We are in the central valley in California. We typically do not see any prolonged freezing periods in the winter, but have very warm and dry summers. We have the combined HT and LT circuits as a result and do not use glycol at all. We only add a corrosion inhibitor to our radiator water.
In our logic the LT pump/heater turns on automatically when temps outside dip below the 35F point. To date, we have been fortunate that this is all we have concerned ourselves with. We do have a combined cycle plant on the same property that is our base load plant, so I am sure there is some residual heat in the local vicinity that adds to our freeze buffer.