For hard water areas, running only a DI only unit is not going to be cost effective because you will run through the DI Resin incredibly quickly. Instead, you need a multi stage system that will draw out a majority of the impurities before the water hits the final DI chamber for that final polish.
Here's an example:
If your water is 222 ppm you could expect about 700 gallons for 23 working hours per resin refill which is around $139. A Multi Stage system like the XERO Pure will give you 5,500 gallons for 184 hours and it would only be $79 to refill the resin. To accomplish that same 5,500 gallons with only a DI Tank, it would cost you close to $1,100. You can easily see how that would add up.
Multi Stage systems utilize at least a pre-filter, RO Membrane, and a DI Housing to effectively purify hard water.
XERO offers the Pure Line with options for any budget or workload.