I have a Smith GV100W Propane Gas Boiler that is 13 years old. I getting a loud hot water pipe banging at the beginning of the cycle before the boiler comes. The gauge on the boiler shows less than 10lbs pressure currently. The hot water baseboard heating works fine except for this.
I understand the banging is from air in the pipes and the pressure should be between 10 and 20lbs.
What should I do to correct this and how do I bleed the air out of the pipes if this is the problem?

Thanks

I am assuming the loop runs from the basement up, so that tells me that your system is condensing air as the water cools going away from the boiler, (as water cools it contracts) leaving air gaps if you find the hi point in the loop the fix is to install an air bleed valve. (at the hi point) these valves are relatively inexpensive and if the pipes are copper will require some soldering or comp fittings to install. This is one suggestion to your trouble