Coolant flush

Could never figure why they would make people think anti freeze is a coolant. Water has a higher cooling factor. You do need the rust and lubrication additives.

 

Never thought about using distilled water, but that makes sense....The water here is awful hard and the mineral content is likely not a good addition to the cooling system.....I've got a '96 Ford F-250 with a 351 that developed a coolant leak.....Took the darn thing apart and found that pinholes had developed in the head, from the inside out, next to the number 2 cylinder...Never saw that before and wouldn't have believed it if I hadn't seen it.. Also, holes developed in the core plug in the rear of the same head...

 

Antifreeze lowers the boiling point of the coolant, 55% antifreeze and a 15 lb cap gives you a boiling point of 243 degrees. Water even under pressure will boil at the valve area, and will pit the hell out of the liners, from Cavitation.
Doing a flush of the cooling system is always a great idea, you want to remove the lower block plugs to get the scale out. I use a pail of warm water with Arm and Hammer washing soda, mixed, with a paint mixer,or bent welding rod, as strong as possible, pore into another pail and add some dishwasher soap, fill the unit with every drop you can, that way it cleans the inside of the recovery/surge tank. doing it this way will even remove the scale.
When you flush it out do the heater backwards to clean it as much as possible.