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[allis48]

I want to change water in my boiler but should I drain it and leave empty till fall, wait till fall, or drain and fill now?

[mlappin]

Any metal exposed to air will rust. Drain it now if desired, refill and add your treatment.

I think I would wait till fall, drain before starting it, refill then once it's up to temp and full, add your treatment

[Sprinter]

I wouldn't drain unless it needs to be stored for a very long time. Every time the metal is exposed to. Air it starts a new round of oxidation. This is why indoor boilers last so long. If your really concerned, I would put a filter in line and clean debris from the system in the summer. If possible  check to see if you have build up sitting on the bottom of the water jacket. Those little inspection cameras are handy for this. I use a rigid mini see snake , or jet the bottom and drain a little off.
Next is to cycle the circulators once a month or 2 weeks, as the stagnate water is when things grow or corrode and lock up circs. If its gonna happen at all.

[allis48]

the only reason I am changing it, is cause my ph level is really high

[big cat]

Does it make sense to flush with something like CLR before refilling and adding treatment?

[slimjim]

I don't think I would use anything like CLR, how about using rainwater from a barrel and treating it right away.