Which RODI part needs to be replaced first?

298 is just slightly above the national average of about 250. Not that bad really. mine is usually over 800, never ever drops below 500, and can get as high as 1200.

Using your TDS figures it shows your membrane is only 85% removal efficiency or rejection rate. Not good if you tested directly from the RO membrane and not from a faucet or pressure tank.

Don't waste any money on DI resin replacement if the membarne is in fact that bad.
A simple rule of thumb says for every 2% you reduce the membrane efficiency you cut your DI life in half! Imagine how long DI would last if you were operating at 98% rejection rate which is where you could/should be. A new Spectrapure membrane is on sale for $35 for batch tested and only $45 for individually hand tested and guaranteed better than 98% rejection rate 90 GPD membranes.

Lets find out first if your TDS numbers are correct, then tme the waste ratio and get the temperature and pressuer and lets look at the system as a whole instead of throwing pieces at it that may not last.

 

Pressure is about 55 the temp I am unsure of but I would imagine its about 50 - I didn't let it run at all - I measured it right away! The DI comes directly off the RO and I have no tank. And I sampled the RO right from the membrane - about a 6" piece of tubing that come off of it

 

Let it run several minutes before sampling so the TDS creep passes and sample again to get a better reading. You will see it continue to drop for as long as 5 minutes, especially with colder waters. It should hit a low point and stay there.
When you are making topoff wate rthe longer you can let it run the better so you don't keep fouling the DI resin with high TDS creep water. Fill a 5G jug and use it until its empty so you get nice long filter runs and reduce the effect of TDS creep.

 

Yeah, thats a little problem. I have a water topoff that runs directly off the RODI system. NOT TOO BAD though because its on a timer that only allows it to run for one 2 hour time frame per day. I will try retesting though and see what I get.

 

Plumbing directly to a sump or tank is highly discouraged, timer or not. I also hate float valves as they fail just like toilet or swamp cooler floats. I use float switches and a solenoid in my top off storage container so it has to drop 11 gallons before refilling and and the sump has an optical/air sensor system made by Spectrapure (UPLC-II) which can be adjusted several inches between start and stop. No TDS creep problems for me.

 

I have a float and a solenoid so it cannot overflow if the float doesn't work.

 

OK, retested everything doing it as you specified. I cleaned the container with very hot water several times and let the unit run 5-6 minutes. OK, the tap water is 280, RO is 16 and Final RODI is 7. So that seems much better.
I think I will wait on the membrane considering its at aroun 95% efficiency. I will do the other stuff though soon.

 

AZ did you actually invent the RO/DI system design cuz you are THE MAN. Do you do service calls. LOL

 

Thats much better for the RO TDS. The DI is still in need of replacement, actually past the point of needing it. What many people don't realize is DI resin starts to release weakly ionized substances even before it is exhausted, a few of these include nitrates, silicates and phosphates, so change your resain as soon as you start seeing any reading at all. If not you could be doing more harm than good since weakly ionized substances may not even register on a hobbyist grade TDS meter and exhausted resin can release it in huge spurts many times higher than the tap was to begin with since it has been collecting and concentrating the substances for many gallons.
A house call to South Carolina might get expensive! Never been there though so it might be fun. I have a cousin in Akin if you know where that is?

 

Well, I finally got around to doing the maintenece on my RODI unit today. I only replaced the sediment filter and the DI resin for now. In 6 months or so I am going to replace the carbon, sediment, DI and the RO membrane to start out fresh but this should be good until then. Actually I have enough DI to do it again so I will probably do that in between just to band aid it in the meantime. I have a vacuum packer for food so I resealed it up air tight. I am reading 0 TDS once again.