Boiler water sulfite level, conductivity, pH, and return line treatment are all good places to start. I'm assuming we're speaking of a steam boiler system vs. closed loop hot water. Any time water has to be made up to a boiler/steam system it will need treatment. Regular blowdowns are also essential to keep solids in check.
On my boilers I try to keep sulfite levels above 40ppm, conductivity levels between 3,000 and 4,000 ppm. Find yourself a good chemical company as they will know your local water supply and can recommend proper treatment procedures. Ask them to get you the necessary test kits so you can take daily samples of your boiler water and know the status of the treatment. Minimizing steam and return loop leaks will also reduce amount of treatment needed to maintain a healthy steam system.
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.