A friend has an RO system that seems to have lost capacity. They can draw about 40 oz.s of water, 1st thing in the AM, before flow slows to a trickle. The system was in the house when they bought 5 years ago, and no maintenance has been done in that time. I’m sure the filters need to be replaced, so…Is it all 3 filters that need replaced? Are they a pretty standard thing, available at my local big-box home-center? The two filters mounted vertically are in cylinders of identical size (the horizontal one is smaller)and I may have already switched their position. Does it matter which is first, in relation to the flow? Thanks for any input.

I’ve replaced filters in several RO units. The larger canisters at the bottom are pre-filters. They comprise of a
sediment pre-filter, hopefully you’ll see a micron size on it. The next filter should be a carbon filter. The water flows from sediment to carbon, this removes the chlorine which can damage the RO membrane. Then from the carbon the water flows to the inlet of the RD cartridge. It won’t look anything like the other filters which should make it stand apart. In the RO filter there is an inlet, a waste line and a good water line. Sometimes they are color coded. I’ve seen blue as good water, black as waste.
Take a picture of it before you disassemble it. Its really easy to get confused about how the water flows. Not so much on the pre filters but on the RO membrane. You wouldn’t want to be drinking from the waste line. You can try to just replace the sediment and carbon and see if it works. Get a water testing kit and test the output. That will show if the RO membrane needs replacing. If the flow is still bad then it may mean the RO membrane needs replacing too. Graingers has RO filter membranes, sediment filters and carbon filters if you can’t find the right type or size anywhere else.