Living the midwest - the cold water pipe can get pretty darned cold. I'm eyeballing 2 RO/DI units on ebay - and not sure if I'd go perma fixture in bathroom - or mud room hose connector where I can control water temp.

I'd really prefer to do permanent install - but not sure how that will go over in the cold of winter for producing usable water from the RO membrane.

Still useful during the peak of winter chills?

Production rate is proportional to water temperature.
Expected GPD=Rated GPD x PCF x TCF
PCF is the pressure correction factor
TCF is the tamperature correction factor

So a 90 GPD unit will produce 90 GPD @ 60 PSI @ 77 deg. F

PCF = line pressure ( in PSI ) / 60

So say you are looking at a 90 GPD unit, your incomming temp. is 41 deg F and your line pressure is 50 PSI.

Expected GPD = 90 x .83 x .521

So your 90 GPD unit is now producing roughly 39 GPD.

You can look up a temperature correction factor table and work your own numbers.

Personally, I would look into adding a booster pump to increase production in the winter time, mixing hot and cold water for a good ratio could be risky, if its too hot, youll ruin the membrane.

Yeah, that was a concern I had thought of too. Plus if somebody flushes the toilet - or starts a load of laundry - there could be a sudden flash of hot water that wouldn' be good.

So you would stick with the cold line feed - regardless of snowy winters?

Depending on your demand, it should not be too big of a deal. Even with low temps. You should be able to produce about 5 gallons in about 3 hours with a 90 GPD unit without a booster pump.

I prefer a permanent install, but some people enjoy the portability of their units. Wherever you decide I would only plumb it to cold water side.

Steer clear of ebay units, they are cheap for a reason. With RO/DI you really do get what you pay for and cheap is not where you want to be with water quality.

Cold water does affect GPD output as well as rejection rate. www.buckeyefieldsupply.com has a very easy to use calculator at the top of their home page. Enter a membrane size (75 is the standard size most vendors use), a pressure and a temperature and it will spit out a very accurate flow rate. As for rejection rate, colder water is more dense so the rejection rate goes down. Do not confuse GPD with rejection they are two different things. Rejection rate is the percentage of contaminants that are removed ie 96-98% rejection or removal efficiency. GPD is output regardless of rejection or removal efficiency. Either way cold water affects both.

If your water temperature changes significantly from season to season you may want to have two capillary tube type flow restrictors, one trimmed for summer and one for winter so you keep the waste ratio at 4:1 year round. They are only about $4 and can be found from this vendor here:
http://www.spectrapure.com/email/customer-...eciation.html#1

I would also highly recommend their RO/DI units too, you can't beat the ProPlus at $149 and the MaxCap is the Cadillac of the industry.

They also offer booster pumps which will overcome any cold water problems you have.

Do you have any affiliation with SpectraPure. I noticed from your profile / and SpectraPure are from the same geographical area Phoenix / Tempe Arizona. YOu are the first person that I've read say anything negative about Ebay RO/DI units but I haven't looked in a while. At the other Reef site that goes unmentioned - I've read posts about the Water General unit I'm looking at and if memory serves me they were generally positive.

For 152 bucks with all the accessories / water storage / faucet - that's kindof hard to beat.
http://cgi.ebay.com/WATER-GENERAL-REEF-HOM...=item2ea8e749bd

I generally don't like to spend extra money on commodity type purchases - but open to learning reasons why if there's a legitimate reason to pay a little extra. No way I can spend 200 bucks on a filter system though. Just not in my budget. 100-150 is about my max for this item - and the water filter is a Bonus I wouldn't mind having for drinking water.

thats a long time for 5G of water? i have a buckeye 100GPD unit. in the warmer times i get about 5G in about a hour. in winter time i get 5G in 1-2 hours depending on the time of the day. no booster pump for me. but then again mine being 10gpd more could probley make a huge diffrence along with how much minerals are in the water depening on the area which i or the op lives in. lots of things can factor in i suppose.

I am not affiliated with Spectrapure or any other RO vendor but being a water treatement plant supervisor by profession I do know quality when I use it. I have personally owned 5 different RO or RO/DI units over the years and nothing compares.

The ebay units almost without exception use low quality inferior filters, both prefilter and carbon. Many use off brand imported membranes which are not even ANSI/NSF listed for use in the US. Many use low quality housings and the old twist compression type fittings instead of name brand tested components like John Guest and Amatek/Pentek/US Filter. While some may look the same they are made of more brittle plastics which break easily. I know this from experience. You also do not get things like pressure gauges, TDS meters, adjustable flow restrictors etc with most ebay quality systems. DI resins may be old and outdated since they come over from China by the ship load and sit in huge warehouses. DI has a short shelf life even when sealed.

You really do get what you pay for.


You don't want most 100 GPD membranes. The 100 GPD Dow Filmtec is not even and RO membrane but is considerd a nano filter which is not approved for drinking water use in the US. The difference is 90% rejection or removal efficiency vs 96-98% rejection rate. The life of DI resin is cut considerably with the Dow 100. The Spectrapure 90 GPD starts life as a Dow 75 GPD but is enhanced to produce 90 GPD at 60 psi and at 98% or higher efficiency depending on the model. Some such as the Select series are individually hand tested and guaranteed to be better than 98% rejection. This can be significant since every 2% increase in RO membrane efficiency DOUBLES the life of your DI resin.

Don't doubt AZ. He is the king of RO-DI, he helped me out years ago on RC and helped me find the Spectrapure monthly deals add where I just got my ProPlus from.

