Installing Undersink RO/DI

i thought 0 on a tds is what you want

 

Hopefully your rejection rate improves as the membrane gets broken in or seasoned. If I am reading the meter right you have 591 in your tap and 65 in the RO only, thats only 89% rejection which is pretty low. With a new Dow membrane it should be in the 96-98% range.
Does the DI filter pull water out of the pressure tank or does it come directly from the RO membrane? This can make a huge difference in DI life and water quality since pressure tanks suffer immensely from TDS creep. Every time you shut off a RO membrane the water inside the filter housing osmoses or equalizes on both sides of the membrane meaning tap water moves to the treated side raising the TDS. So, every time you draw a glass or two of water and the RO starts again to replenish the drop in pressure and volume in the pressure tank it dumps a few pints to a quart of high TDS water into the tank which gets more and more concentrated with time. The easiest way to help this situation is to completely drain the pressure tank every week or two and let it fill overnight or another long undisturbed time so it can refill with fresh low TDS water.

The reason you don't want this stored RO water going into your DI is the added TDS in the tank eats DI resin like crazy, it may only last a few gallons with a small horizontal throwaway DI like comes with that unit. At a TDS of 65 I would expect your DI to be exhausted in about 46 gallons by my calculations if that DI cartridge is a full 16 oz of resin which it may only be 12 oz in which case the DI may only last about 34 gallons.

If the DI does come from the pressure tank you need to do a little replumbing with a tee and check valve so the DI comes directly from the RO membrane. This diagram shows how to seperate the RO and RO/DI systems for drinking and reefkeeping and make your DIl ast longer and work better.
http://www.spectrapure.com/huds/4-STAGE-DWK-RODI-NAG.pdf

This unit:
http://www.buckeyefieldsupply.com/showproducts.asp?Category=171&Sub=166
comes with the RO and RO/DI plumbed seperately plus is more of a reef quality system versus the AWI unit which is a drinking water system with an added small horizontal DI. The differences are in the filter micron ratings and efficiencies, the full size 20 oz vertical refillable DI and the addition of an inline pressure gauge and a TDS meter standard. For the $277 for a 75 GPD its really a better value but at least you are on the right track by getting a RO/DI system to begin with. Yours can be improved over time as filter replacements become necessary and you could even convert one of the useless carbon filters to a full size vertical refillable DI for about $10-$15 and a few minutes time.

Good install thread though and I applaud you for taking the time to show everyone! I want to know how you found that much room under the sink? My wife has so much stuff under the sink there is no way I would have room so I have both my RO and RO/DI units mounted in the garage over my laundy sink.

 

Well alot has changed since this install, mostly I moved...

Also the whole system underwent an "upgrade", the three filter cartridges hold a .5 micron sediment and carbon block and carbon "polishing" filter for the drinking water. RO membrane is a Spectrapure 75gpd with appropriate flow restrictor. Also replaced the DI with a Spectrapure MaxCap...

That is the exact diagram I used for the "replumb", except the inline carbon is a full size...

Starting out over 600 I thought 2 was quite acceptable considering the inexpense of the unit and inline DI not being the best. After the upgrade I do get 0...

Thanks for the validation, this is exactly what I do and now I know why (besides intuition)!

 

Whoa, there's your problem with 2 TDS output water! The MaxCap isn't good alone as a DI! You need to have a SilicaBuster DI in line after the MaxCap!

 

The 2 TDS was with the inline DI, which has been replaced with the MaxCap Dual-Stage Add-on:

MAXCAP D2 ADD ON KIT

 

Blackraven is correct, the MaxCap cannot be used alone. It is intended to be used as a roughing cartridge or pre DI if you will for a downstream mixed bed DI cartridge. It will not work correctly on its own and will alter pH. When you use it in front of another DI the last DI will last at least 3 or 4 times longer and you can usually go through 3 MaxCap refills or cartridges before you have to replace your first regular mixed bed DI cartridge. It may and probably will give you 0 TDS but it will alter other properties you don't want in your finished water.

My tap TDS often exceeds 800 and I get 0 TDS for over 1000 gallons per MaxCap cartridge and over 3000 gallons per SilicaBuster cartridge compared to only 150 gallons when I owned a Typhoon III, the difference is amazing.

EDIT. You beat me to it, I was typing as you posted your last message!