Originally Posted by qman

Distilled is the water of choice because it has no minerals in it. Minerals build up over time is all. Where I live our hose water was snow only hours ago so about the only minerals we have (which is nearly none) are what wash off the rocks as the snow melts and the water runs down the mountains. The tap water here is so soft that the chem labs use it as distilled water. Now if you're pumping it out of a well and it's loaded with heavy minerals like iron, etc. then you'll want to get distilled water in there eventually. But don't waste it flushing trash out of your engine block.

A cooling system is nothing more than a bunch of passageways that coolant (water/antifreeze) flow through to take heat from the engine. It's not a precision system like the oil where you can't afford to have hunks of stuff floating around in it. You won't hurt anything long term using tap water.

Your system doesn't need to be spotless. It would be nice but it just doesn't matter. What matters is that it doesn't over heat. I just drained my system to put new manifold gaskets in it. I've owned this truck 17 years and every time I have to drain the coolant it comes out with a tad bit of rust. When I pour the old coolant back in I just don't put in the last few ounces that are loaded up with rust dust.