You should probably order a new membrane as well if you didn't already. If your prefilters are 18 months old + then your membrane is likely shot too since the prefilters probably stopped doing their jobs months ago.
We suggest changing the sediment and carbon cartridges every 3-6 months depending on the quality of the source water and/or usage (use more water = change more often).
If your cartridge canisters are clear, you should be able to see when the sediment/micron cartridge starts looking grubby, that's a good time to change both that and the carbon. The DI cartridges, for the most part, will change colours as they exhaust, so if you have clear canisters you can see that too (I don't like units with white or solid coloured canisters, you have to open them to get an idea what's happening in there).
$25 invested in an in-line dual TDS meter is a good idea too. You can push a button and toggle a switch to see what the source water TDS is, and the product water TDS, which will tell you in a second or two, if everything is performing as it should.
Something else too - now that it's really cold outside, your RO unit will work more slowly. Colder water = molecules are closer together. Takes longer for it to go through the membrane.
Jenn
