He takes all the gas out, vacuum checks the system for leaks and removes any moisture in the system then refills with the correct weight of gas. The old stuff is sucked into a different gas bottle so I assume it gets reprocessed. Nothing leaks out or is wasted.
I use a chap who's company is called Brighton Autogas. He comes to your house and will remove all the old gas, suck out any water in the system and re-gas with new stuff... He told me to refresh gas every two years and charges £45..
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