[mike31154]

I can understand it being a problem if your pre filter or carbon block filter housings are not filling completely, but the DI stage on most systems is generally a refillable cartridge and it has openings only at the top & bottom, that is, the water can only enter and exit at the bottom/top since the rest of the filter is a non permeable plastic casing. Therefore, as far as I can tell, there shouldn't be a need for the DI housing to be completely full of water for it to work correctly. As long as there's sufficient water going through the housing to enter one side of the DI case and exit the other, I don't see the need to have the entire housing completely full. Carbon blocks & poly pre filters allow the water to permeate along the entire length of the filter, so yes, those housings need to be full for best performance. I'm just thinking out loud here and could be very wrong, but as I mentioned, I get 0 TDS out of my system and my DI housing has never been full. The resin is slowly changing colour from bottom to top which would indicate that the water moves from the outside of the cartridge, into the bottom of the DI casing and out the top. Pretty sure all the other housings work the same way as far as which way the water travels... from the outside of the filter, then through the center top to the next housing.

Having said that, I also had a refillable carbon cartridge between my 1st stage poly pre filter and my solid carbon block filter. I've since replaced it with another poly pre filter. These pre filters are always completely full and the refillable carbon filter looks to be the same type of housing as the refillable DI cartridge, so not sure why one would be full and not the other. Maybe because there's higher pressure at the pre filter housings before the RO membrane. I reckon by the time the water gets to the final DI stage, there's not a whole lot of pressure left. The pre filters, RO membrane and waste line bleed off a pile of psi. Makes sense that the DI housing may not fill up since there's such a slow output after both the RO membrane and out of your DI line. There's no back pressure there until your auto shutoff on the DI line kicks in, if so equipped. Quite certain all my fittings are secure and water tight, otherwise there would be bubbling or water leaking out somewhere with just over 80 psi source pressure and 60 psi before the RO membrane.