Much of your winterizing success will depend on the initial condition of your existing water system. If it is nearly sterile (effectively chlorinated water), there may be too few bacteria and fungus present for infection to start. However, if you use less than 20 percent alcohol or glycol (after dilution with water left in the system) you are at risk of biological growth; the lower the concentration the greater the risk. This is the reason why vodka and other alcohol-based winterizing fluids should be avoided. In our testing these solutions acted like an invitation to a microbial feast.