How often do I need to flush the boiler and add boiler treatment to the system to expand the life of the boiler from expensive replacements? I just did the flushing in Dec 2005, and I just have a plumber in to inspect the furnace condition, they changed the gas value and when I ask them when I need to flush the system, they say in about another 5 years. Isn't that too long, since scale buildup in pipe? And how often do I need to change the thermocouple? Is there any sites to look up where you can do it yourself to add boiler treatments to keep the boiler clean? thanks!

No you don't need to flush a low pressure boiler every year, actually it is better if you don't. Once the water has loaded up with metals from the system it can no longer leach anything more, the water has become super saturated with metal ions.