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I have a 7 yr old FHW by gas heating system that makes a banging sound even at modest boiler temperature well below 212. I have heard that minerals can b build up at the bottom of the cast iron boiler that can inhibit heated water from escaping the bottom of the boiler, which gets overheated to locally over 212. The banging is due to this water vapor escaping.

Does this sound plausible? How can I treat the boiler to get rid of the minerals without damaging the cast iron and causing a breakthrough?

I purchased a quart of Hydronic Formula #5 SK471 with silicone (Silver King Mfg. Company, Warrington, PS). Is anyone familiar with this product, might know what chemical is in it, and know if it is effective? How susceptible am I to causing damage to the boiler so severe that a leak might result from treating the boiler water with Formula #5?
 

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Few things can cause this. Most common is bad flow. A power flush can normally fix this issue. In the UK they call it boiling up. The pump can also cause this issue by not moving the water through the boiler fast enough. Anything that can restrict the flow through the boiler causing it to reach boiling point before it leaves will cause this.

Any system cleaner can cause problems. The build up of sludge can cover pin holes and once moved you may see some leaks or you may not. The rads take quite a beating if done correctly because you use a special sds attachment that vibrates the rads. This can also cause its own problems. I have blown this tool through a few rad panels where they were completely corroded inside.
 
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