Senior TN Member
Join Date: Apr 2008
Location: Austin, Texas
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To get the most old coolant out of it, you can open the block drains as well as remove the thermostat. Remove a heater hose going in to the heater core, and run water through with a hose, tap water OK. Let it all drain out of the block and radiator. If you are worried about a little tap water in the heater core, you can pull the other heater hose off and get most of it out.
Some shops have flushing machines, they have to pay for those and sell a coolant flush as a serrvice. I doubt they use distilled water for flushing and many use a "universal" type coolant pumped out of a 55 gallon drum to refill the system. Often, an actual flush is not necessary.
If your system is contaminated with stop leak or, lots of corrosion, you may want to drain it, fill it with water and a flushing agent like BG. Then drain, flush with water, drain etc etc. However, excessive stop leak or, corrosion is pretty much impossible to remove.
If the coolant you initially drain out looks reasonably clean and not contaminated, just drain as much as you can and refill with the correct coolant.
The most important thing is to use the correct coolant for your vehicle.