How often should boiler water be replaced on a 5 yr old unit?

Steam or hot water boiler?

(Disclaimer - I'm just a semi-educated homeowner.)

If this is a steam boiler, you need not ever change the water. This assumes the boiler has been properly skimmed, etc. You're trying to introduce as little freash water as possible - to avoid the corrosive quality of the oxygenated water. Of course, over time you will need to add make-up water and of course you need to blow down the LWCO routinely and top the boiler up after. That, at least is my understanding.

For hot water, the same corrosion issues would seem to come into play, but a system with antifreeze would change the approach, and (for all I know) so might some other circumstance.

Just my understanding....

(a later edit to my lunchtime post:)

If the boiler water is filthy, foamy & bouncing in the glass, I believe it does need treatment/changing/skimming, etc.

As far as air, I'm fairly full of hot air, but I don't believe I'm the sole source around here.

OEM air...

... works even better. However, especially for vintage and antique car collectors, finding OEM air (if possible, from the year the vehicle was manufactured) is becoming ever more difficult.

Shouldn't a cleaning chemical be added to the boiler before draining and replacing the water?

Is there any way to flush the (1-pipe) runs and risers?

-Charles