Awesome unit. IMO id buy from Spectrapure or the Filter guys and nobody else. Service is excellent from both.
I will say I really like the handbuilt quality of the unit I just got.

While I agree that you shouldn't bother with an eBay RO/DI system, I dispute AZ's contention that Dow membranes are second rate. Spectrapure is like the little yapping Chiuahua claiming he's better than the Great Dane of Dow. There is nothing wrong with Dow membranes, I've used them for over 20 years on industrial sized systems and they are as good or better than any other membranes out there. I can gurantee you that on my 150gpm and 250gpm systems if we got too low of a rejection rate we'd be wasting money fast. We consistently get 98% and greater rejection rates, and I've seen no worse in my small home system. where I get the same results as I do with my big systems.
Spectrapure is not the only system out there, though I'd be saying some jive too if I was trying to sell my RO membranes at nearly twice what a Dow costs.
And no, I am not affiliated with any retailer of membranes, just that I've spent hundreds of thousands of dollars on them over the years, and they haven't even given me a t-shirt.

I am not in any way bad mouthing Dow Filmtec RO membranes, they are the industry leader by far. I AM dissing the 100 GPD, TW 30-1812-100 membrane in particular though.
Take a look here and you will see Dow does not list it with their other tap water membranes up to 75 GPD.
http://www.dow.com/PublishedLiterature/dh_...fromPage=GetDoc

Then take a look here and you will find the 100 listed at only 90% rejection:
http://www.dow.com/PublishedLiterature/dh_...fromPage=GetDoc

Notice Dow states the 75 and below meet the ANSI 58 standard but there is no mention of this with the 100. The reason is the removal efficiency of nano filtration vs Reverse Osmisis which is a step higher.
Take a look at the filtration spectrum found here and you see the difference in particle size they remove.
http://www.dow.com/liquidseps/prod/mfs2.htm

The US EPA does not recognize nano filtration by itself to be sufficient for drinking water 3 or 4 log reduction.

why not add feed line and run it coiled it in your sw mix bin (which I assume is heated), then feed your ro unit. That will heat the feed water to the optimal temp.

Heating water is very expensive and inefficient. When you calculate the BTUs or watts required to heat even one gallon of water one degree its not worth it. Plus the fact plastic tubing is a very poor conductor or heat and cold so you gain very little heat. All the math is around here somewhere or on RC since its been posted many times.

AZ, I agree that heating additional water makes no sense. but since I have my fresh SW heating anyways, there is no downside in down to dowing what I just mentioned. Running water through 50' of additional tubing is worth the price of the tubing on my end.

The cost is in the power to run a heater continously versus intermittently. Calculate what 300 watts or whatever you have is over 24 hours vs maybe an hour or two a day if that and it adds up to quite a bit over a years time. My power cost keeps going up every few months due to some tariff, adjustment, fuel surcharge or whatever so I try to be as efficient as possible. The cost of a booster ends up less than the cost to run a heater in year or two's time.
Many try similar things using a dorm fridge as a chiller and wrap some tubing around inside it but again they are not efficient plus not designed for continous service. I love DIY myself but in the end some ideas just don't pan out!

again, since I am already heating the water overnight for a waterchange, I don't really see a downside.

The down side is run time and associated power consumption on the heater. If its overnight its not so bad but some of us have autotopoffs so we can't control the times when we are making water.
I keep 23 gallons of RO/DI in my topoff reservoir at all times and its starts and stops automatically on level, I wouldn't want to heat it 24/7 since its located in my garage.

You will see about a 10 PSI drop in pressure if you go through 50 ft of tubing that is flat and level. You psi drop will be much more once you start to coil the tubing. Considering you start out at 60 psi at the faucet, you will be at much less than 40 psi by the time you get though the tubing, your pre-filter and carbon block. So in the end your gonna loose water production anyways due to your psi drop, regardless of the water temp. Might as well just forget all the bull and just get a booster pump. Spectrapure sells one for 130 bucks, made in the USA too.

I felt I should defend Dow in general, not a specific membrane. The issue with the 100 has been around since it was first made, not sure why they continue making it. I misunderstood what you were saying.

Correct if I'm wrong, but Spectra uses DOW membranes, just alters them for their specifications and needs.

Most RO/DI units you'll encounter out there (Spectra like the basic MAX Cap and Pro+, Water General, Pure H20, etc) all use the same housings. This was confirmed to me by Charles @ SpectraPure's booth at Reef-A-Poolza here in So. Cali last month. Housings for systems are generally rated for up to around 80psi, and that's mostly for the seals, not the actual plastic of the housing. So filters and connectors removed, Spectra units are NOT superior to even Ebay units, ok SOME Ebay units since many are actually crap indeed. Where Spectra shines is in the filters they use and provide, as well as the R&D into their systems as well as the value for what's offered. At $150 for the Pro+, you get the RO/DI unit, built-in pressure gauge AND an included dual probe inline TDS meter. Alone those would run you almost $50 if you added them onto an Ebay unit (Water General included) and thus you'd be paying more. Till the sale ceases, the Spectra units cannot be beat. Even then they still really can't be for the quality. But I wouldn't knock WaterGeneral either. They are the exception to a typical Ebay unit. They di use quality filters and membranes, though not the same ones as Spectra. GE membranes are just as good as DOW, maybe a hair below and that hair probably isn't terribly noticeable.

But anyways, if you have a big summer to winter differance, I'd definitely do as AZ suggested by incorporating two seperate flow restrictors into the system, one for summer and one for winter. Great idea bud!