Horizontal DI housings are a design intended to minimize the original cost of the system - you should be prepared for the tradeoffs. Horizontal DI units typically contain 8 oz. to 16 oz. of resin. Typical vertical DI cartridges contain 20 oz of resin. Obviously the more resin contained in the housing the longer it will last and the better treatment it will provide.
Some horizontal DI housings are not refillable - you'll therefore have to pay for a new housing every time you need to replace the DI resin. The cost of repeatedly replacing the horizontal housing will far outweigh any money saved up-front in purchasing the unit.
Perhaps most importantly, horizontal DI housings are a less than ideal arrangement for water treatment. DI resin beds shrink through normal use over their life span. You'll note that a cartridge that was full when new can sometimes have a ¼ inch of empty space in it when fully expended. When DI resin settles in a horizontal housing, it leaves a pathway (of least resistance) along the top of the housing where water can flow while coming into minimal contact with the DI resin.
You’ll note that the output from the DI housing is at the center of the end of the housing. Depending upon how your system is configured, RO water may enter the DI housing in port, fill up the housing until the water level reaches the out port (i.e., fill up the bottom half of housing), and then exit the DI housing. Your RO water has been in contact only with half the resin in the housing.
Russ
