[mike31154]

My RODI system has been running for close to a year with no filter changes. I have two carbon stages though, one is 0.5 Micron Thick Wall Carbon Chlorine guzzler Model MAXCL-975 good for 20,000 gals @ 1 gpm and the other is a 1 Micron Carbon Block filter. I think this unit was designed to handle both Chlorine and Chloramine. Chloramine is not used by our city to treat the water, so I guess it's overkill. At any rate, I've kept track of the number of gallons produced (just over 800) so I think the 'chlorine guzzler' should be good for a few more.... years in fact. Why would I change it after 6 months? I think that would be a waste of my cash. I've purchased a Chlorine test kit from a hot tub supply outlet, any place that sells swimming pool paraphernalia will probably have these kits as well. So at some point I'll do a little test after the carbon stages, just to make sure.

I also have two pressure guages installed on my system. One monitors the input pressure which is usually between 83 to 85 psi. The second is installed before the RO membrane (after the 3 pre filters, poly & 2 carbons). The reading there has been in the neighbourhood of 50 to 55 psi, dependent on the input pressure of course. This gives me a pressure differential of 30 to 33 psi. Once that drops significantly, it should be a good indicator that one or all of the pre filters are clogging up. So with the initial investment of a couple of $18 pressure guages from Princess Auto, I can save money over time by not throwing out perfectly good pre filters and carbon filters every 6 months.

Of course everyone's source water is different (my input TDS is around 210) and your mileage will vary, but my point is, invest in some monitoring equipment besides the standard TDS meter, such as pressure guages, chlorine test kits and you'll be better able to make the call as to when filters have truly reached the end of their useful life. Not good for suppliers of filters etc, but good for your pocketbook and less stuff in the trash.

Apologies for the long answer, but as stated by hillegom, as long as your TDS is still 0, you should be ok. The 6 month number is a good one to be safe if you have no other way of telling, but it's a rule of thumb and everyone will have different results depending on source water quality and actual throughput of your system. If your RO membrane loses efficiency, your DI media will work harder and need changing sooner.