Raw Dodge is giving good advice, but.......... The equipment manufacturer, Ventrac, seems to recommend very short service intervals as if they believe their machine is under severe service routinely. For example, the engine manufacturer, Kubota, recommends motor oil and filter changes at 200 hours of service where Ventrac states 50 hours. Kubota specifies coolant changes every other year, Ventrac, as I noted in my OP specifies annual coolant changes with flush. (Kubota does not mention flush agent or flush). The equipment is under Ventrac's consumer warranty. I will follow the Ventrac specification for those 3 years. So, I drained the radiator while the engine was very warm. The approx 6+ quarts of Asian "Pink" (Toyota?) coolant was pristine clean and clear in the waste bucket. There was, by inference, about 3 pints of "old" coolant in the engine block and plumbing. I installed the flush agent and distilled water before operating to full temp. Let it cool to ambient temp and installed the flush fixture to the radiator and flushed with tap water with engine at idle and circulating. The flush was clear and clean from the beginning, it did NOT need a flush. After a thorough "flush" with tap water the system was filled with distilled water and run briefly to operational temp, allowed to cool to very warm and drained again, the water again clean and clear. When cool, added a gallon of Prestone yellow :blends with anything type of silicate free coolant" and topped-up with distilled water; this step to prevent dilution of the final coolant due to retained water in block and plumbing. Ran the machine as I mowed a field. Let cool to very warm and drained nice clean yellow coolant mix. When cool, I added new Asian type super long life pre-mix coolant (Honda Blue, same as Toyota Pink). What a waste of time, coolant, flush agent, distilled water, and $. But, I followed the Ventrac directive. When the warranty is out, I will service the cooling system when the coolant tests less than new, of at 3 years and NO flush. And even at that the procedure will be somewhat wasteful. If you are not quite as nutty as I, you could get by with less time and work by using pure antifreeze instead of 50:50 and then topping up with distilled water. But I decided to use the top quality Asian type coolant as was in the new machine, but could not find any other than premix. I had the Prestone yellow on my shelf for years, so "used" it to correct the trapped water to 50:50. prs