guaranteed. All you need to add is a handheld TDS meter for $25 and you are in business.

You might also consider the BFS-161 found here:
http://www.buckeyefieldsupply.com/showproducts.asp?Category=168&Sub=166

Or maybe the Optima series found here:
RO/DI Filter Systems

It comes with a handheld TDS meter and is ready to go.

The different outputs are the differences in the RO membranes themselves, you can find them anywhere from 15 GPD to 150 GPD with the Dow Filmtec 75 GPD (the starting point for Spectrapures treated and tested 90 GPD) the most popular choice hands down. It probably sells 100 for every 1 of the others combined.

The number of stages has little effect on the GPD, if anything more stages reduces the GPD due to headloss. Pressure does have an effect as does temperature but the 75 GPD Dow is rated at 75 GPD at 77 degrees F water temp and 50 psi water pressure. Higher pressure or warmer water increases the flow and colder water or lower pressure reduces the flow. Its still labelled a 75 GPD regardless. Spectrapure rates theirs at 77 degrees F and at 60 psi which is more normally found in most US homes.

With any RO or RO/DI you want to make water in larger batches so the membrane has a chance to rinse up andclean itself, short runs are not advised and will lead to premature failure and lesser quality water. It is better to make 5-10 galons once a week than to make 1/2 gallon every day. You should use the unit at least every 10-14 days to keep it fresh and it will last a long long time.

 

Thanks for the info/responses. Hey AZDesertRat, If I don't want to run a pressure pump and I am on a municipal water supply, don't you think I will be limited by whatever my water supply pressure is? And how, without buying a guage for a one-time use, would I pre-determine my output pressure and what size I can accomodate?

 

All RO and RO/DI units will operate down to slightly less than 40 psi. The average municipal system is between 50 and 60 psi since they must also provide fire protection via fire hydrants as well as domestic usage.
A pressure gauge and hose thread adapter runs about $8-$12 at any hardware store and is a good thing to have. You can also call your local water utility and they should be able to give you a very accurate delivery pressure figure.

It does not matter if its a 15 GPD or a 90 GPD RO/DI system, they all require the same 40 psi minimum. If you had a 15 GPD and only 40 psi plus cold water it would maybe do 5-10 GPD, but a 75 GPD would still do 50 GPD so is always going to be the better deal.

If you look at the top right side of the page here you will find a calculator where you can plug RO membrane sizes, pressures and temperatures in and get an idea of what to expect:
www.buckeyefieldsupply.com