Do you have one with a hosebarb? or threaded fitting. If it's threaded then it's 5/8" CGA. If not, I think it's 5/16 hose barb. Usually 1/4" can be fit onto 5/16" hosebarb if there is any flex at all in the line. We've been using 5/8" CGA for a while now on most of our units.