Many people do not measure ...

Many people do not measure silica in water supplies.   Many boreholes have silica present but it is much lower than the hardness often as much as 1/10.  There are waters where the hardness levels are relatively low but the silica levels are proportionately high i.e. 20-25% of the hardness.  Scales can form including some calcium and magnesium but silica and iron and these form heat resistant scales. It doesn't need much hardness slippage to allow these complexes to occur.  To control silica in a boiler, a minimum of 2.4 times of free caustic is required.  Phosphate programs are better than chelant to aid removal.  I have also used organic sequestrants in conjunction with phosphate polymers and caustic levels 3 times the silica content.  This can aid removal, by causing cracking of scale deposits.  Care must be taken because lumps of scale can bridge the tubes causing localised overheating.  Once the scale is breached normal acid cleaning using inhibited hydrochloric acid can be used.  If the scale has not cracked then hydroflouric is the answer.  It is extremely nasty to carry out and should only be done by a specialist company under strictly controlled conditions.  

I disagree with the comment that often chemical suppliers motive is to push as much chemical as they can.  Reputable companies, of which there are many, dose the required amount of chemical to do the job in question.  There is a British Standard for the treatment of steam boilers, which is a useful guide for the correct treatment.