Those who are experiencing a hiss with HFCs are experiencing it as a result of their exhaust and HFC pipes not mating. Some members are running a HFC with a 2.25 in outlet and then are running a 2.5" exhaust. Or some are running a 2.5" HFC and then a 2.25" inch exhaust. The variations in piping diameter at the flanges are causing air to seep out and creating a hissing sound.
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