Deionized water classifications are Class 1, Class 2 and Class 4. The lower the number the greater insulative quality and higher the cost. ChemWorld is one producer lab / wholesaler of coolant, DI and distilled water. A 55 Gallon Drum of Class 1 DI currently goes for over $500 (yikes) and has electrical specs of conductivity of < 20 uS per cm and resistance of > 18 Mega ohms per cm. Their Type II goes for around $360 for a 55 gallon drum of DI with specs for conductivity of < 1 uS per cm and resistance of > 1 mega ohms resistance per cm. Compare the conductivity of typical drinking water in the range of 5-50 mS/m, while sea water about 5 S/m[2] (i.e., sea water's conductivity is one million times higher than that of deionized water). Why Not Stick with Distilled? Keep in mind some basic myth busting: Distilled and Deionized water are the same. MYTH Deionized water does not have the correct PH content and will rot your cooling system. MYTH Water run through a softener / drinking water filter will work on my customer 's Chevy Volt's power electronics cooling system. MYTH The time honored method for making water pure is distillation: Heat water until it boils. Trap the steam created from the boiling process. Pull off the condensation from the steam. Filter the condensation. The biggest reason to use DI water is that's what some OEMs are mandating. (Fig. 11 & 12) To make sure you really know what kind of water is in your premix coolant, do a little digging. Look at the owner 's manual and the factory service manual as well as TSBs to determine what the OEM Courtesy AC Delco Photos Dave Hobbs The problem is when you distill (boil) water, the salts don't evaporate. So when you collect the water, it has less minerals in it but it is NOT free of minerals. Figure 11 and 12: This ACDelco DEX-COOL premix gallon jug (left) has deionized as the other half of the 50/50 mix. ACDelco further states regarding antifreeze concentrate to: "Use clean drinking water to dilute as impurities in the water can cause a reaction to the coolant and harm the cooling system. Deionized water is recommended." The same exact GM p/n applies to the GM dealer premix DEX-COOL. The PI (GM Preliminary Information bulletin) mentions the Chevy Volt's requirement for DI water and NOT tap or distilled water when not using one of the DEX-COOL coolants that are premixed. May 2017 7 MACS Service Reports